Slide 1

Knowing the Heavens

Chapter Two

Naked-eye astronomy had an important place
in ancient civilizations
• Positional astronomy
– the study of the positions of objects in the sky and
how these positions change

• Naked-eye astronomy
– the sort that requires no equipment but human
vision

• Extends far back in time
–
–
–
–

British Isles Stonehenge
Native American Medicine Wheel
Aztec, Mayan and Incan temples
Egyptian pyramids

Eighty-eight constellations cover the
entire sky
• Ancient peoples looked
at the stars and
imagined groupings
made pictures in the
sky
• We still refer to many of
these groupings
• Astronomers call them
constellations (from
the Latin for “group of
stars”)

Modern Constellations
• On modern star charts,
the entire sky is divided
into 88 regions. Each
is a constellation
• Most stars in a
constellation are
nowhere near one
another
• They only appear to be
close together because
they are in nearly the
same direction as seen
from Earth

The appearance of the sky changes during the
course of the night and from one night to the next
• Stars appear to rise
in the east, slowly
rotate about the
earth and set in the
west.
• This diurnal or daily
motion of the stars
is actually caused
by the 24-hour
rotation of the earth.

Animation of constellation movement
• To represent what we have just discussed,
follow this animation from the vantage
point of our Californian observer.

Annual Motion
• The stars also appear
to slowly shift in position
throughout the year
• This is due to the orbit
of the earth around the
sun
• If you follow a particular
star on successive
evenings, you will find
that it rises
approximately 4
minutes earlier each
night, or 2 hours earlier
each month

It is convenient to imagine that the stars are
located on a celestial sphere
• The celestial sphere is
an imaginary object
that has no basis in
physical reality
• However it is still a
model that remains a
useful tool of positional
astronomy
• Landmarks on the
celestial sphere are
projections of those on
the Earth

•Celestial equator
divides the sky into
northern and southern
hemispheres
•Celestial poles are
where the Earth’s axis
of rotation would
intersect the celestial
sphere
•Polaris is less than 1°
away from the north
celestial pole, which is
why it is called the
North Star or the Pole
Star.
•Point in the sky directly
overhead an observer
anywhere on Earth is
called that observer’s
zenith.

The Celestial Coordinate System
• Again, let us see what we have just
determined in a more 3-dimension manner.

Positional astronomy plays an important
role in keeping track of time
• Apparent solar time is based on the apparent
motion of the Sun across the celestial sphere,
which varies over the course of the year
• Mean solar time is based on the motion of an
imaginary mean sun along the celestial
equator, which produces a uniform mean
solar day of 24 hours
• Ordinary watches and clocks measure mean
solar time
• Sidereal time is based on the apparent
motion of the celestial sphere

• Local noon is defined to be when the Sun crosses the
upper meridian, which is the half of the meridian
above the horizon

Sidereal and Solar Days
• Appreciating the difference between a solar
day and a sidereal day is a challenging
concept. See if this helps.

Circumpolar
stars

• At any time, an observer can see only half of the celestial sphere
• The other half is below the horizon, hidden by the body of the Earth

Last thought on coordinates
• The Equatorial System of Coordinates is what most
astronomers use when specifying the location of an
object on the Celestial Sphere
• Right Ascension (measured eastwards from the
Vernal Equinox) goes from 0h to 24h
• Declination (measured north or south from the
celestial equator goes from -90° to +90 °.
• The hour angle (HA) of an object is the angle
between the meridian on which the object is situated
and the (observer’s) celestial meridian
• ST = RA + HA

The seasons are caused by the tilt of
Earth’s axis of rotation
• The Earth’s axis of rotation is not
perpendicular to the plane of the Earth’s
orbit
• It is tilted about 23½° away from the
perpendicular & is called the obliquity.
• The Earth maintains this tilt as it orbits
the Sun, with the Earth’s north pole
pointing toward the north celestial pole

The Earth’s orbit

• Seasons do NOT arise from the
distance the Earth is from the Sun but
rather as a result of the Earth’s annual
motion and axial inclination – the tip
of our planet with respect to its orbital
plane. As we move around the Sun,
the orientation of our planet gives us
seasons.

Seasons
• During part of the year the northern hemisphere of
the Earth is tilted toward the Sun
• As the Earth spins on its axis, a point in the
northern hemisphere spends more than 12 hours in
the sunlight
• The days there are long and the nights are short,
and it is summer in the northern hemisphere and
winter in the southern hemisphere
• The summer is hot not only because of the
extended daylight hours but also because the Sun
is high in the northern hemisphere’s sky
• As a result, sunlight strikes the ground at a nearly
perpendicular angle that heats the ground efficiently
• This situation reverses six months later

Sept
21
June
21

Dec
21
March
31

• The Sun appears to
trace out a circular path
called the ecliptic on
the celestial sphere tilted
at 23 ½ degrees to the
equator
• The ecliptic and the
celestial equator
intersect at only two
points
• Each point is called an
equinox
• The point on the ecliptic
farthest north of the
celestial equator that
marks the location of the
Sun at the beginning of
summer in the northern
hemisphere is called the
summer solstice
• At the beginning of the
northern hemisphere’s
winter the Sun is farthest
south of the celestial
equator at a point called
the winter solstice

Landmarks on the Earth’s surface are
marked by the Sun’s position in the sky
throughout the year

The Moon helps to cause precession, a
slow, conical motion of Earth’s axis of
rotation

Precession causes the gradual change of the
star that marks the North Celestial Pole

Astronomical observations led to the
development of the modern calendar
•
•
•
•

The day is based on the Earth’s rotation
The year is based on the Earth’s orbit
The month is based on the lunar cycle
None of these are exactly the same as
nature so astronomers use the average
or mean day and leap years to keep the
calendar and time consistent

The different types of ‘year’.
• The sidereal year (year with respect to
the stars) measured in solar time is
365d 6h 9m 10s (365.2564d) in length.
• The tropical year (successive passages
of the Sun through the Vernal Equinox)
is 365d 5h 48m 46s (365.2422d) in length.
• Due to precession, the tropical year is
20m 24s shorter than the sidereal year.

Calendars
• Caesar introduced the 365.25 days calendar
and thus the Leap Year (February 29)
• However, this is 11m 14s longer than the real
tropical year. This accumulates to 3 days in
4 centuries error.
• To correct, October 4 was followed by
October 15, in 1562 and the century rule was
invoked.


Slide 2

Knowing the Heavens

Chapter Two

Naked-eye astronomy had an important place
in ancient civilizations
• Positional astronomy
– the study of the positions of objects in the sky and
how these positions change

• Naked-eye astronomy
– the sort that requires no equipment but human
vision

• Extends far back in time
–
–
–
–

British Isles Stonehenge
Native American Medicine Wheel
Aztec, Mayan and Incan temples
Egyptian pyramids

Eighty-eight constellations cover the
entire sky
• Ancient peoples looked
at the stars and
imagined groupings
made pictures in the
sky
• We still refer to many of
these groupings
• Astronomers call them
constellations (from
the Latin for “group of
stars”)

Modern Constellations
• On modern star charts,
the entire sky is divided
into 88 regions. Each
is a constellation
• Most stars in a
constellation are
nowhere near one
another
• They only appear to be
close together because
they are in nearly the
same direction as seen
from Earth

The appearance of the sky changes during the
course of the night and from one night to the next
• Stars appear to rise
in the east, slowly
rotate about the
earth and set in the
west.
• This diurnal or daily
motion of the stars
is actually caused
by the 24-hour
rotation of the earth.

Animation of constellation movement
• To represent what we have just discussed,
follow this animation from the vantage
point of our Californian observer.

Annual Motion
• The stars also appear
to slowly shift in position
throughout the year
• This is due to the orbit
of the earth around the
sun
• If you follow a particular
star on successive
evenings, you will find
that it rises
approximately 4
minutes earlier each
night, or 2 hours earlier
each month

It is convenient to imagine that the stars are
located on a celestial sphere
• The celestial sphere is
an imaginary object
that has no basis in
physical reality
• However it is still a
model that remains a
useful tool of positional
astronomy
• Landmarks on the
celestial sphere are
projections of those on
the Earth

•Celestial equator
divides the sky into
northern and southern
hemispheres
•Celestial poles are
where the Earth’s axis
of rotation would
intersect the celestial
sphere
•Polaris is less than 1°
away from the north
celestial pole, which is
why it is called the
North Star or the Pole
Star.
•Point in the sky directly
overhead an observer
anywhere on Earth is
called that observer’s
zenith.

The Celestial Coordinate System
• Again, let us see what we have just
determined in a more 3-dimension manner.

Positional astronomy plays an important
role in keeping track of time
• Apparent solar time is based on the apparent
motion of the Sun across the celestial sphere,
which varies over the course of the year
• Mean solar time is based on the motion of an
imaginary mean sun along the celestial
equator, which produces a uniform mean
solar day of 24 hours
• Ordinary watches and clocks measure mean
solar time
• Sidereal time is based on the apparent
motion of the celestial sphere

• Local noon is defined to be when the Sun crosses the
upper meridian, which is the half of the meridian
above the horizon

Sidereal and Solar Days
• Appreciating the difference between a solar
day and a sidereal day is a challenging
concept. See if this helps.

Circumpolar
stars

• At any time, an observer can see only half of the celestial sphere
• The other half is below the horizon, hidden by the body of the Earth

Last thought on coordinates
• The Equatorial System of Coordinates is what most
astronomers use when specifying the location of an
object on the Celestial Sphere
• Right Ascension (measured eastwards from the
Vernal Equinox) goes from 0h to 24h
• Declination (measured north or south from the
celestial equator goes from -90° to +90 °.
• The hour angle (HA) of an object is the angle
between the meridian on which the object is situated
and the (observer’s) celestial meridian
• ST = RA + HA

The seasons are caused by the tilt of
Earth’s axis of rotation
• The Earth’s axis of rotation is not
perpendicular to the plane of the Earth’s
orbit
• It is tilted about 23½° away from the
perpendicular & is called the obliquity.
• The Earth maintains this tilt as it orbits
the Sun, with the Earth’s north pole
pointing toward the north celestial pole

The Earth’s orbit

• Seasons do NOT arise from the
distance the Earth is from the Sun but
rather as a result of the Earth’s annual
motion and axial inclination – the tip
of our planet with respect to its orbital
plane. As we move around the Sun,
the orientation of our planet gives us
seasons.

Seasons
• During part of the year the northern hemisphere of
the Earth is tilted toward the Sun
• As the Earth spins on its axis, a point in the
northern hemisphere spends more than 12 hours in
the sunlight
• The days there are long and the nights are short,
and it is summer in the northern hemisphere and
winter in the southern hemisphere
• The summer is hot not only because of the
extended daylight hours but also because the Sun
is high in the northern hemisphere’s sky
• As a result, sunlight strikes the ground at a nearly
perpendicular angle that heats the ground efficiently
• This situation reverses six months later

Sept
21
June
21

Dec
21
March
31

• The Sun appears to
trace out a circular path
called the ecliptic on
the celestial sphere tilted
at 23 ½ degrees to the
equator
• The ecliptic and the
celestial equator
intersect at only two
points
• Each point is called an
equinox
• The point on the ecliptic
farthest north of the
celestial equator that
marks the location of the
Sun at the beginning of
summer in the northern
hemisphere is called the
summer solstice
• At the beginning of the
northern hemisphere’s
winter the Sun is farthest
south of the celestial
equator at a point called
the winter solstice

Landmarks on the Earth’s surface are
marked by the Sun’s position in the sky
throughout the year

The Moon helps to cause precession, a
slow, conical motion of Earth’s axis of
rotation

Precession causes the gradual change of the
star that marks the North Celestial Pole

Astronomical observations led to the
development of the modern calendar
•
•
•
•

The day is based on the Earth’s rotation
The year is based on the Earth’s orbit
The month is based on the lunar cycle
None of these are exactly the same as
nature so astronomers use the average
or mean day and leap years to keep the
calendar and time consistent

The different types of ‘year’.
• The sidereal year (year with respect to
the stars) measured in solar time is
365d 6h 9m 10s (365.2564d) in length.
• The tropical year (successive passages
of the Sun through the Vernal Equinox)
is 365d 5h 48m 46s (365.2422d) in length.
• Due to precession, the tropical year is
20m 24s shorter than the sidereal year.

Calendars
• Caesar introduced the 365.25 days calendar
and thus the Leap Year (February 29)
• However, this is 11m 14s longer than the real
tropical year. This accumulates to 3 days in
4 centuries error.
• To correct, October 4 was followed by
October 15, in 1562 and the century rule was
invoked.


Slide 3

Knowing the Heavens

Chapter Two

Naked-eye astronomy had an important place
in ancient civilizations
• Positional astronomy
– the study of the positions of objects in the sky and
how these positions change

• Naked-eye astronomy
– the sort that requires no equipment but human
vision

• Extends far back in time
–
–
–
–

British Isles Stonehenge
Native American Medicine Wheel
Aztec, Mayan and Incan temples
Egyptian pyramids

Eighty-eight constellations cover the
entire sky
• Ancient peoples looked
at the stars and
imagined groupings
made pictures in the
sky
• We still refer to many of
these groupings
• Astronomers call them
constellations (from
the Latin for “group of
stars”)

Modern Constellations
• On modern star charts,
the entire sky is divided
into 88 regions. Each
is a constellation
• Most stars in a
constellation are
nowhere near one
another
• They only appear to be
close together because
they are in nearly the
same direction as seen
from Earth

The appearance of the sky changes during the
course of the night and from one night to the next
• Stars appear to rise
in the east, slowly
rotate about the
earth and set in the
west.
• This diurnal or daily
motion of the stars
is actually caused
by the 24-hour
rotation of the earth.

Animation of constellation movement
• To represent what we have just discussed,
follow this animation from the vantage
point of our Californian observer.

Annual Motion
• The stars also appear
to slowly shift in position
throughout the year
• This is due to the orbit
of the earth around the
sun
• If you follow a particular
star on successive
evenings, you will find
that it rises
approximately 4
minutes earlier each
night, or 2 hours earlier
each month

It is convenient to imagine that the stars are
located on a celestial sphere
• The celestial sphere is
an imaginary object
that has no basis in
physical reality
• However it is still a
model that remains a
useful tool of positional
astronomy
• Landmarks on the
celestial sphere are
projections of those on
the Earth

•Celestial equator
divides the sky into
northern and southern
hemispheres
•Celestial poles are
where the Earth’s axis
of rotation would
intersect the celestial
sphere
•Polaris is less than 1°
away from the north
celestial pole, which is
why it is called the
North Star or the Pole
Star.
•Point in the sky directly
overhead an observer
anywhere on Earth is
called that observer’s
zenith.

The Celestial Coordinate System
• Again, let us see what we have just
determined in a more 3-dimension manner.

Positional astronomy plays an important
role in keeping track of time
• Apparent solar time is based on the apparent
motion of the Sun across the celestial sphere,
which varies over the course of the year
• Mean solar time is based on the motion of an
imaginary mean sun along the celestial
equator, which produces a uniform mean
solar day of 24 hours
• Ordinary watches and clocks measure mean
solar time
• Sidereal time is based on the apparent
motion of the celestial sphere

• Local noon is defined to be when the Sun crosses the
upper meridian, which is the half of the meridian
above the horizon

Sidereal and Solar Days
• Appreciating the difference between a solar
day and a sidereal day is a challenging
concept. See if this helps.

Circumpolar
stars

• At any time, an observer can see only half of the celestial sphere
• The other half is below the horizon, hidden by the body of the Earth

Last thought on coordinates
• The Equatorial System of Coordinates is what most
astronomers use when specifying the location of an
object on the Celestial Sphere
• Right Ascension (measured eastwards from the
Vernal Equinox) goes from 0h to 24h
• Declination (measured north or south from the
celestial equator goes from -90° to +90 °.
• The hour angle (HA) of an object is the angle
between the meridian on which the object is situated
and the (observer’s) celestial meridian
• ST = RA + HA

The seasons are caused by the tilt of
Earth’s axis of rotation
• The Earth’s axis of rotation is not
perpendicular to the plane of the Earth’s
orbit
• It is tilted about 23½° away from the
perpendicular & is called the obliquity.
• The Earth maintains this tilt as it orbits
the Sun, with the Earth’s north pole
pointing toward the north celestial pole

The Earth’s orbit

• Seasons do NOT arise from the
distance the Earth is from the Sun but
rather as a result of the Earth’s annual
motion and axial inclination – the tip
of our planet with respect to its orbital
plane. As we move around the Sun,
the orientation of our planet gives us
seasons.

Seasons
• During part of the year the northern hemisphere of
the Earth is tilted toward the Sun
• As the Earth spins on its axis, a point in the
northern hemisphere spends more than 12 hours in
the sunlight
• The days there are long and the nights are short,
and it is summer in the northern hemisphere and
winter in the southern hemisphere
• The summer is hot not only because of the
extended daylight hours but also because the Sun
is high in the northern hemisphere’s sky
• As a result, sunlight strikes the ground at a nearly
perpendicular angle that heats the ground efficiently
• This situation reverses six months later

Sept
21
June
21

Dec
21
March
31

• The Sun appears to
trace out a circular path
called the ecliptic on
the celestial sphere tilted
at 23 ½ degrees to the
equator
• The ecliptic and the
celestial equator
intersect at only two
points
• Each point is called an
equinox
• The point on the ecliptic
farthest north of the
celestial equator that
marks the location of the
Sun at the beginning of
summer in the northern
hemisphere is called the
summer solstice
• At the beginning of the
northern hemisphere’s
winter the Sun is farthest
south of the celestial
equator at a point called
the winter solstice

Landmarks on the Earth’s surface are
marked by the Sun’s position in the sky
throughout the year

The Moon helps to cause precession, a
slow, conical motion of Earth’s axis of
rotation

Precession causes the gradual change of the
star that marks the North Celestial Pole

Astronomical observations led to the
development of the modern calendar
•
•
•
•

The day is based on the Earth’s rotation
The year is based on the Earth’s orbit
The month is based on the lunar cycle
None of these are exactly the same as
nature so astronomers use the average
or mean day and leap years to keep the
calendar and time consistent

The different types of ‘year’.
• The sidereal year (year with respect to
the stars) measured in solar time is
365d 6h 9m 10s (365.2564d) in length.
• The tropical year (successive passages
of the Sun through the Vernal Equinox)
is 365d 5h 48m 46s (365.2422d) in length.
• Due to precession, the tropical year is
20m 24s shorter than the sidereal year.

Calendars
• Caesar introduced the 365.25 days calendar
and thus the Leap Year (February 29)
• However, this is 11m 14s longer than the real
tropical year. This accumulates to 3 days in
4 centuries error.
• To correct, October 4 was followed by
October 15, in 1562 and the century rule was
invoked.


Slide 4

Knowing the Heavens

Chapter Two

Naked-eye astronomy had an important place
in ancient civilizations
• Positional astronomy
– the study of the positions of objects in the sky and
how these positions change

• Naked-eye astronomy
– the sort that requires no equipment but human
vision

• Extends far back in time
–
–
–
–

British Isles Stonehenge
Native American Medicine Wheel
Aztec, Mayan and Incan temples
Egyptian pyramids

Eighty-eight constellations cover the
entire sky
• Ancient peoples looked
at the stars and
imagined groupings
made pictures in the
sky
• We still refer to many of
these groupings
• Astronomers call them
constellations (from
the Latin for “group of
stars”)

Modern Constellations
• On modern star charts,
the entire sky is divided
into 88 regions. Each
is a constellation
• Most stars in a
constellation are
nowhere near one
another
• They only appear to be
close together because
they are in nearly the
same direction as seen
from Earth

The appearance of the sky changes during the
course of the night and from one night to the next
• Stars appear to rise
in the east, slowly
rotate about the
earth and set in the
west.
• This diurnal or daily
motion of the stars
is actually caused
by the 24-hour
rotation of the earth.

Animation of constellation movement
• To represent what we have just discussed,
follow this animation from the vantage
point of our Californian observer.

Annual Motion
• The stars also appear
to slowly shift in position
throughout the year
• This is due to the orbit
of the earth around the
sun
• If you follow a particular
star on successive
evenings, you will find
that it rises
approximately 4
minutes earlier each
night, or 2 hours earlier
each month

It is convenient to imagine that the stars are
located on a celestial sphere
• The celestial sphere is
an imaginary object
that has no basis in
physical reality
• However it is still a
model that remains a
useful tool of positional
astronomy
• Landmarks on the
celestial sphere are
projections of those on
the Earth

•Celestial equator
divides the sky into
northern and southern
hemispheres
•Celestial poles are
where the Earth’s axis
of rotation would
intersect the celestial
sphere
•Polaris is less than 1°
away from the north
celestial pole, which is
why it is called the
North Star or the Pole
Star.
•Point in the sky directly
overhead an observer
anywhere on Earth is
called that observer’s
zenith.

The Celestial Coordinate System
• Again, let us see what we have just
determined in a more 3-dimension manner.

Positional astronomy plays an important
role in keeping track of time
• Apparent solar time is based on the apparent
motion of the Sun across the celestial sphere,
which varies over the course of the year
• Mean solar time is based on the motion of an
imaginary mean sun along the celestial
equator, which produces a uniform mean
solar day of 24 hours
• Ordinary watches and clocks measure mean
solar time
• Sidereal time is based on the apparent
motion of the celestial sphere

• Local noon is defined to be when the Sun crosses the
upper meridian, which is the half of the meridian
above the horizon

Sidereal and Solar Days
• Appreciating the difference between a solar
day and a sidereal day is a challenging
concept. See if this helps.

Circumpolar
stars

• At any time, an observer can see only half of the celestial sphere
• The other half is below the horizon, hidden by the body of the Earth

Last thought on coordinates
• The Equatorial System of Coordinates is what most
astronomers use when specifying the location of an
object on the Celestial Sphere
• Right Ascension (measured eastwards from the
Vernal Equinox) goes from 0h to 24h
• Declination (measured north or south from the
celestial equator goes from -90° to +90 °.
• The hour angle (HA) of an object is the angle
between the meridian on which the object is situated
and the (observer’s) celestial meridian
• ST = RA + HA

The seasons are caused by the tilt of
Earth’s axis of rotation
• The Earth’s axis of rotation is not
perpendicular to the plane of the Earth’s
orbit
• It is tilted about 23½° away from the
perpendicular & is called the obliquity.
• The Earth maintains this tilt as it orbits
the Sun, with the Earth’s north pole
pointing toward the north celestial pole

The Earth’s orbit

• Seasons do NOT arise from the
distance the Earth is from the Sun but
rather as a result of the Earth’s annual
motion and axial inclination – the tip
of our planet with respect to its orbital
plane. As we move around the Sun,
the orientation of our planet gives us
seasons.

Seasons
• During part of the year the northern hemisphere of
the Earth is tilted toward the Sun
• As the Earth spins on its axis, a point in the
northern hemisphere spends more than 12 hours in
the sunlight
• The days there are long and the nights are short,
and it is summer in the northern hemisphere and
winter in the southern hemisphere
• The summer is hot not only because of the
extended daylight hours but also because the Sun
is high in the northern hemisphere’s sky
• As a result, sunlight strikes the ground at a nearly
perpendicular angle that heats the ground efficiently
• This situation reverses six months later

Sept
21
June
21

Dec
21
March
31

• The Sun appears to
trace out a circular path
called the ecliptic on
the celestial sphere tilted
at 23 ½ degrees to the
equator
• The ecliptic and the
celestial equator
intersect at only two
points
• Each point is called an
equinox
• The point on the ecliptic
farthest north of the
celestial equator that
marks the location of the
Sun at the beginning of
summer in the northern
hemisphere is called the
summer solstice
• At the beginning of the
northern hemisphere’s
winter the Sun is farthest
south of the celestial
equator at a point called
the winter solstice

Landmarks on the Earth’s surface are
marked by the Sun’s position in the sky
throughout the year

The Moon helps to cause precession, a
slow, conical motion of Earth’s axis of
rotation

Precession causes the gradual change of the
star that marks the North Celestial Pole

Astronomical observations led to the
development of the modern calendar
•
•
•
•

The day is based on the Earth’s rotation
The year is based on the Earth’s orbit
The month is based on the lunar cycle
None of these are exactly the same as
nature so astronomers use the average
or mean day and leap years to keep the
calendar and time consistent

The different types of ‘year’.
• The sidereal year (year with respect to
the stars) measured in solar time is
365d 6h 9m 10s (365.2564d) in length.
• The tropical year (successive passages
of the Sun through the Vernal Equinox)
is 365d 5h 48m 46s (365.2422d) in length.
• Due to precession, the tropical year is
20m 24s shorter than the sidereal year.

Calendars
• Caesar introduced the 365.25 days calendar
and thus the Leap Year (February 29)
• However, this is 11m 14s longer than the real
tropical year. This accumulates to 3 days in
4 centuries error.
• To correct, October 4 was followed by
October 15, in 1562 and the century rule was
invoked.


Slide 5

Knowing the Heavens

Chapter Two

Naked-eye astronomy had an important place
in ancient civilizations
• Positional astronomy
– the study of the positions of objects in the sky and
how these positions change

• Naked-eye astronomy
– the sort that requires no equipment but human
vision

• Extends far back in time
–
–
–
–

British Isles Stonehenge
Native American Medicine Wheel
Aztec, Mayan and Incan temples
Egyptian pyramids

Eighty-eight constellations cover the
entire sky
• Ancient peoples looked
at the stars and
imagined groupings
made pictures in the
sky
• We still refer to many of
these groupings
• Astronomers call them
constellations (from
the Latin for “group of
stars”)

Modern Constellations
• On modern star charts,
the entire sky is divided
into 88 regions. Each
is a constellation
• Most stars in a
constellation are
nowhere near one
another
• They only appear to be
close together because
they are in nearly the
same direction as seen
from Earth

The appearance of the sky changes during the
course of the night and from one night to the next
• Stars appear to rise
in the east, slowly
rotate about the
earth and set in the
west.
• This diurnal or daily
motion of the stars
is actually caused
by the 24-hour
rotation of the earth.

Animation of constellation movement
• To represent what we have just discussed,
follow this animation from the vantage
point of our Californian observer.

Annual Motion
• The stars also appear
to slowly shift in position
throughout the year
• This is due to the orbit
of the earth around the
sun
• If you follow a particular
star on successive
evenings, you will find
that it rises
approximately 4
minutes earlier each
night, or 2 hours earlier
each month

It is convenient to imagine that the stars are
located on a celestial sphere
• The celestial sphere is
an imaginary object
that has no basis in
physical reality
• However it is still a
model that remains a
useful tool of positional
astronomy
• Landmarks on the
celestial sphere are
projections of those on
the Earth

•Celestial equator
divides the sky into
northern and southern
hemispheres
•Celestial poles are
where the Earth’s axis
of rotation would
intersect the celestial
sphere
•Polaris is less than 1°
away from the north
celestial pole, which is
why it is called the
North Star or the Pole
Star.
•Point in the sky directly
overhead an observer
anywhere on Earth is
called that observer’s
zenith.

The Celestial Coordinate System
• Again, let us see what we have just
determined in a more 3-dimension manner.

Positional astronomy plays an important
role in keeping track of time
• Apparent solar time is based on the apparent
motion of the Sun across the celestial sphere,
which varies over the course of the year
• Mean solar time is based on the motion of an
imaginary mean sun along the celestial
equator, which produces a uniform mean
solar day of 24 hours
• Ordinary watches and clocks measure mean
solar time
• Sidereal time is based on the apparent
motion of the celestial sphere

• Local noon is defined to be when the Sun crosses the
upper meridian, which is the half of the meridian
above the horizon

Sidereal and Solar Days
• Appreciating the difference between a solar
day and a sidereal day is a challenging
concept. See if this helps.

Circumpolar
stars

• At any time, an observer can see only half of the celestial sphere
• The other half is below the horizon, hidden by the body of the Earth

Last thought on coordinates
• The Equatorial System of Coordinates is what most
astronomers use when specifying the location of an
object on the Celestial Sphere
• Right Ascension (measured eastwards from the
Vernal Equinox) goes from 0h to 24h
• Declination (measured north or south from the
celestial equator goes from -90° to +90 °.
• The hour angle (HA) of an object is the angle
between the meridian on which the object is situated
and the (observer’s) celestial meridian
• ST = RA + HA

The seasons are caused by the tilt of
Earth’s axis of rotation
• The Earth’s axis of rotation is not
perpendicular to the plane of the Earth’s
orbit
• It is tilted about 23½° away from the
perpendicular & is called the obliquity.
• The Earth maintains this tilt as it orbits
the Sun, with the Earth’s north pole
pointing toward the north celestial pole

The Earth’s orbit

• Seasons do NOT arise from the
distance the Earth is from the Sun but
rather as a result of the Earth’s annual
motion and axial inclination – the tip
of our planet with respect to its orbital
plane. As we move around the Sun,
the orientation of our planet gives us
seasons.

Seasons
• During part of the year the northern hemisphere of
the Earth is tilted toward the Sun
• As the Earth spins on its axis, a point in the
northern hemisphere spends more than 12 hours in
the sunlight
• The days there are long and the nights are short,
and it is summer in the northern hemisphere and
winter in the southern hemisphere
• The summer is hot not only because of the
extended daylight hours but also because the Sun
is high in the northern hemisphere’s sky
• As a result, sunlight strikes the ground at a nearly
perpendicular angle that heats the ground efficiently
• This situation reverses six months later

Sept
21
June
21

Dec
21
March
31

• The Sun appears to
trace out a circular path
called the ecliptic on
the celestial sphere tilted
at 23 ½ degrees to the
equator
• The ecliptic and the
celestial equator
intersect at only two
points
• Each point is called an
equinox
• The point on the ecliptic
farthest north of the
celestial equator that
marks the location of the
Sun at the beginning of
summer in the northern
hemisphere is called the
summer solstice
• At the beginning of the
northern hemisphere’s
winter the Sun is farthest
south of the celestial
equator at a point called
the winter solstice

Landmarks on the Earth’s surface are
marked by the Sun’s position in the sky
throughout the year

The Moon helps to cause precession, a
slow, conical motion of Earth’s axis of
rotation

Precession causes the gradual change of the
star that marks the North Celestial Pole

Astronomical observations led to the
development of the modern calendar
•
•
•
•

The day is based on the Earth’s rotation
The year is based on the Earth’s orbit
The month is based on the lunar cycle
None of these are exactly the same as
nature so astronomers use the average
or mean day and leap years to keep the
calendar and time consistent

The different types of ‘year’.
• The sidereal year (year with respect to
the stars) measured in solar time is
365d 6h 9m 10s (365.2564d) in length.
• The tropical year (successive passages
of the Sun through the Vernal Equinox)
is 365d 5h 48m 46s (365.2422d) in length.
• Due to precession, the tropical year is
20m 24s shorter than the sidereal year.

Calendars
• Caesar introduced the 365.25 days calendar
and thus the Leap Year (February 29)
• However, this is 11m 14s longer than the real
tropical year. This accumulates to 3 days in
4 centuries error.
• To correct, October 4 was followed by
October 15, in 1562 and the century rule was
invoked.


Slide 6

Knowing the Heavens

Chapter Two

Naked-eye astronomy had an important place
in ancient civilizations
• Positional astronomy
– the study of the positions of objects in the sky and
how these positions change

• Naked-eye astronomy
– the sort that requires no equipment but human
vision

• Extends far back in time
–
–
–
–

British Isles Stonehenge
Native American Medicine Wheel
Aztec, Mayan and Incan temples
Egyptian pyramids

Eighty-eight constellations cover the
entire sky
• Ancient peoples looked
at the stars and
imagined groupings
made pictures in the
sky
• We still refer to many of
these groupings
• Astronomers call them
constellations (from
the Latin for “group of
stars”)

Modern Constellations
• On modern star charts,
the entire sky is divided
into 88 regions. Each
is a constellation
• Most stars in a
constellation are
nowhere near one
another
• They only appear to be
close together because
they are in nearly the
same direction as seen
from Earth

The appearance of the sky changes during the
course of the night and from one night to the next
• Stars appear to rise
in the east, slowly
rotate about the
earth and set in the
west.
• This diurnal or daily
motion of the stars
is actually caused
by the 24-hour
rotation of the earth.

Animation of constellation movement
• To represent what we have just discussed,
follow this animation from the vantage
point of our Californian observer.

Annual Motion
• The stars also appear
to slowly shift in position
throughout the year
• This is due to the orbit
of the earth around the
sun
• If you follow a particular
star on successive
evenings, you will find
that it rises
approximately 4
minutes earlier each
night, or 2 hours earlier
each month

It is convenient to imagine that the stars are
located on a celestial sphere
• The celestial sphere is
an imaginary object
that has no basis in
physical reality
• However it is still a
model that remains a
useful tool of positional
astronomy
• Landmarks on the
celestial sphere are
projections of those on
the Earth

•Celestial equator
divides the sky into
northern and southern
hemispheres
•Celestial poles are
where the Earth’s axis
of rotation would
intersect the celestial
sphere
•Polaris is less than 1°
away from the north
celestial pole, which is
why it is called the
North Star or the Pole
Star.
•Point in the sky directly
overhead an observer
anywhere on Earth is
called that observer’s
zenith.

The Celestial Coordinate System
• Again, let us see what we have just
determined in a more 3-dimension manner.

Positional astronomy plays an important
role in keeping track of time
• Apparent solar time is based on the apparent
motion of the Sun across the celestial sphere,
which varies over the course of the year
• Mean solar time is based on the motion of an
imaginary mean sun along the celestial
equator, which produces a uniform mean
solar day of 24 hours
• Ordinary watches and clocks measure mean
solar time
• Sidereal time is based on the apparent
motion of the celestial sphere

• Local noon is defined to be when the Sun crosses the
upper meridian, which is the half of the meridian
above the horizon

Sidereal and Solar Days
• Appreciating the difference between a solar
day and a sidereal day is a challenging
concept. See if this helps.

Circumpolar
stars

• At any time, an observer can see only half of the celestial sphere
• The other half is below the horizon, hidden by the body of the Earth

Last thought on coordinates
• The Equatorial System of Coordinates is what most
astronomers use when specifying the location of an
object on the Celestial Sphere
• Right Ascension (measured eastwards from the
Vernal Equinox) goes from 0h to 24h
• Declination (measured north or south from the
celestial equator goes from -90° to +90 °.
• The hour angle (HA) of an object is the angle
between the meridian on which the object is situated
and the (observer’s) celestial meridian
• ST = RA + HA

The seasons are caused by the tilt of
Earth’s axis of rotation
• The Earth’s axis of rotation is not
perpendicular to the plane of the Earth’s
orbit
• It is tilted about 23½° away from the
perpendicular & is called the obliquity.
• The Earth maintains this tilt as it orbits
the Sun, with the Earth’s north pole
pointing toward the north celestial pole

The Earth’s orbit

• Seasons do NOT arise from the
distance the Earth is from the Sun but
rather as a result of the Earth’s annual
motion and axial inclination – the tip
of our planet with respect to its orbital
plane. As we move around the Sun,
the orientation of our planet gives us
seasons.

Seasons
• During part of the year the northern hemisphere of
the Earth is tilted toward the Sun
• As the Earth spins on its axis, a point in the
northern hemisphere spends more than 12 hours in
the sunlight
• The days there are long and the nights are short,
and it is summer in the northern hemisphere and
winter in the southern hemisphere
• The summer is hot not only because of the
extended daylight hours but also because the Sun
is high in the northern hemisphere’s sky
• As a result, sunlight strikes the ground at a nearly
perpendicular angle that heats the ground efficiently
• This situation reverses six months later

Sept
21
June
21

Dec
21
March
31

• The Sun appears to
trace out a circular path
called the ecliptic on
the celestial sphere tilted
at 23 ½ degrees to the
equator
• The ecliptic and the
celestial equator
intersect at only two
points
• Each point is called an
equinox
• The point on the ecliptic
farthest north of the
celestial equator that
marks the location of the
Sun at the beginning of
summer in the northern
hemisphere is called the
summer solstice
• At the beginning of the
northern hemisphere’s
winter the Sun is farthest
south of the celestial
equator at a point called
the winter solstice

Landmarks on the Earth’s surface are
marked by the Sun’s position in the sky
throughout the year

The Moon helps to cause precession, a
slow, conical motion of Earth’s axis of
rotation

Precession causes the gradual change of the
star that marks the North Celestial Pole

Astronomical observations led to the
development of the modern calendar
•
•
•
•

The day is based on the Earth’s rotation
The year is based on the Earth’s orbit
The month is based on the lunar cycle
None of these are exactly the same as
nature so astronomers use the average
or mean day and leap years to keep the
calendar and time consistent

The different types of ‘year’.
• The sidereal year (year with respect to
the stars) measured in solar time is
365d 6h 9m 10s (365.2564d) in length.
• The tropical year (successive passages
of the Sun through the Vernal Equinox)
is 365d 5h 48m 46s (365.2422d) in length.
• Due to precession, the tropical year is
20m 24s shorter than the sidereal year.

Calendars
• Caesar introduced the 365.25 days calendar
and thus the Leap Year (February 29)
• However, this is 11m 14s longer than the real
tropical year. This accumulates to 3 days in
4 centuries error.
• To correct, October 4 was followed by
October 15, in 1562 and the century rule was
invoked.


Slide 7

Knowing the Heavens

Chapter Two

Naked-eye astronomy had an important place
in ancient civilizations
• Positional astronomy
– the study of the positions of objects in the sky and
how these positions change

• Naked-eye astronomy
– the sort that requires no equipment but human
vision

• Extends far back in time
–
–
–
–

British Isles Stonehenge
Native American Medicine Wheel
Aztec, Mayan and Incan temples
Egyptian pyramids

Eighty-eight constellations cover the
entire sky
• Ancient peoples looked
at the stars and
imagined groupings
made pictures in the
sky
• We still refer to many of
these groupings
• Astronomers call them
constellations (from
the Latin for “group of
stars”)

Modern Constellations
• On modern star charts,
the entire sky is divided
into 88 regions. Each
is a constellation
• Most stars in a
constellation are
nowhere near one
another
• They only appear to be
close together because
they are in nearly the
same direction as seen
from Earth

The appearance of the sky changes during the
course of the night and from one night to the next
• Stars appear to rise
in the east, slowly
rotate about the
earth and set in the
west.
• This diurnal or daily
motion of the stars
is actually caused
by the 24-hour
rotation of the earth.

Animation of constellation movement
• To represent what we have just discussed,
follow this animation from the vantage
point of our Californian observer.

Annual Motion
• The stars also appear
to slowly shift in position
throughout the year
• This is due to the orbit
of the earth around the
sun
• If you follow a particular
star on successive
evenings, you will find
that it rises
approximately 4
minutes earlier each
night, or 2 hours earlier
each month

It is convenient to imagine that the stars are
located on a celestial sphere
• The celestial sphere is
an imaginary object
that has no basis in
physical reality
• However it is still a
model that remains a
useful tool of positional
astronomy
• Landmarks on the
celestial sphere are
projections of those on
the Earth

•Celestial equator
divides the sky into
northern and southern
hemispheres
•Celestial poles are
where the Earth’s axis
of rotation would
intersect the celestial
sphere
•Polaris is less than 1°
away from the north
celestial pole, which is
why it is called the
North Star or the Pole
Star.
•Point in the sky directly
overhead an observer
anywhere on Earth is
called that observer’s
zenith.

The Celestial Coordinate System
• Again, let us see what we have just
determined in a more 3-dimension manner.

Positional astronomy plays an important
role in keeping track of time
• Apparent solar time is based on the apparent
motion of the Sun across the celestial sphere,
which varies over the course of the year
• Mean solar time is based on the motion of an
imaginary mean sun along the celestial
equator, which produces a uniform mean
solar day of 24 hours
• Ordinary watches and clocks measure mean
solar time
• Sidereal time is based on the apparent
motion of the celestial sphere

• Local noon is defined to be when the Sun crosses the
upper meridian, which is the half of the meridian
above the horizon

Sidereal and Solar Days
• Appreciating the difference between a solar
day and a sidereal day is a challenging
concept. See if this helps.

Circumpolar
stars

• At any time, an observer can see only half of the celestial sphere
• The other half is below the horizon, hidden by the body of the Earth

Last thought on coordinates
• The Equatorial System of Coordinates is what most
astronomers use when specifying the location of an
object on the Celestial Sphere
• Right Ascension (measured eastwards from the
Vernal Equinox) goes from 0h to 24h
• Declination (measured north or south from the
celestial equator goes from -90° to +90 °.
• The hour angle (HA) of an object is the angle
between the meridian on which the object is situated
and the (observer’s) celestial meridian
• ST = RA + HA

The seasons are caused by the tilt of
Earth’s axis of rotation
• The Earth’s axis of rotation is not
perpendicular to the plane of the Earth’s
orbit
• It is tilted about 23½° away from the
perpendicular & is called the obliquity.
• The Earth maintains this tilt as it orbits
the Sun, with the Earth’s north pole
pointing toward the north celestial pole

The Earth’s orbit

• Seasons do NOT arise from the
distance the Earth is from the Sun but
rather as a result of the Earth’s annual
motion and axial inclination – the tip
of our planet with respect to its orbital
plane. As we move around the Sun,
the orientation of our planet gives us
seasons.

Seasons
• During part of the year the northern hemisphere of
the Earth is tilted toward the Sun
• As the Earth spins on its axis, a point in the
northern hemisphere spends more than 12 hours in
the sunlight
• The days there are long and the nights are short,
and it is summer in the northern hemisphere and
winter in the southern hemisphere
• The summer is hot not only because of the
extended daylight hours but also because the Sun
is high in the northern hemisphere’s sky
• As a result, sunlight strikes the ground at a nearly
perpendicular angle that heats the ground efficiently
• This situation reverses six months later

Sept
21
June
21

Dec
21
March
31

• The Sun appears to
trace out a circular path
called the ecliptic on
the celestial sphere tilted
at 23 ½ degrees to the
equator
• The ecliptic and the
celestial equator
intersect at only two
points
• Each point is called an
equinox
• The point on the ecliptic
farthest north of the
celestial equator that
marks the location of the
Sun at the beginning of
summer in the northern
hemisphere is called the
summer solstice
• At the beginning of the
northern hemisphere’s
winter the Sun is farthest
south of the celestial
equator at a point called
the winter solstice

Landmarks on the Earth’s surface are
marked by the Sun’s position in the sky
throughout the year

The Moon helps to cause precession, a
slow, conical motion of Earth’s axis of
rotation

Precession causes the gradual change of the
star that marks the North Celestial Pole

Astronomical observations led to the
development of the modern calendar
•
•
•
•

The day is based on the Earth’s rotation
The year is based on the Earth’s orbit
The month is based on the lunar cycle
None of these are exactly the same as
nature so astronomers use the average
or mean day and leap years to keep the
calendar and time consistent

The different types of ‘year’.
• The sidereal year (year with respect to
the stars) measured in solar time is
365d 6h 9m 10s (365.2564d) in length.
• The tropical year (successive passages
of the Sun through the Vernal Equinox)
is 365d 5h 48m 46s (365.2422d) in length.
• Due to precession, the tropical year is
20m 24s shorter than the sidereal year.

Calendars
• Caesar introduced the 365.25 days calendar
and thus the Leap Year (February 29)
• However, this is 11m 14s longer than the real
tropical year. This accumulates to 3 days in
4 centuries error.
• To correct, October 4 was followed by
October 15, in 1562 and the century rule was
invoked.


Slide 8

Knowing the Heavens

Chapter Two

Naked-eye astronomy had an important place
in ancient civilizations
• Positional astronomy
– the study of the positions of objects in the sky and
how these positions change

• Naked-eye astronomy
– the sort that requires no equipment but human
vision

• Extends far back in time
–
–
–
–

British Isles Stonehenge
Native American Medicine Wheel
Aztec, Mayan and Incan temples
Egyptian pyramids

Eighty-eight constellations cover the
entire sky
• Ancient peoples looked
at the stars and
imagined groupings
made pictures in the
sky
• We still refer to many of
these groupings
• Astronomers call them
constellations (from
the Latin for “group of
stars”)

Modern Constellations
• On modern star charts,
the entire sky is divided
into 88 regions. Each
is a constellation
• Most stars in a
constellation are
nowhere near one
another
• They only appear to be
close together because
they are in nearly the
same direction as seen
from Earth

The appearance of the sky changes during the
course of the night and from one night to the next
• Stars appear to rise
in the east, slowly
rotate about the
earth and set in the
west.
• This diurnal or daily
motion of the stars
is actually caused
by the 24-hour
rotation of the earth.

Animation of constellation movement
• To represent what we have just discussed,
follow this animation from the vantage
point of our Californian observer.

Annual Motion
• The stars also appear
to slowly shift in position
throughout the year
• This is due to the orbit
of the earth around the
sun
• If you follow a particular
star on successive
evenings, you will find
that it rises
approximately 4
minutes earlier each
night, or 2 hours earlier
each month

It is convenient to imagine that the stars are
located on a celestial sphere
• The celestial sphere is
an imaginary object
that has no basis in
physical reality
• However it is still a
model that remains a
useful tool of positional
astronomy
• Landmarks on the
celestial sphere are
projections of those on
the Earth

•Celestial equator
divides the sky into
northern and southern
hemispheres
•Celestial poles are
where the Earth’s axis
of rotation would
intersect the celestial
sphere
•Polaris is less than 1°
away from the north
celestial pole, which is
why it is called the
North Star or the Pole
Star.
•Point in the sky directly
overhead an observer
anywhere on Earth is
called that observer’s
zenith.

The Celestial Coordinate System
• Again, let us see what we have just
determined in a more 3-dimension manner.

Positional astronomy plays an important
role in keeping track of time
• Apparent solar time is based on the apparent
motion of the Sun across the celestial sphere,
which varies over the course of the year
• Mean solar time is based on the motion of an
imaginary mean sun along the celestial
equator, which produces a uniform mean
solar day of 24 hours
• Ordinary watches and clocks measure mean
solar time
• Sidereal time is based on the apparent
motion of the celestial sphere

• Local noon is defined to be when the Sun crosses the
upper meridian, which is the half of the meridian
above the horizon

Sidereal and Solar Days
• Appreciating the difference between a solar
day and a sidereal day is a challenging
concept. See if this helps.

Circumpolar
stars

• At any time, an observer can see only half of the celestial sphere
• The other half is below the horizon, hidden by the body of the Earth

Last thought on coordinates
• The Equatorial System of Coordinates is what most
astronomers use when specifying the location of an
object on the Celestial Sphere
• Right Ascension (measured eastwards from the
Vernal Equinox) goes from 0h to 24h
• Declination (measured north or south from the
celestial equator goes from -90° to +90 °.
• The hour angle (HA) of an object is the angle
between the meridian on which the object is situated
and the (observer’s) celestial meridian
• ST = RA + HA

The seasons are caused by the tilt of
Earth’s axis of rotation
• The Earth’s axis of rotation is not
perpendicular to the plane of the Earth’s
orbit
• It is tilted about 23½° away from the
perpendicular & is called the obliquity.
• The Earth maintains this tilt as it orbits
the Sun, with the Earth’s north pole
pointing toward the north celestial pole

The Earth’s orbit

• Seasons do NOT arise from the
distance the Earth is from the Sun but
rather as a result of the Earth’s annual
motion and axial inclination – the tip
of our planet with respect to its orbital
plane. As we move around the Sun,
the orientation of our planet gives us
seasons.

Seasons
• During part of the year the northern hemisphere of
the Earth is tilted toward the Sun
• As the Earth spins on its axis, a point in the
northern hemisphere spends more than 12 hours in
the sunlight
• The days there are long and the nights are short,
and it is summer in the northern hemisphere and
winter in the southern hemisphere
• The summer is hot not only because of the
extended daylight hours but also because the Sun
is high in the northern hemisphere’s sky
• As a result, sunlight strikes the ground at a nearly
perpendicular angle that heats the ground efficiently
• This situation reverses six months later

Sept
21
June
21

Dec
21
March
31

• The Sun appears to
trace out a circular path
called the ecliptic on
the celestial sphere tilted
at 23 ½ degrees to the
equator
• The ecliptic and the
celestial equator
intersect at only two
points
• Each point is called an
equinox
• The point on the ecliptic
farthest north of the
celestial equator that
marks the location of the
Sun at the beginning of
summer in the northern
hemisphere is called the
summer solstice
• At the beginning of the
northern hemisphere’s
winter the Sun is farthest
south of the celestial
equator at a point called
the winter solstice

Landmarks on the Earth’s surface are
marked by the Sun’s position in the sky
throughout the year

The Moon helps to cause precession, a
slow, conical motion of Earth’s axis of
rotation

Precession causes the gradual change of the
star that marks the North Celestial Pole

Astronomical observations led to the
development of the modern calendar
•
•
•
•

The day is based on the Earth’s rotation
The year is based on the Earth’s orbit
The month is based on the lunar cycle
None of these are exactly the same as
nature so astronomers use the average
or mean day and leap years to keep the
calendar and time consistent

The different types of ‘year’.
• The sidereal year (year with respect to
the stars) measured in solar time is
365d 6h 9m 10s (365.2564d) in length.
• The tropical year (successive passages
of the Sun through the Vernal Equinox)
is 365d 5h 48m 46s (365.2422d) in length.
• Due to precession, the tropical year is
20m 24s shorter than the sidereal year.

Calendars
• Caesar introduced the 365.25 days calendar
and thus the Leap Year (February 29)
• However, this is 11m 14s longer than the real
tropical year. This accumulates to 3 days in
4 centuries error.
• To correct, October 4 was followed by
October 15, in 1562 and the century rule was
invoked.


Slide 9

Knowing the Heavens

Chapter Two

Naked-eye astronomy had an important place
in ancient civilizations
• Positional astronomy
– the study of the positions of objects in the sky and
how these positions change

• Naked-eye astronomy
– the sort that requires no equipment but human
vision

• Extends far back in time
–
–
–
–

British Isles Stonehenge
Native American Medicine Wheel
Aztec, Mayan and Incan temples
Egyptian pyramids

Eighty-eight constellations cover the
entire sky
• Ancient peoples looked
at the stars and
imagined groupings
made pictures in the
sky
• We still refer to many of
these groupings
• Astronomers call them
constellations (from
the Latin for “group of
stars”)

Modern Constellations
• On modern star charts,
the entire sky is divided
into 88 regions. Each
is a constellation
• Most stars in a
constellation are
nowhere near one
another
• They only appear to be
close together because
they are in nearly the
same direction as seen
from Earth

The appearance of the sky changes during the
course of the night and from one night to the next
• Stars appear to rise
in the east, slowly
rotate about the
earth and set in the
west.
• This diurnal or daily
motion of the stars
is actually caused
by the 24-hour
rotation of the earth.

Animation of constellation movement
• To represent what we have just discussed,
follow this animation from the vantage
point of our Californian observer.

Annual Motion
• The stars also appear
to slowly shift in position
throughout the year
• This is due to the orbit
of the earth around the
sun
• If you follow a particular
star on successive
evenings, you will find
that it rises
approximately 4
minutes earlier each
night, or 2 hours earlier
each month

It is convenient to imagine that the stars are
located on a celestial sphere
• The celestial sphere is
an imaginary object
that has no basis in
physical reality
• However it is still a
model that remains a
useful tool of positional
astronomy
• Landmarks on the
celestial sphere are
projections of those on
the Earth

•Celestial equator
divides the sky into
northern and southern
hemispheres
•Celestial poles are
where the Earth’s axis
of rotation would
intersect the celestial
sphere
•Polaris is less than 1°
away from the north
celestial pole, which is
why it is called the
North Star or the Pole
Star.
•Point in the sky directly
overhead an observer
anywhere on Earth is
called that observer’s
zenith.

The Celestial Coordinate System
• Again, let us see what we have just
determined in a more 3-dimension manner.

Positional astronomy plays an important
role in keeping track of time
• Apparent solar time is based on the apparent
motion of the Sun across the celestial sphere,
which varies over the course of the year
• Mean solar time is based on the motion of an
imaginary mean sun along the celestial
equator, which produces a uniform mean
solar day of 24 hours
• Ordinary watches and clocks measure mean
solar time
• Sidereal time is based on the apparent
motion of the celestial sphere

• Local noon is defined to be when the Sun crosses the
upper meridian, which is the half of the meridian
above the horizon

Sidereal and Solar Days
• Appreciating the difference between a solar
day and a sidereal day is a challenging
concept. See if this helps.

Circumpolar
stars

• At any time, an observer can see only half of the celestial sphere
• The other half is below the horizon, hidden by the body of the Earth

Last thought on coordinates
• The Equatorial System of Coordinates is what most
astronomers use when specifying the location of an
object on the Celestial Sphere
• Right Ascension (measured eastwards from the
Vernal Equinox) goes from 0h to 24h
• Declination (measured north or south from the
celestial equator goes from -90° to +90 °.
• The hour angle (HA) of an object is the angle
between the meridian on which the object is situated
and the (observer’s) celestial meridian
• ST = RA + HA

The seasons are caused by the tilt of
Earth’s axis of rotation
• The Earth’s axis of rotation is not
perpendicular to the plane of the Earth’s
orbit
• It is tilted about 23½° away from the
perpendicular & is called the obliquity.
• The Earth maintains this tilt as it orbits
the Sun, with the Earth’s north pole
pointing toward the north celestial pole

The Earth’s orbit

• Seasons do NOT arise from the
distance the Earth is from the Sun but
rather as a result of the Earth’s annual
motion and axial inclination – the tip
of our planet with respect to its orbital
plane. As we move around the Sun,
the orientation of our planet gives us
seasons.

Seasons
• During part of the year the northern hemisphere of
the Earth is tilted toward the Sun
• As the Earth spins on its axis, a point in the
northern hemisphere spends more than 12 hours in
the sunlight
• The days there are long and the nights are short,
and it is summer in the northern hemisphere and
winter in the southern hemisphere
• The summer is hot not only because of the
extended daylight hours but also because the Sun
is high in the northern hemisphere’s sky
• As a result, sunlight strikes the ground at a nearly
perpendicular angle that heats the ground efficiently
• This situation reverses six months later

Sept
21
June
21

Dec
21
March
31

• The Sun appears to
trace out a circular path
called the ecliptic on
the celestial sphere tilted
at 23 ½ degrees to the
equator
• The ecliptic and the
celestial equator
intersect at only two
points
• Each point is called an
equinox
• The point on the ecliptic
farthest north of the
celestial equator that
marks the location of the
Sun at the beginning of
summer in the northern
hemisphere is called the
summer solstice
• At the beginning of the
northern hemisphere’s
winter the Sun is farthest
south of the celestial
equator at a point called
the winter solstice

Landmarks on the Earth’s surface are
marked by the Sun’s position in the sky
throughout the year

The Moon helps to cause precession, a
slow, conical motion of Earth’s axis of
rotation

Precession causes the gradual change of the
star that marks the North Celestial Pole

Astronomical observations led to the
development of the modern calendar
•
•
•
•

The day is based on the Earth’s rotation
The year is based on the Earth’s orbit
The month is based on the lunar cycle
None of these are exactly the same as
nature so astronomers use the average
or mean day and leap years to keep the
calendar and time consistent

The different types of ‘year’.
• The sidereal year (year with respect to
the stars) measured in solar time is
365d 6h 9m 10s (365.2564d) in length.
• The tropical year (successive passages
of the Sun through the Vernal Equinox)
is 365d 5h 48m 46s (365.2422d) in length.
• Due to precession, the tropical year is
20m 24s shorter than the sidereal year.

Calendars
• Caesar introduced the 365.25 days calendar
and thus the Leap Year (February 29)
• However, this is 11m 14s longer than the real
tropical year. This accumulates to 3 days in
4 centuries error.
• To correct, October 4 was followed by
October 15, in 1562 and the century rule was
invoked.


Slide 10

Knowing the Heavens

Chapter Two

Naked-eye astronomy had an important place
in ancient civilizations
• Positional astronomy
– the study of the positions of objects in the sky and
how these positions change

• Naked-eye astronomy
– the sort that requires no equipment but human
vision

• Extends far back in time
–
–
–
–

British Isles Stonehenge
Native American Medicine Wheel
Aztec, Mayan and Incan temples
Egyptian pyramids

Eighty-eight constellations cover the
entire sky
• Ancient peoples looked
at the stars and
imagined groupings
made pictures in the
sky
• We still refer to many of
these groupings
• Astronomers call them
constellations (from
the Latin for “group of
stars”)

Modern Constellations
• On modern star charts,
the entire sky is divided
into 88 regions. Each
is a constellation
• Most stars in a
constellation are
nowhere near one
another
• They only appear to be
close together because
they are in nearly the
same direction as seen
from Earth

The appearance of the sky changes during the
course of the night and from one night to the next
• Stars appear to rise
in the east, slowly
rotate about the
earth and set in the
west.
• This diurnal or daily
motion of the stars
is actually caused
by the 24-hour
rotation of the earth.

Animation of constellation movement
• To represent what we have just discussed,
follow this animation from the vantage
point of our Californian observer.

Annual Motion
• The stars also appear
to slowly shift in position
throughout the year
• This is due to the orbit
of the earth around the
sun
• If you follow a particular
star on successive
evenings, you will find
that it rises
approximately 4
minutes earlier each
night, or 2 hours earlier
each month

It is convenient to imagine that the stars are
located on a celestial sphere
• The celestial sphere is
an imaginary object
that has no basis in
physical reality
• However it is still a
model that remains a
useful tool of positional
astronomy
• Landmarks on the
celestial sphere are
projections of those on
the Earth

•Celestial equator
divides the sky into
northern and southern
hemispheres
•Celestial poles are
where the Earth’s axis
of rotation would
intersect the celestial
sphere
•Polaris is less than 1°
away from the north
celestial pole, which is
why it is called the
North Star or the Pole
Star.
•Point in the sky directly
overhead an observer
anywhere on Earth is
called that observer’s
zenith.

The Celestial Coordinate System
• Again, let us see what we have just
determined in a more 3-dimension manner.

Positional astronomy plays an important
role in keeping track of time
• Apparent solar time is based on the apparent
motion of the Sun across the celestial sphere,
which varies over the course of the year
• Mean solar time is based on the motion of an
imaginary mean sun along the celestial
equator, which produces a uniform mean
solar day of 24 hours
• Ordinary watches and clocks measure mean
solar time
• Sidereal time is based on the apparent
motion of the celestial sphere

• Local noon is defined to be when the Sun crosses the
upper meridian, which is the half of the meridian
above the horizon

Sidereal and Solar Days
• Appreciating the difference between a solar
day and a sidereal day is a challenging
concept. See if this helps.

Circumpolar
stars

• At any time, an observer can see only half of the celestial sphere
• The other half is below the horizon, hidden by the body of the Earth

Last thought on coordinates
• The Equatorial System of Coordinates is what most
astronomers use when specifying the location of an
object on the Celestial Sphere
• Right Ascension (measured eastwards from the
Vernal Equinox) goes from 0h to 24h
• Declination (measured north or south from the
celestial equator goes from -90° to +90 °.
• The hour angle (HA) of an object is the angle
between the meridian on which the object is situated
and the (observer’s) celestial meridian
• ST = RA + HA

The seasons are caused by the tilt of
Earth’s axis of rotation
• The Earth’s axis of rotation is not
perpendicular to the plane of the Earth’s
orbit
• It is tilted about 23½° away from the
perpendicular & is called the obliquity.
• The Earth maintains this tilt as it orbits
the Sun, with the Earth’s north pole
pointing toward the north celestial pole

The Earth’s orbit

• Seasons do NOT arise from the
distance the Earth is from the Sun but
rather as a result of the Earth’s annual
motion and axial inclination – the tip
of our planet with respect to its orbital
plane. As we move around the Sun,
the orientation of our planet gives us
seasons.

Seasons
• During part of the year the northern hemisphere of
the Earth is tilted toward the Sun
• As the Earth spins on its axis, a point in the
northern hemisphere spends more than 12 hours in
the sunlight
• The days there are long and the nights are short,
and it is summer in the northern hemisphere and
winter in the southern hemisphere
• The summer is hot not only because of the
extended daylight hours but also because the Sun
is high in the northern hemisphere’s sky
• As a result, sunlight strikes the ground at a nearly
perpendicular angle that heats the ground efficiently
• This situation reverses six months later

Sept
21
June
21

Dec
21
March
31

• The Sun appears to
trace out a circular path
called the ecliptic on
the celestial sphere tilted
at 23 ½ degrees to the
equator
• The ecliptic and the
celestial equator
intersect at only two
points
• Each point is called an
equinox
• The point on the ecliptic
farthest north of the
celestial equator that
marks the location of the
Sun at the beginning of
summer in the northern
hemisphere is called the
summer solstice
• At the beginning of the
northern hemisphere’s
winter the Sun is farthest
south of the celestial
equator at a point called
the winter solstice

Landmarks on the Earth’s surface are
marked by the Sun’s position in the sky
throughout the year

The Moon helps to cause precession, a
slow, conical motion of Earth’s axis of
rotation

Precession causes the gradual change of the
star that marks the North Celestial Pole

Astronomical observations led to the
development of the modern calendar
•
•
•
•

The day is based on the Earth’s rotation
The year is based on the Earth’s orbit
The month is based on the lunar cycle
None of these are exactly the same as
nature so astronomers use the average
or mean day and leap years to keep the
calendar and time consistent

The different types of ‘year’.
• The sidereal year (year with respect to
the stars) measured in solar time is
365d 6h 9m 10s (365.2564d) in length.
• The tropical year (successive passages
of the Sun through the Vernal Equinox)
is 365d 5h 48m 46s (365.2422d) in length.
• Due to precession, the tropical year is
20m 24s shorter than the sidereal year.

Calendars
• Caesar introduced the 365.25 days calendar
and thus the Leap Year (February 29)
• However, this is 11m 14s longer than the real
tropical year. This accumulates to 3 days in
4 centuries error.
• To correct, October 4 was followed by
October 15, in 1562 and the century rule was
invoked.


Slide 11

Knowing the Heavens

Chapter Two

Naked-eye astronomy had an important place
in ancient civilizations
• Positional astronomy
– the study of the positions of objects in the sky and
how these positions change

• Naked-eye astronomy
– the sort that requires no equipment but human
vision

• Extends far back in time
–
–
–
–

British Isles Stonehenge
Native American Medicine Wheel
Aztec, Mayan and Incan temples
Egyptian pyramids

Eighty-eight constellations cover the
entire sky
• Ancient peoples looked
at the stars and
imagined groupings
made pictures in the
sky
• We still refer to many of
these groupings
• Astronomers call them
constellations (from
the Latin for “group of
stars”)

Modern Constellations
• On modern star charts,
the entire sky is divided
into 88 regions. Each
is a constellation
• Most stars in a
constellation are
nowhere near one
another
• They only appear to be
close together because
they are in nearly the
same direction as seen
from Earth

The appearance of the sky changes during the
course of the night and from one night to the next
• Stars appear to rise
in the east, slowly
rotate about the
earth and set in the
west.
• This diurnal or daily
motion of the stars
is actually caused
by the 24-hour
rotation of the earth.

Animation of constellation movement
• To represent what we have just discussed,
follow this animation from the vantage
point of our Californian observer.

Annual Motion
• The stars also appear
to slowly shift in position
throughout the year
• This is due to the orbit
of the earth around the
sun
• If you follow a particular
star on successive
evenings, you will find
that it rises
approximately 4
minutes earlier each
night, or 2 hours earlier
each month

It is convenient to imagine that the stars are
located on a celestial sphere
• The celestial sphere is
an imaginary object
that has no basis in
physical reality
• However it is still a
model that remains a
useful tool of positional
astronomy
• Landmarks on the
celestial sphere are
projections of those on
the Earth

•Celestial equator
divides the sky into
northern and southern
hemispheres
•Celestial poles are
where the Earth’s axis
of rotation would
intersect the celestial
sphere
•Polaris is less than 1°
away from the north
celestial pole, which is
why it is called the
North Star or the Pole
Star.
•Point in the sky directly
overhead an observer
anywhere on Earth is
called that observer’s
zenith.

The Celestial Coordinate System
• Again, let us see what we have just
determined in a more 3-dimension manner.

Positional astronomy plays an important
role in keeping track of time
• Apparent solar time is based on the apparent
motion of the Sun across the celestial sphere,
which varies over the course of the year
• Mean solar time is based on the motion of an
imaginary mean sun along the celestial
equator, which produces a uniform mean
solar day of 24 hours
• Ordinary watches and clocks measure mean
solar time
• Sidereal time is based on the apparent
motion of the celestial sphere

• Local noon is defined to be when the Sun crosses the
upper meridian, which is the half of the meridian
above the horizon

Sidereal and Solar Days
• Appreciating the difference between a solar
day and a sidereal day is a challenging
concept. See if this helps.

Circumpolar
stars

• At any time, an observer can see only half of the celestial sphere
• The other half is below the horizon, hidden by the body of the Earth

Last thought on coordinates
• The Equatorial System of Coordinates is what most
astronomers use when specifying the location of an
object on the Celestial Sphere
• Right Ascension (measured eastwards from the
Vernal Equinox) goes from 0h to 24h
• Declination (measured north or south from the
celestial equator goes from -90° to +90 °.
• The hour angle (HA) of an object is the angle
between the meridian on which the object is situated
and the (observer’s) celestial meridian
• ST = RA + HA

The seasons are caused by the tilt of
Earth’s axis of rotation
• The Earth’s axis of rotation is not
perpendicular to the plane of the Earth’s
orbit
• It is tilted about 23½° away from the
perpendicular & is called the obliquity.
• The Earth maintains this tilt as it orbits
the Sun, with the Earth’s north pole
pointing toward the north celestial pole

The Earth’s orbit

• Seasons do NOT arise from the
distance the Earth is from the Sun but
rather as a result of the Earth’s annual
motion and axial inclination – the tip
of our planet with respect to its orbital
plane. As we move around the Sun,
the orientation of our planet gives us
seasons.

Seasons
• During part of the year the northern hemisphere of
the Earth is tilted toward the Sun
• As the Earth spins on its axis, a point in the
northern hemisphere spends more than 12 hours in
the sunlight
• The days there are long and the nights are short,
and it is summer in the northern hemisphere and
winter in the southern hemisphere
• The summer is hot not only because of the
extended daylight hours but also because the Sun
is high in the northern hemisphere’s sky
• As a result, sunlight strikes the ground at a nearly
perpendicular angle that heats the ground efficiently
• This situation reverses six months later

Sept
21
June
21

Dec
21
March
31

• The Sun appears to
trace out a circular path
called the ecliptic on
the celestial sphere tilted
at 23 ½ degrees to the
equator
• The ecliptic and the
celestial equator
intersect at only two
points
• Each point is called an
equinox
• The point on the ecliptic
farthest north of the
celestial equator that
marks the location of the
Sun at the beginning of
summer in the northern
hemisphere is called the
summer solstice
• At the beginning of the
northern hemisphere’s
winter the Sun is farthest
south of the celestial
equator at a point called
the winter solstice

Landmarks on the Earth’s surface are
marked by the Sun’s position in the sky
throughout the year

The Moon helps to cause precession, a
slow, conical motion of Earth’s axis of
rotation

Precession causes the gradual change of the
star that marks the North Celestial Pole

Astronomical observations led to the
development of the modern calendar
•
•
•
•

The day is based on the Earth’s rotation
The year is based on the Earth’s orbit
The month is based on the lunar cycle
None of these are exactly the same as
nature so astronomers use the average
or mean day and leap years to keep the
calendar and time consistent

The different types of ‘year’.
• The sidereal year (year with respect to
the stars) measured in solar time is
365d 6h 9m 10s (365.2564d) in length.
• The tropical year (successive passages
of the Sun through the Vernal Equinox)
is 365d 5h 48m 46s (365.2422d) in length.
• Due to precession, the tropical year is
20m 24s shorter than the sidereal year.

Calendars
• Caesar introduced the 365.25 days calendar
and thus the Leap Year (February 29)
• However, this is 11m 14s longer than the real
tropical year. This accumulates to 3 days in
4 centuries error.
• To correct, October 4 was followed by
October 15, in 1562 and the century rule was
invoked.


Slide 12

Knowing the Heavens

Chapter Two

Naked-eye astronomy had an important place
in ancient civilizations
• Positional astronomy
– the study of the positions of objects in the sky and
how these positions change

• Naked-eye astronomy
– the sort that requires no equipment but human
vision

• Extends far back in time
–
–
–
–

British Isles Stonehenge
Native American Medicine Wheel
Aztec, Mayan and Incan temples
Egyptian pyramids

Eighty-eight constellations cover the
entire sky
• Ancient peoples looked
at the stars and
imagined groupings
made pictures in the
sky
• We still refer to many of
these groupings
• Astronomers call them
constellations (from
the Latin for “group of
stars”)

Modern Constellations
• On modern star charts,
the entire sky is divided
into 88 regions. Each
is a constellation
• Most stars in a
constellation are
nowhere near one
another
• They only appear to be
close together because
they are in nearly the
same direction as seen
from Earth

The appearance of the sky changes during the
course of the night and from one night to the next
• Stars appear to rise
in the east, slowly
rotate about the
earth and set in the
west.
• This diurnal or daily
motion of the stars
is actually caused
by the 24-hour
rotation of the earth.

Animation of constellation movement
• To represent what we have just discussed,
follow this animation from the vantage
point of our Californian observer.

Annual Motion
• The stars also appear
to slowly shift in position
throughout the year
• This is due to the orbit
of the earth around the
sun
• If you follow a particular
star on successive
evenings, you will find
that it rises
approximately 4
minutes earlier each
night, or 2 hours earlier
each month

It is convenient to imagine that the stars are
located on a celestial sphere
• The celestial sphere is
an imaginary object
that has no basis in
physical reality
• However it is still a
model that remains a
useful tool of positional
astronomy
• Landmarks on the
celestial sphere are
projections of those on
the Earth

•Celestial equator
divides the sky into
northern and southern
hemispheres
•Celestial poles are
where the Earth’s axis
of rotation would
intersect the celestial
sphere
•Polaris is less than 1°
away from the north
celestial pole, which is
why it is called the
North Star or the Pole
Star.
•Point in the sky directly
overhead an observer
anywhere on Earth is
called that observer’s
zenith.

The Celestial Coordinate System
• Again, let us see what we have just
determined in a more 3-dimension manner.

Positional astronomy plays an important
role in keeping track of time
• Apparent solar time is based on the apparent
motion of the Sun across the celestial sphere,
which varies over the course of the year
• Mean solar time is based on the motion of an
imaginary mean sun along the celestial
equator, which produces a uniform mean
solar day of 24 hours
• Ordinary watches and clocks measure mean
solar time
• Sidereal time is based on the apparent
motion of the celestial sphere

• Local noon is defined to be when the Sun crosses the
upper meridian, which is the half of the meridian
above the horizon

Sidereal and Solar Days
• Appreciating the difference between a solar
day and a sidereal day is a challenging
concept. See if this helps.

Circumpolar
stars

• At any time, an observer can see only half of the celestial sphere
• The other half is below the horizon, hidden by the body of the Earth

Last thought on coordinates
• The Equatorial System of Coordinates is what most
astronomers use when specifying the location of an
object on the Celestial Sphere
• Right Ascension (measured eastwards from the
Vernal Equinox) goes from 0h to 24h
• Declination (measured north or south from the
celestial equator goes from -90° to +90 °.
• The hour angle (HA) of an object is the angle
between the meridian on which the object is situated
and the (observer’s) celestial meridian
• ST = RA + HA

The seasons are caused by the tilt of
Earth’s axis of rotation
• The Earth’s axis of rotation is not
perpendicular to the plane of the Earth’s
orbit
• It is tilted about 23½° away from the
perpendicular & is called the obliquity.
• The Earth maintains this tilt as it orbits
the Sun, with the Earth’s north pole
pointing toward the north celestial pole

The Earth’s orbit

• Seasons do NOT arise from the
distance the Earth is from the Sun but
rather as a result of the Earth’s annual
motion and axial inclination – the tip
of our planet with respect to its orbital
plane. As we move around the Sun,
the orientation of our planet gives us
seasons.

Seasons
• During part of the year the northern hemisphere of
the Earth is tilted toward the Sun
• As the Earth spins on its axis, a point in the
northern hemisphere spends more than 12 hours in
the sunlight
• The days there are long and the nights are short,
and it is summer in the northern hemisphere and
winter in the southern hemisphere
• The summer is hot not only because of the
extended daylight hours but also because the Sun
is high in the northern hemisphere’s sky
• As a result, sunlight strikes the ground at a nearly
perpendicular angle that heats the ground efficiently
• This situation reverses six months later

Sept
21
June
21

Dec
21
March
31

• The Sun appears to
trace out a circular path
called the ecliptic on
the celestial sphere tilted
at 23 ½ degrees to the
equator
• The ecliptic and the
celestial equator
intersect at only two
points
• Each point is called an
equinox
• The point on the ecliptic
farthest north of the
celestial equator that
marks the location of the
Sun at the beginning of
summer in the northern
hemisphere is called the
summer solstice
• At the beginning of the
northern hemisphere’s
winter the Sun is farthest
south of the celestial
equator at a point called
the winter solstice

Landmarks on the Earth’s surface are
marked by the Sun’s position in the sky
throughout the year

The Moon helps to cause precession, a
slow, conical motion of Earth’s axis of
rotation

Precession causes the gradual change of the
star that marks the North Celestial Pole

Astronomical observations led to the
development of the modern calendar
•
•
•
•

The day is based on the Earth’s rotation
The year is based on the Earth’s orbit
The month is based on the lunar cycle
None of these are exactly the same as
nature so astronomers use the average
or mean day and leap years to keep the
calendar and time consistent

The different types of ‘year’.
• The sidereal year (year with respect to
the stars) measured in solar time is
365d 6h 9m 10s (365.2564d) in length.
• The tropical year (successive passages
of the Sun through the Vernal Equinox)
is 365d 5h 48m 46s (365.2422d) in length.
• Due to precession, the tropical year is
20m 24s shorter than the sidereal year.

Calendars
• Caesar introduced the 365.25 days calendar
and thus the Leap Year (February 29)
• However, this is 11m 14s longer than the real
tropical year. This accumulates to 3 days in
4 centuries error.
• To correct, October 4 was followed by
October 15, in 1562 and the century rule was
invoked.


Slide 13

Knowing the Heavens

Chapter Two

Naked-eye astronomy had an important place
in ancient civilizations
• Positional astronomy
– the study of the positions of objects in the sky and
how these positions change

• Naked-eye astronomy
– the sort that requires no equipment but human
vision

• Extends far back in time
–
–
–
–

British Isles Stonehenge
Native American Medicine Wheel
Aztec, Mayan and Incan temples
Egyptian pyramids

Eighty-eight constellations cover the
entire sky
• Ancient peoples looked
at the stars and
imagined groupings
made pictures in the
sky
• We still refer to many of
these groupings
• Astronomers call them
constellations (from
the Latin for “group of
stars”)

Modern Constellations
• On modern star charts,
the entire sky is divided
into 88 regions. Each
is a constellation
• Most stars in a
constellation are
nowhere near one
another
• They only appear to be
close together because
they are in nearly the
same direction as seen
from Earth

The appearance of the sky changes during the
course of the night and from one night to the next
• Stars appear to rise
in the east, slowly
rotate about the
earth and set in the
west.
• This diurnal or daily
motion of the stars
is actually caused
by the 24-hour
rotation of the earth.

Animation of constellation movement
• To represent what we have just discussed,
follow this animation from the vantage
point of our Californian observer.

Annual Motion
• The stars also appear
to slowly shift in position
throughout the year
• This is due to the orbit
of the earth around the
sun
• If you follow a particular
star on successive
evenings, you will find
that it rises
approximately 4
minutes earlier each
night, or 2 hours earlier
each month

It is convenient to imagine that the stars are
located on a celestial sphere
• The celestial sphere is
an imaginary object
that has no basis in
physical reality
• However it is still a
model that remains a
useful tool of positional
astronomy
• Landmarks on the
celestial sphere are
projections of those on
the Earth

•Celestial equator
divides the sky into
northern and southern
hemispheres
•Celestial poles are
where the Earth’s axis
of rotation would
intersect the celestial
sphere
•Polaris is less than 1°
away from the north
celestial pole, which is
why it is called the
North Star or the Pole
Star.
•Point in the sky directly
overhead an observer
anywhere on Earth is
called that observer’s
zenith.

The Celestial Coordinate System
• Again, let us see what we have just
determined in a more 3-dimension manner.

Positional astronomy plays an important
role in keeping track of time
• Apparent solar time is based on the apparent
motion of the Sun across the celestial sphere,
which varies over the course of the year
• Mean solar time is based on the motion of an
imaginary mean sun along the celestial
equator, which produces a uniform mean
solar day of 24 hours
• Ordinary watches and clocks measure mean
solar time
• Sidereal time is based on the apparent
motion of the celestial sphere

• Local noon is defined to be when the Sun crosses the
upper meridian, which is the half of the meridian
above the horizon

Sidereal and Solar Days
• Appreciating the difference between a solar
day and a sidereal day is a challenging
concept. See if this helps.

Circumpolar
stars

• At any time, an observer can see only half of the celestial sphere
• The other half is below the horizon, hidden by the body of the Earth

Last thought on coordinates
• The Equatorial System of Coordinates is what most
astronomers use when specifying the location of an
object on the Celestial Sphere
• Right Ascension (measured eastwards from the
Vernal Equinox) goes from 0h to 24h
• Declination (measured north or south from the
celestial equator goes from -90° to +90 °.
• The hour angle (HA) of an object is the angle
between the meridian on which the object is situated
and the (observer’s) celestial meridian
• ST = RA + HA

The seasons are caused by the tilt of
Earth’s axis of rotation
• The Earth’s axis of rotation is not
perpendicular to the plane of the Earth’s
orbit
• It is tilted about 23½° away from the
perpendicular & is called the obliquity.
• The Earth maintains this tilt as it orbits
the Sun, with the Earth’s north pole
pointing toward the north celestial pole

The Earth’s orbit

• Seasons do NOT arise from the
distance the Earth is from the Sun but
rather as a result of the Earth’s annual
motion and axial inclination – the tip
of our planet with respect to its orbital
plane. As we move around the Sun,
the orientation of our planet gives us
seasons.

Seasons
• During part of the year the northern hemisphere of
the Earth is tilted toward the Sun
• As the Earth spins on its axis, a point in the
northern hemisphere spends more than 12 hours in
the sunlight
• The days there are long and the nights are short,
and it is summer in the northern hemisphere and
winter in the southern hemisphere
• The summer is hot not only because of the
extended daylight hours but also because the Sun
is high in the northern hemisphere’s sky
• As a result, sunlight strikes the ground at a nearly
perpendicular angle that heats the ground efficiently
• This situation reverses six months later

Sept
21
June
21

Dec
21
March
31

• The Sun appears to
trace out a circular path
called the ecliptic on
the celestial sphere tilted
at 23 ½ degrees to the
equator
• The ecliptic and the
celestial equator
intersect at only two
points
• Each point is called an
equinox
• The point on the ecliptic
farthest north of the
celestial equator that
marks the location of the
Sun at the beginning of
summer in the northern
hemisphere is called the
summer solstice
• At the beginning of the
northern hemisphere’s
winter the Sun is farthest
south of the celestial
equator at a point called
the winter solstice

Landmarks on the Earth’s surface are
marked by the Sun’s position in the sky
throughout the year

The Moon helps to cause precession, a
slow, conical motion of Earth’s axis of
rotation

Precession causes the gradual change of the
star that marks the North Celestial Pole

Astronomical observations led to the
development of the modern calendar
•
•
•
•

The day is based on the Earth’s rotation
The year is based on the Earth’s orbit
The month is based on the lunar cycle
None of these are exactly the same as
nature so astronomers use the average
or mean day and leap years to keep the
calendar and time consistent

The different types of ‘year’.
• The sidereal year (year with respect to
the stars) measured in solar time is
365d 6h 9m 10s (365.2564d) in length.
• The tropical year (successive passages
of the Sun through the Vernal Equinox)
is 365d 5h 48m 46s (365.2422d) in length.
• Due to precession, the tropical year is
20m 24s shorter than the sidereal year.

Calendars
• Caesar introduced the 365.25 days calendar
and thus the Leap Year (February 29)
• However, this is 11m 14s longer than the real
tropical year. This accumulates to 3 days in
4 centuries error.
• To correct, October 4 was followed by
October 15, in 1562 and the century rule was
invoked.


Slide 14

Knowing the Heavens

Chapter Two

Naked-eye astronomy had an important place
in ancient civilizations
• Positional astronomy
– the study of the positions of objects in the sky and
how these positions change

• Naked-eye astronomy
– the sort that requires no equipment but human
vision

• Extends far back in time
–
–
–
–

British Isles Stonehenge
Native American Medicine Wheel
Aztec, Mayan and Incan temples
Egyptian pyramids

Eighty-eight constellations cover the
entire sky
• Ancient peoples looked
at the stars and
imagined groupings
made pictures in the
sky
• We still refer to many of
these groupings
• Astronomers call them
constellations (from
the Latin for “group of
stars”)

Modern Constellations
• On modern star charts,
the entire sky is divided
into 88 regions. Each
is a constellation
• Most stars in a
constellation are
nowhere near one
another
• They only appear to be
close together because
they are in nearly the
same direction as seen
from Earth

The appearance of the sky changes during the
course of the night and from one night to the next
• Stars appear to rise
in the east, slowly
rotate about the
earth and set in the
west.
• This diurnal or daily
motion of the stars
is actually caused
by the 24-hour
rotation of the earth.

Animation of constellation movement
• To represent what we have just discussed,
follow this animation from the vantage
point of our Californian observer.

Annual Motion
• The stars also appear
to slowly shift in position
throughout the year
• This is due to the orbit
of the earth around the
sun
• If you follow a particular
star on successive
evenings, you will find
that it rises
approximately 4
minutes earlier each
night, or 2 hours earlier
each month

It is convenient to imagine that the stars are
located on a celestial sphere
• The celestial sphere is
an imaginary object
that has no basis in
physical reality
• However it is still a
model that remains a
useful tool of positional
astronomy
• Landmarks on the
celestial sphere are
projections of those on
the Earth

•Celestial equator
divides the sky into
northern and southern
hemispheres
•Celestial poles are
where the Earth’s axis
of rotation would
intersect the celestial
sphere
•Polaris is less than 1°
away from the north
celestial pole, which is
why it is called the
North Star or the Pole
Star.
•Point in the sky directly
overhead an observer
anywhere on Earth is
called that observer’s
zenith.

The Celestial Coordinate System
• Again, let us see what we have just
determined in a more 3-dimension manner.

Positional astronomy plays an important
role in keeping track of time
• Apparent solar time is based on the apparent
motion of the Sun across the celestial sphere,
which varies over the course of the year
• Mean solar time is based on the motion of an
imaginary mean sun along the celestial
equator, which produces a uniform mean
solar day of 24 hours
• Ordinary watches and clocks measure mean
solar time
• Sidereal time is based on the apparent
motion of the celestial sphere

• Local noon is defined to be when the Sun crosses the
upper meridian, which is the half of the meridian
above the horizon

Sidereal and Solar Days
• Appreciating the difference between a solar
day and a sidereal day is a challenging
concept. See if this helps.

Circumpolar
stars

• At any time, an observer can see only half of the celestial sphere
• The other half is below the horizon, hidden by the body of the Earth

Last thought on coordinates
• The Equatorial System of Coordinates is what most
astronomers use when specifying the location of an
object on the Celestial Sphere
• Right Ascension (measured eastwards from the
Vernal Equinox) goes from 0h to 24h
• Declination (measured north or south from the
celestial equator goes from -90° to +90 °.
• The hour angle (HA) of an object is the angle
between the meridian on which the object is situated
and the (observer’s) celestial meridian
• ST = RA + HA

The seasons are caused by the tilt of
Earth’s axis of rotation
• The Earth’s axis of rotation is not
perpendicular to the plane of the Earth’s
orbit
• It is tilted about 23½° away from the
perpendicular & is called the obliquity.
• The Earth maintains this tilt as it orbits
the Sun, with the Earth’s north pole
pointing toward the north celestial pole

The Earth’s orbit

• Seasons do NOT arise from the
distance the Earth is from the Sun but
rather as a result of the Earth’s annual
motion and axial inclination – the tip
of our planet with respect to its orbital
plane. As we move around the Sun,
the orientation of our planet gives us
seasons.

Seasons
• During part of the year the northern hemisphere of
the Earth is tilted toward the Sun
• As the Earth spins on its axis, a point in the
northern hemisphere spends more than 12 hours in
the sunlight
• The days there are long and the nights are short,
and it is summer in the northern hemisphere and
winter in the southern hemisphere
• The summer is hot not only because of the
extended daylight hours but also because the Sun
is high in the northern hemisphere’s sky
• As a result, sunlight strikes the ground at a nearly
perpendicular angle that heats the ground efficiently
• This situation reverses six months later

Sept
21
June
21

Dec
21
March
31

• The Sun appears to
trace out a circular path
called the ecliptic on
the celestial sphere tilted
at 23 ½ degrees to the
equator
• The ecliptic and the
celestial equator
intersect at only two
points
• Each point is called an
equinox
• The point on the ecliptic
farthest north of the
celestial equator that
marks the location of the
Sun at the beginning of
summer in the northern
hemisphere is called the
summer solstice
• At the beginning of the
northern hemisphere’s
winter the Sun is farthest
south of the celestial
equator at a point called
the winter solstice

Landmarks on the Earth’s surface are
marked by the Sun’s position in the sky
throughout the year

The Moon helps to cause precession, a
slow, conical motion of Earth’s axis of
rotation

Precession causes the gradual change of the
star that marks the North Celestial Pole

Astronomical observations led to the
development of the modern calendar
•
•
•
•

The day is based on the Earth’s rotation
The year is based on the Earth’s orbit
The month is based on the lunar cycle
None of these are exactly the same as
nature so astronomers use the average
or mean day and leap years to keep the
calendar and time consistent

The different types of ‘year’.
• The sidereal year (year with respect to
the stars) measured in solar time is
365d 6h 9m 10s (365.2564d) in length.
• The tropical year (successive passages
of the Sun through the Vernal Equinox)
is 365d 5h 48m 46s (365.2422d) in length.
• Due to precession, the tropical year is
20m 24s shorter than the sidereal year.

Calendars
• Caesar introduced the 365.25 days calendar
and thus the Leap Year (February 29)
• However, this is 11m 14s longer than the real
tropical year. This accumulates to 3 days in
4 centuries error.
• To correct, October 4 was followed by
October 15, in 1562 and the century rule was
invoked.


Slide 15

Knowing the Heavens

Chapter Two

Naked-eye astronomy had an important place
in ancient civilizations
• Positional astronomy
– the study of the positions of objects in the sky and
how these positions change

• Naked-eye astronomy
– the sort that requires no equipment but human
vision

• Extends far back in time
–
–
–
–

British Isles Stonehenge
Native American Medicine Wheel
Aztec, Mayan and Incan temples
Egyptian pyramids

Eighty-eight constellations cover the
entire sky
• Ancient peoples looked
at the stars and
imagined groupings
made pictures in the
sky
• We still refer to many of
these groupings
• Astronomers call them
constellations (from
the Latin for “group of
stars”)

Modern Constellations
• On modern star charts,
the entire sky is divided
into 88 regions. Each
is a constellation
• Most stars in a
constellation are
nowhere near one
another
• They only appear to be
close together because
they are in nearly the
same direction as seen
from Earth

The appearance of the sky changes during the
course of the night and from one night to the next
• Stars appear to rise
in the east, slowly
rotate about the
earth and set in the
west.
• This diurnal or daily
motion of the stars
is actually caused
by the 24-hour
rotation of the earth.

Animation of constellation movement
• To represent what we have just discussed,
follow this animation from the vantage
point of our Californian observer.

Annual Motion
• The stars also appear
to slowly shift in position
throughout the year
• This is due to the orbit
of the earth around the
sun
• If you follow a particular
star on successive
evenings, you will find
that it rises
approximately 4
minutes earlier each
night, or 2 hours earlier
each month

It is convenient to imagine that the stars are
located on a celestial sphere
• The celestial sphere is
an imaginary object
that has no basis in
physical reality
• However it is still a
model that remains a
useful tool of positional
astronomy
• Landmarks on the
celestial sphere are
projections of those on
the Earth

•Celestial equator
divides the sky into
northern and southern
hemispheres
•Celestial poles are
where the Earth’s axis
of rotation would
intersect the celestial
sphere
•Polaris is less than 1°
away from the north
celestial pole, which is
why it is called the
North Star or the Pole
Star.
•Point in the sky directly
overhead an observer
anywhere on Earth is
called that observer’s
zenith.

The Celestial Coordinate System
• Again, let us see what we have just
determined in a more 3-dimension manner.

Positional astronomy plays an important
role in keeping track of time
• Apparent solar time is based on the apparent
motion of the Sun across the celestial sphere,
which varies over the course of the year
• Mean solar time is based on the motion of an
imaginary mean sun along the celestial
equator, which produces a uniform mean
solar day of 24 hours
• Ordinary watches and clocks measure mean
solar time
• Sidereal time is based on the apparent
motion of the celestial sphere

• Local noon is defined to be when the Sun crosses the
upper meridian, which is the half of the meridian
above the horizon

Sidereal and Solar Days
• Appreciating the difference between a solar
day and a sidereal day is a challenging
concept. See if this helps.

Circumpolar
stars

• At any time, an observer can see only half of the celestial sphere
• The other half is below the horizon, hidden by the body of the Earth

Last thought on coordinates
• The Equatorial System of Coordinates is what most
astronomers use when specifying the location of an
object on the Celestial Sphere
• Right Ascension (measured eastwards from the
Vernal Equinox) goes from 0h to 24h
• Declination (measured north or south from the
celestial equator goes from -90° to +90 °.
• The hour angle (HA) of an object is the angle
between the meridian on which the object is situated
and the (observer’s) celestial meridian
• ST = RA + HA

The seasons are caused by the tilt of
Earth’s axis of rotation
• The Earth’s axis of rotation is not
perpendicular to the plane of the Earth’s
orbit
• It is tilted about 23½° away from the
perpendicular & is called the obliquity.
• The Earth maintains this tilt as it orbits
the Sun, with the Earth’s north pole
pointing toward the north celestial pole

The Earth’s orbit

• Seasons do NOT arise from the
distance the Earth is from the Sun but
rather as a result of the Earth’s annual
motion and axial inclination – the tip
of our planet with respect to its orbital
plane. As we move around the Sun,
the orientation of our planet gives us
seasons.

Seasons
• During part of the year the northern hemisphere of
the Earth is tilted toward the Sun
• As the Earth spins on its axis, a point in the
northern hemisphere spends more than 12 hours in
the sunlight
• The days there are long and the nights are short,
and it is summer in the northern hemisphere and
winter in the southern hemisphere
• The summer is hot not only because of the
extended daylight hours but also because the Sun
is high in the northern hemisphere’s sky
• As a result, sunlight strikes the ground at a nearly
perpendicular angle that heats the ground efficiently
• This situation reverses six months later

Sept
21
June
21

Dec
21
March
31

• The Sun appears to
trace out a circular path
called the ecliptic on
the celestial sphere tilted
at 23 ½ degrees to the
equator
• The ecliptic and the
celestial equator
intersect at only two
points
• Each point is called an
equinox
• The point on the ecliptic
farthest north of the
celestial equator that
marks the location of the
Sun at the beginning of
summer in the northern
hemisphere is called the
summer solstice
• At the beginning of the
northern hemisphere’s
winter the Sun is farthest
south of the celestial
equator at a point called
the winter solstice

Landmarks on the Earth’s surface are
marked by the Sun’s position in the sky
throughout the year

The Moon helps to cause precession, a
slow, conical motion of Earth’s axis of
rotation

Precession causes the gradual change of the
star that marks the North Celestial Pole

Astronomical observations led to the
development of the modern calendar
•
•
•
•

The day is based on the Earth’s rotation
The year is based on the Earth’s orbit
The month is based on the lunar cycle
None of these are exactly the same as
nature so astronomers use the average
or mean day and leap years to keep the
calendar and time consistent

The different types of ‘year’.
• The sidereal year (year with respect to
the stars) measured in solar time is
365d 6h 9m 10s (365.2564d) in length.
• The tropical year (successive passages
of the Sun through the Vernal Equinox)
is 365d 5h 48m 46s (365.2422d) in length.
• Due to precession, the tropical year is
20m 24s shorter than the sidereal year.

Calendars
• Caesar introduced the 365.25 days calendar
and thus the Leap Year (February 29)
• However, this is 11m 14s longer than the real
tropical year. This accumulates to 3 days in
4 centuries error.
• To correct, October 4 was followed by
October 15, in 1562 and the century rule was
invoked.


Slide 16

Knowing the Heavens

Chapter Two

Naked-eye astronomy had an important place
in ancient civilizations
• Positional astronomy
– the study of the positions of objects in the sky and
how these positions change

• Naked-eye astronomy
– the sort that requires no equipment but human
vision

• Extends far back in time
–
–
–
–

British Isles Stonehenge
Native American Medicine Wheel
Aztec, Mayan and Incan temples
Egyptian pyramids

Eighty-eight constellations cover the
entire sky
• Ancient peoples looked
at the stars and
imagined groupings
made pictures in the
sky
• We still refer to many of
these groupings
• Astronomers call them
constellations (from
the Latin for “group of
stars”)

Modern Constellations
• On modern star charts,
the entire sky is divided
into 88 regions. Each
is a constellation
• Most stars in a
constellation are
nowhere near one
another
• They only appear to be
close together because
they are in nearly the
same direction as seen
from Earth

The appearance of the sky changes during the
course of the night and from one night to the next
• Stars appear to rise
in the east, slowly
rotate about the
earth and set in the
west.
• This diurnal or daily
motion of the stars
is actually caused
by the 24-hour
rotation of the earth.

Animation of constellation movement
• To represent what we have just discussed,
follow this animation from the vantage
point of our Californian observer.

Annual Motion
• The stars also appear
to slowly shift in position
throughout the year
• This is due to the orbit
of the earth around the
sun
• If you follow a particular
star on successive
evenings, you will find
that it rises
approximately 4
minutes earlier each
night, or 2 hours earlier
each month

It is convenient to imagine that the stars are
located on a celestial sphere
• The celestial sphere is
an imaginary object
that has no basis in
physical reality
• However it is still a
model that remains a
useful tool of positional
astronomy
• Landmarks on the
celestial sphere are
projections of those on
the Earth

•Celestial equator
divides the sky into
northern and southern
hemispheres
•Celestial poles are
where the Earth’s axis
of rotation would
intersect the celestial
sphere
•Polaris is less than 1°
away from the north
celestial pole, which is
why it is called the
North Star or the Pole
Star.
•Point in the sky directly
overhead an observer
anywhere on Earth is
called that observer’s
zenith.

The Celestial Coordinate System
• Again, let us see what we have just
determined in a more 3-dimension manner.

Positional astronomy plays an important
role in keeping track of time
• Apparent solar time is based on the apparent
motion of the Sun across the celestial sphere,
which varies over the course of the year
• Mean solar time is based on the motion of an
imaginary mean sun along the celestial
equator, which produces a uniform mean
solar day of 24 hours
• Ordinary watches and clocks measure mean
solar time
• Sidereal time is based on the apparent
motion of the celestial sphere

• Local noon is defined to be when the Sun crosses the
upper meridian, which is the half of the meridian
above the horizon

Sidereal and Solar Days
• Appreciating the difference between a solar
day and a sidereal day is a challenging
concept. See if this helps.

Circumpolar
stars

• At any time, an observer can see only half of the celestial sphere
• The other half is below the horizon, hidden by the body of the Earth

Last thought on coordinates
• The Equatorial System of Coordinates is what most
astronomers use when specifying the location of an
object on the Celestial Sphere
• Right Ascension (measured eastwards from the
Vernal Equinox) goes from 0h to 24h
• Declination (measured north or south from the
celestial equator goes from -90° to +90 °.
• The hour angle (HA) of an object is the angle
between the meridian on which the object is situated
and the (observer’s) celestial meridian
• ST = RA + HA

The seasons are caused by the tilt of
Earth’s axis of rotation
• The Earth’s axis of rotation is not
perpendicular to the plane of the Earth’s
orbit
• It is tilted about 23½° away from the
perpendicular & is called the obliquity.
• The Earth maintains this tilt as it orbits
the Sun, with the Earth’s north pole
pointing toward the north celestial pole

The Earth’s orbit

• Seasons do NOT arise from the
distance the Earth is from the Sun but
rather as a result of the Earth’s annual
motion and axial inclination – the tip
of our planet with respect to its orbital
plane. As we move around the Sun,
the orientation of our planet gives us
seasons.

Seasons
• During part of the year the northern hemisphere of
the Earth is tilted toward the Sun
• As the Earth spins on its axis, a point in the
northern hemisphere spends more than 12 hours in
the sunlight
• The days there are long and the nights are short,
and it is summer in the northern hemisphere and
winter in the southern hemisphere
• The summer is hot not only because of the
extended daylight hours but also because the Sun
is high in the northern hemisphere’s sky
• As a result, sunlight strikes the ground at a nearly
perpendicular angle that heats the ground efficiently
• This situation reverses six months later

Sept
21
June
21

Dec
21
March
31

• The Sun appears to
trace out a circular path
called the ecliptic on
the celestial sphere tilted
at 23 ½ degrees to the
equator
• The ecliptic and the
celestial equator
intersect at only two
points
• Each point is called an
equinox
• The point on the ecliptic
farthest north of the
celestial equator that
marks the location of the
Sun at the beginning of
summer in the northern
hemisphere is called the
summer solstice
• At the beginning of the
northern hemisphere’s
winter the Sun is farthest
south of the celestial
equator at a point called
the winter solstice

Landmarks on the Earth’s surface are
marked by the Sun’s position in the sky
throughout the year

The Moon helps to cause precession, a
slow, conical motion of Earth’s axis of
rotation

Precession causes the gradual change of the
star that marks the North Celestial Pole

Astronomical observations led to the
development of the modern calendar
•
•
•
•

The day is based on the Earth’s rotation
The year is based on the Earth’s orbit
The month is based on the lunar cycle
None of these are exactly the same as
nature so astronomers use the average
or mean day and leap years to keep the
calendar and time consistent

The different types of ‘year’.
• The sidereal year (year with respect to
the stars) measured in solar time is
365d 6h 9m 10s (365.2564d) in length.
• The tropical year (successive passages
of the Sun through the Vernal Equinox)
is 365d 5h 48m 46s (365.2422d) in length.
• Due to precession, the tropical year is
20m 24s shorter than the sidereal year.

Calendars
• Caesar introduced the 365.25 days calendar
and thus the Leap Year (February 29)
• However, this is 11m 14s longer than the real
tropical year. This accumulates to 3 days in
4 centuries error.
• To correct, October 4 was followed by
October 15, in 1562 and the century rule was
invoked.


Slide 17

Knowing the Heavens

Chapter Two

Naked-eye astronomy had an important place
in ancient civilizations
• Positional astronomy
– the study of the positions of objects in the sky and
how these positions change

• Naked-eye astronomy
– the sort that requires no equipment but human
vision

• Extends far back in time
–
–
–
–

British Isles Stonehenge
Native American Medicine Wheel
Aztec, Mayan and Incan temples
Egyptian pyramids

Eighty-eight constellations cover the
entire sky
• Ancient peoples looked
at the stars and
imagined groupings
made pictures in the
sky
• We still refer to many of
these groupings
• Astronomers call them
constellations (from
the Latin for “group of
stars”)

Modern Constellations
• On modern star charts,
the entire sky is divided
into 88 regions. Each
is a constellation
• Most stars in a
constellation are
nowhere near one
another
• They only appear to be
close together because
they are in nearly the
same direction as seen
from Earth

The appearance of the sky changes during the
course of the night and from one night to the next
• Stars appear to rise
in the east, slowly
rotate about the
earth and set in the
west.
• This diurnal or daily
motion of the stars
is actually caused
by the 24-hour
rotation of the earth.

Animation of constellation movement
• To represent what we have just discussed,
follow this animation from the vantage
point of our Californian observer.

Annual Motion
• The stars also appear
to slowly shift in position
throughout the year
• This is due to the orbit
of the earth around the
sun
• If you follow a particular
star on successive
evenings, you will find
that it rises
approximately 4
minutes earlier each
night, or 2 hours earlier
each month

It is convenient to imagine that the stars are
located on a celestial sphere
• The celestial sphere is
an imaginary object
that has no basis in
physical reality
• However it is still a
model that remains a
useful tool of positional
astronomy
• Landmarks on the
celestial sphere are
projections of those on
the Earth

•Celestial equator
divides the sky into
northern and southern
hemispheres
•Celestial poles are
where the Earth’s axis
of rotation would
intersect the celestial
sphere
•Polaris is less than 1°
away from the north
celestial pole, which is
why it is called the
North Star or the Pole
Star.
•Point in the sky directly
overhead an observer
anywhere on Earth is
called that observer’s
zenith.

The Celestial Coordinate System
• Again, let us see what we have just
determined in a more 3-dimension manner.

Positional astronomy plays an important
role in keeping track of time
• Apparent solar time is based on the apparent
motion of the Sun across the celestial sphere,
which varies over the course of the year
• Mean solar time is based on the motion of an
imaginary mean sun along the celestial
equator, which produces a uniform mean
solar day of 24 hours
• Ordinary watches and clocks measure mean
solar time
• Sidereal time is based on the apparent
motion of the celestial sphere

• Local noon is defined to be when the Sun crosses the
upper meridian, which is the half of the meridian
above the horizon

Sidereal and Solar Days
• Appreciating the difference between a solar
day and a sidereal day is a challenging
concept. See if this helps.

Circumpolar
stars

• At any time, an observer can see only half of the celestial sphere
• The other half is below the horizon, hidden by the body of the Earth

Last thought on coordinates
• The Equatorial System of Coordinates is what most
astronomers use when specifying the location of an
object on the Celestial Sphere
• Right Ascension (measured eastwards from the
Vernal Equinox) goes from 0h to 24h
• Declination (measured north or south from the
celestial equator goes from -90° to +90 °.
• The hour angle (HA) of an object is the angle
between the meridian on which the object is situated
and the (observer’s) celestial meridian
• ST = RA + HA

The seasons are caused by the tilt of
Earth’s axis of rotation
• The Earth’s axis of rotation is not
perpendicular to the plane of the Earth’s
orbit
• It is tilted about 23½° away from the
perpendicular & is called the obliquity.
• The Earth maintains this tilt as it orbits
the Sun, with the Earth’s north pole
pointing toward the north celestial pole

The Earth’s orbit

• Seasons do NOT arise from the
distance the Earth is from the Sun but
rather as a result of the Earth’s annual
motion and axial inclination – the tip
of our planet with respect to its orbital
plane. As we move around the Sun,
the orientation of our planet gives us
seasons.

Seasons
• During part of the year the northern hemisphere of
the Earth is tilted toward the Sun
• As the Earth spins on its axis, a point in the
northern hemisphere spends more than 12 hours in
the sunlight
• The days there are long and the nights are short,
and it is summer in the northern hemisphere and
winter in the southern hemisphere
• The summer is hot not only because of the
extended daylight hours but also because the Sun
is high in the northern hemisphere’s sky
• As a result, sunlight strikes the ground at a nearly
perpendicular angle that heats the ground efficiently
• This situation reverses six months later

Sept
21
June
21

Dec
21
March
31

• The Sun appears to
trace out a circular path
called the ecliptic on
the celestial sphere tilted
at 23 ½ degrees to the
equator
• The ecliptic and the
celestial equator
intersect at only two
points
• Each point is called an
equinox
• The point on the ecliptic
farthest north of the
celestial equator that
marks the location of the
Sun at the beginning of
summer in the northern
hemisphere is called the
summer solstice
• At the beginning of the
northern hemisphere’s
winter the Sun is farthest
south of the celestial
equator at a point called
the winter solstice

Landmarks on the Earth’s surface are
marked by the Sun’s position in the sky
throughout the year

The Moon helps to cause precession, a
slow, conical motion of Earth’s axis of
rotation

Precession causes the gradual change of the
star that marks the North Celestial Pole

Astronomical observations led to the
development of the modern calendar
•
•
•
•

The day is based on the Earth’s rotation
The year is based on the Earth’s orbit
The month is based on the lunar cycle
None of these are exactly the same as
nature so astronomers use the average
or mean day and leap years to keep the
calendar and time consistent

The different types of ‘year’.
• The sidereal year (year with respect to
the stars) measured in solar time is
365d 6h 9m 10s (365.2564d) in length.
• The tropical year (successive passages
of the Sun through the Vernal Equinox)
is 365d 5h 48m 46s (365.2422d) in length.
• Due to precession, the tropical year is
20m 24s shorter than the sidereal year.

Calendars
• Caesar introduced the 365.25 days calendar
and thus the Leap Year (February 29)
• However, this is 11m 14s longer than the real
tropical year. This accumulates to 3 days in
4 centuries error.
• To correct, October 4 was followed by
October 15, in 1562 and the century rule was
invoked.


Slide 18

Knowing the Heavens

Chapter Two

Naked-eye astronomy had an important place
in ancient civilizations
• Positional astronomy
– the study of the positions of objects in the sky and
how these positions change

• Naked-eye astronomy
– the sort that requires no equipment but human
vision

• Extends far back in time
–
–
–
–

British Isles Stonehenge
Native American Medicine Wheel
Aztec, Mayan and Incan temples
Egyptian pyramids

Eighty-eight constellations cover the
entire sky
• Ancient peoples looked
at the stars and
imagined groupings
made pictures in the
sky
• We still refer to many of
these groupings
• Astronomers call them
constellations (from
the Latin for “group of
stars”)

Modern Constellations
• On modern star charts,
the entire sky is divided
into 88 regions. Each
is a constellation
• Most stars in a
constellation are
nowhere near one
another
• They only appear to be
close together because
they are in nearly the
same direction as seen
from Earth

The appearance of the sky changes during the
course of the night and from one night to the next
• Stars appear to rise
in the east, slowly
rotate about the
earth and set in the
west.
• This diurnal or daily
motion of the stars
is actually caused
by the 24-hour
rotation of the earth.

Animation of constellation movement
• To represent what we have just discussed,
follow this animation from the vantage
point of our Californian observer.

Annual Motion
• The stars also appear
to slowly shift in position
throughout the year
• This is due to the orbit
of the earth around the
sun
• If you follow a particular
star on successive
evenings, you will find
that it rises
approximately 4
minutes earlier each
night, or 2 hours earlier
each month

It is convenient to imagine that the stars are
located on a celestial sphere
• The celestial sphere is
an imaginary object
that has no basis in
physical reality
• However it is still a
model that remains a
useful tool of positional
astronomy
• Landmarks on the
celestial sphere are
projections of those on
the Earth

•Celestial equator
divides the sky into
northern and southern
hemispheres
•Celestial poles are
where the Earth’s axis
of rotation would
intersect the celestial
sphere
•Polaris is less than 1°
away from the north
celestial pole, which is
why it is called the
North Star or the Pole
Star.
•Point in the sky directly
overhead an observer
anywhere on Earth is
called that observer’s
zenith.

The Celestial Coordinate System
• Again, let us see what we have just
determined in a more 3-dimension manner.

Positional astronomy plays an important
role in keeping track of time
• Apparent solar time is based on the apparent
motion of the Sun across the celestial sphere,
which varies over the course of the year
• Mean solar time is based on the motion of an
imaginary mean sun along the celestial
equator, which produces a uniform mean
solar day of 24 hours
• Ordinary watches and clocks measure mean
solar time
• Sidereal time is based on the apparent
motion of the celestial sphere

• Local noon is defined to be when the Sun crosses the
upper meridian, which is the half of the meridian
above the horizon

Sidereal and Solar Days
• Appreciating the difference between a solar
day and a sidereal day is a challenging
concept. See if this helps.

Circumpolar
stars

• At any time, an observer can see only half of the celestial sphere
• The other half is below the horizon, hidden by the body of the Earth

Last thought on coordinates
• The Equatorial System of Coordinates is what most
astronomers use when specifying the location of an
object on the Celestial Sphere
• Right Ascension (measured eastwards from the
Vernal Equinox) goes from 0h to 24h
• Declination (measured north or south from the
celestial equator goes from -90° to +90 °.
• The hour angle (HA) of an object is the angle
between the meridian on which the object is situated
and the (observer’s) celestial meridian
• ST = RA + HA

The seasons are caused by the tilt of
Earth’s axis of rotation
• The Earth’s axis of rotation is not
perpendicular to the plane of the Earth’s
orbit
• It is tilted about 23½° away from the
perpendicular & is called the obliquity.
• The Earth maintains this tilt as it orbits
the Sun, with the Earth’s north pole
pointing toward the north celestial pole

The Earth’s orbit

• Seasons do NOT arise from the
distance the Earth is from the Sun but
rather as a result of the Earth’s annual
motion and axial inclination – the tip
of our planet with respect to its orbital
plane. As we move around the Sun,
the orientation of our planet gives us
seasons.

Seasons
• During part of the year the northern hemisphere of
the Earth is tilted toward the Sun
• As the Earth spins on its axis, a point in the
northern hemisphere spends more than 12 hours in
the sunlight
• The days there are long and the nights are short,
and it is summer in the northern hemisphere and
winter in the southern hemisphere
• The summer is hot not only because of the
extended daylight hours but also because the Sun
is high in the northern hemisphere’s sky
• As a result, sunlight strikes the ground at a nearly
perpendicular angle that heats the ground efficiently
• This situation reverses six months later

Sept
21
June
21

Dec
21
March
31

• The Sun appears to
trace out a circular path
called the ecliptic on
the celestial sphere tilted
at 23 ½ degrees to the
equator
• The ecliptic and the
celestial equator
intersect at only two
points
• Each point is called an
equinox
• The point on the ecliptic
farthest north of the
celestial equator that
marks the location of the
Sun at the beginning of
summer in the northern
hemisphere is called the
summer solstice
• At the beginning of the
northern hemisphere’s
winter the Sun is farthest
south of the celestial
equator at a point called
the winter solstice

Landmarks on the Earth’s surface are
marked by the Sun’s position in the sky
throughout the year

The Moon helps to cause precession, a
slow, conical motion of Earth’s axis of
rotation

Precession causes the gradual change of the
star that marks the North Celestial Pole

Astronomical observations led to the
development of the modern calendar
•
•
•
•

The day is based on the Earth’s rotation
The year is based on the Earth’s orbit
The month is based on the lunar cycle
None of these are exactly the same as
nature so astronomers use the average
or mean day and leap years to keep the
calendar and time consistent

The different types of ‘year’.
• The sidereal year (year with respect to
the stars) measured in solar time is
365d 6h 9m 10s (365.2564d) in length.
• The tropical year (successive passages
of the Sun through the Vernal Equinox)
is 365d 5h 48m 46s (365.2422d) in length.
• Due to precession, the tropical year is
20m 24s shorter than the sidereal year.

Calendars
• Caesar introduced the 365.25 days calendar
and thus the Leap Year (February 29)
• However, this is 11m 14s longer than the real
tropical year. This accumulates to 3 days in
4 centuries error.
• To correct, October 4 was followed by
October 15, in 1562 and the century rule was
invoked.


Slide 19

Knowing the Heavens

Chapter Two

Naked-eye astronomy had an important place
in ancient civilizations
• Positional astronomy
– the study of the positions of objects in the sky and
how these positions change

• Naked-eye astronomy
– the sort that requires no equipment but human
vision

• Extends far back in time
–
–
–
–

British Isles Stonehenge
Native American Medicine Wheel
Aztec, Mayan and Incan temples
Egyptian pyramids

Eighty-eight constellations cover the
entire sky
• Ancient peoples looked
at the stars and
imagined groupings
made pictures in the
sky
• We still refer to many of
these groupings
• Astronomers call them
constellations (from
the Latin for “group of
stars”)

Modern Constellations
• On modern star charts,
the entire sky is divided
into 88 regions. Each
is a constellation
• Most stars in a
constellation are
nowhere near one
another
• They only appear to be
close together because
they are in nearly the
same direction as seen
from Earth

The appearance of the sky changes during the
course of the night and from one night to the next
• Stars appear to rise
in the east, slowly
rotate about the
earth and set in the
west.
• This diurnal or daily
motion of the stars
is actually caused
by the 24-hour
rotation of the earth.

Animation of constellation movement
• To represent what we have just discussed,
follow this animation from the vantage
point of our Californian observer.

Annual Motion
• The stars also appear
to slowly shift in position
throughout the year
• This is due to the orbit
of the earth around the
sun
• If you follow a particular
star on successive
evenings, you will find
that it rises
approximately 4
minutes earlier each
night, or 2 hours earlier
each month

It is convenient to imagine that the stars are
located on a celestial sphere
• The celestial sphere is
an imaginary object
that has no basis in
physical reality
• However it is still a
model that remains a
useful tool of positional
astronomy
• Landmarks on the
celestial sphere are
projections of those on
the Earth

•Celestial equator
divides the sky into
northern and southern
hemispheres
•Celestial poles are
where the Earth’s axis
of rotation would
intersect the celestial
sphere
•Polaris is less than 1°
away from the north
celestial pole, which is
why it is called the
North Star or the Pole
Star.
•Point in the sky directly
overhead an observer
anywhere on Earth is
called that observer’s
zenith.

The Celestial Coordinate System
• Again, let us see what we have just
determined in a more 3-dimension manner.

Positional astronomy plays an important
role in keeping track of time
• Apparent solar time is based on the apparent
motion of the Sun across the celestial sphere,
which varies over the course of the year
• Mean solar time is based on the motion of an
imaginary mean sun along the celestial
equator, which produces a uniform mean
solar day of 24 hours
• Ordinary watches and clocks measure mean
solar time
• Sidereal time is based on the apparent
motion of the celestial sphere

• Local noon is defined to be when the Sun crosses the
upper meridian, which is the half of the meridian
above the horizon

Sidereal and Solar Days
• Appreciating the difference between a solar
day and a sidereal day is a challenging
concept. See if this helps.

Circumpolar
stars

• At any time, an observer can see only half of the celestial sphere
• The other half is below the horizon, hidden by the body of the Earth

Last thought on coordinates
• The Equatorial System of Coordinates is what most
astronomers use when specifying the location of an
object on the Celestial Sphere
• Right Ascension (measured eastwards from the
Vernal Equinox) goes from 0h to 24h
• Declination (measured north or south from the
celestial equator goes from -90° to +90 °.
• The hour angle (HA) of an object is the angle
between the meridian on which the object is situated
and the (observer’s) celestial meridian
• ST = RA + HA

The seasons are caused by the tilt of
Earth’s axis of rotation
• The Earth’s axis of rotation is not
perpendicular to the plane of the Earth’s
orbit
• It is tilted about 23½° away from the
perpendicular & is called the obliquity.
• The Earth maintains this tilt as it orbits
the Sun, with the Earth’s north pole
pointing toward the north celestial pole

The Earth’s orbit

• Seasons do NOT arise from the
distance the Earth is from the Sun but
rather as a result of the Earth’s annual
motion and axial inclination – the tip
of our planet with respect to its orbital
plane. As we move around the Sun,
the orientation of our planet gives us
seasons.

Seasons
• During part of the year the northern hemisphere of
the Earth is tilted toward the Sun
• As the Earth spins on its axis, a point in the
northern hemisphere spends more than 12 hours in
the sunlight
• The days there are long and the nights are short,
and it is summer in the northern hemisphere and
winter in the southern hemisphere
• The summer is hot not only because of the
extended daylight hours but also because the Sun
is high in the northern hemisphere’s sky
• As a result, sunlight strikes the ground at a nearly
perpendicular angle that heats the ground efficiently
• This situation reverses six months later

Sept
21
June
21

Dec
21
March
31

• The Sun appears to
trace out a circular path
called the ecliptic on
the celestial sphere tilted
at 23 ½ degrees to the
equator
• The ecliptic and the
celestial equator
intersect at only two
points
• Each point is called an
equinox
• The point on the ecliptic
farthest north of the
celestial equator that
marks the location of the
Sun at the beginning of
summer in the northern
hemisphere is called the
summer solstice
• At the beginning of the
northern hemisphere’s
winter the Sun is farthest
south of the celestial
equator at a point called
the winter solstice

Landmarks on the Earth’s surface are
marked by the Sun’s position in the sky
throughout the year

The Moon helps to cause precession, a
slow, conical motion of Earth’s axis of
rotation

Precession causes the gradual change of the
star that marks the North Celestial Pole

Astronomical observations led to the
development of the modern calendar
•
•
•
•

The day is based on the Earth’s rotation
The year is based on the Earth’s orbit
The month is based on the lunar cycle
None of these are exactly the same as
nature so astronomers use the average
or mean day and leap years to keep the
calendar and time consistent

The different types of ‘year’.
• The sidereal year (year with respect to
the stars) measured in solar time is
365d 6h 9m 10s (365.2564d) in length.
• The tropical year (successive passages
of the Sun through the Vernal Equinox)
is 365d 5h 48m 46s (365.2422d) in length.
• Due to precession, the tropical year is
20m 24s shorter than the sidereal year.

Calendars
• Caesar introduced the 365.25 days calendar
and thus the Leap Year (February 29)
• However, this is 11m 14s longer than the real
tropical year. This accumulates to 3 days in
4 centuries error.
• To correct, October 4 was followed by
October 15, in 1562 and the century rule was
invoked.


Slide 20

Knowing the Heavens

Chapter Two

Naked-eye astronomy had an important place
in ancient civilizations
• Positional astronomy
– the study of the positions of objects in the sky and
how these positions change

• Naked-eye astronomy
– the sort that requires no equipment but human
vision

• Extends far back in time
–
–
–
–

British Isles Stonehenge
Native American Medicine Wheel
Aztec, Mayan and Incan temples
Egyptian pyramids

Eighty-eight constellations cover the
entire sky
• Ancient peoples looked
at the stars and
imagined groupings
made pictures in the
sky
• We still refer to many of
these groupings
• Astronomers call them
constellations (from
the Latin for “group of
stars”)

Modern Constellations
• On modern star charts,
the entire sky is divided
into 88 regions. Each
is a constellation
• Most stars in a
constellation are
nowhere near one
another
• They only appear to be
close together because
they are in nearly the
same direction as seen
from Earth

The appearance of the sky changes during the
course of the night and from one night to the next
• Stars appear to rise
in the east, slowly
rotate about the
earth and set in the
west.
• This diurnal or daily
motion of the stars
is actually caused
by the 24-hour
rotation of the earth.

Animation of constellation movement
• To represent what we have just discussed,
follow this animation from the vantage
point of our Californian observer.

Annual Motion
• The stars also appear
to slowly shift in position
throughout the year
• This is due to the orbit
of the earth around the
sun
• If you follow a particular
star on successive
evenings, you will find
that it rises
approximately 4
minutes earlier each
night, or 2 hours earlier
each month

It is convenient to imagine that the stars are
located on a celestial sphere
• The celestial sphere is
an imaginary object
that has no basis in
physical reality
• However it is still a
model that remains a
useful tool of positional
astronomy
• Landmarks on the
celestial sphere are
projections of those on
the Earth

•Celestial equator
divides the sky into
northern and southern
hemispheres
•Celestial poles are
where the Earth’s axis
of rotation would
intersect the celestial
sphere
•Polaris is less than 1°
away from the north
celestial pole, which is
why it is called the
North Star or the Pole
Star.
•Point in the sky directly
overhead an observer
anywhere on Earth is
called that observer’s
zenith.

The Celestial Coordinate System
• Again, let us see what we have just
determined in a more 3-dimension manner.

Positional astronomy plays an important
role in keeping track of time
• Apparent solar time is based on the apparent
motion of the Sun across the celestial sphere,
which varies over the course of the year
• Mean solar time is based on the motion of an
imaginary mean sun along the celestial
equator, which produces a uniform mean
solar day of 24 hours
• Ordinary watches and clocks measure mean
solar time
• Sidereal time is based on the apparent
motion of the celestial sphere

• Local noon is defined to be when the Sun crosses the
upper meridian, which is the half of the meridian
above the horizon

Sidereal and Solar Days
• Appreciating the difference between a solar
day and a sidereal day is a challenging
concept. See if this helps.

Circumpolar
stars

• At any time, an observer can see only half of the celestial sphere
• The other half is below the horizon, hidden by the body of the Earth

Last thought on coordinates
• The Equatorial System of Coordinates is what most
astronomers use when specifying the location of an
object on the Celestial Sphere
• Right Ascension (measured eastwards from the
Vernal Equinox) goes from 0h to 24h
• Declination (measured north or south from the
celestial equator goes from -90° to +90 °.
• The hour angle (HA) of an object is the angle
between the meridian on which the object is situated
and the (observer’s) celestial meridian
• ST = RA + HA

The seasons are caused by the tilt of
Earth’s axis of rotation
• The Earth’s axis of rotation is not
perpendicular to the plane of the Earth’s
orbit
• It is tilted about 23½° away from the
perpendicular & is called the obliquity.
• The Earth maintains this tilt as it orbits
the Sun, with the Earth’s north pole
pointing toward the north celestial pole

The Earth’s orbit

• Seasons do NOT arise from the
distance the Earth is from the Sun but
rather as a result of the Earth’s annual
motion and axial inclination – the tip
of our planet with respect to its orbital
plane. As we move around the Sun,
the orientation of our planet gives us
seasons.

Seasons
• During part of the year the northern hemisphere of
the Earth is tilted toward the Sun
• As the Earth spins on its axis, a point in the
northern hemisphere spends more than 12 hours in
the sunlight
• The days there are long and the nights are short,
and it is summer in the northern hemisphere and
winter in the southern hemisphere
• The summer is hot not only because of the
extended daylight hours but also because the Sun
is high in the northern hemisphere’s sky
• As a result, sunlight strikes the ground at a nearly
perpendicular angle that heats the ground efficiently
• This situation reverses six months later

Sept
21
June
21

Dec
21
March
31

• The Sun appears to
trace out a circular path
called the ecliptic on
the celestial sphere tilted
at 23 ½ degrees to the
equator
• The ecliptic and the
celestial equator
intersect at only two
points
• Each point is called an
equinox
• The point on the ecliptic
farthest north of the
celestial equator that
marks the location of the
Sun at the beginning of
summer in the northern
hemisphere is called the
summer solstice
• At the beginning of the
northern hemisphere’s
winter the Sun is farthest
south of the celestial
equator at a point called
the winter solstice

Landmarks on the Earth’s surface are
marked by the Sun’s position in the sky
throughout the year

The Moon helps to cause precession, a
slow, conical motion of Earth’s axis of
rotation

Precession causes the gradual change of the
star that marks the North Celestial Pole

Astronomical observations led to the
development of the modern calendar
•
•
•
•

The day is based on the Earth’s rotation
The year is based on the Earth’s orbit
The month is based on the lunar cycle
None of these are exactly the same as
nature so astronomers use the average
or mean day and leap years to keep the
calendar and time consistent

The different types of ‘year’.
• The sidereal year (year with respect to
the stars) measured in solar time is
365d 6h 9m 10s (365.2564d) in length.
• The tropical year (successive passages
of the Sun through the Vernal Equinox)
is 365d 5h 48m 46s (365.2422d) in length.
• Due to precession, the tropical year is
20m 24s shorter than the sidereal year.

Calendars
• Caesar introduced the 365.25 days calendar
and thus the Leap Year (February 29)
• However, this is 11m 14s longer than the real
tropical year. This accumulates to 3 days in
4 centuries error.
• To correct, October 4 was followed by
October 15, in 1562 and the century rule was
invoked.


Slide 21

Knowing the Heavens

Chapter Two

Naked-eye astronomy had an important place
in ancient civilizations
• Positional astronomy
– the study of the positions of objects in the sky and
how these positions change

• Naked-eye astronomy
– the sort that requires no equipment but human
vision

• Extends far back in time
–
–
–
–

British Isles Stonehenge
Native American Medicine Wheel
Aztec, Mayan and Incan temples
Egyptian pyramids

Eighty-eight constellations cover the
entire sky
• Ancient peoples looked
at the stars and
imagined groupings
made pictures in the
sky
• We still refer to many of
these groupings
• Astronomers call them
constellations (from
the Latin for “group of
stars”)

Modern Constellations
• On modern star charts,
the entire sky is divided
into 88 regions. Each
is a constellation
• Most stars in a
constellation are
nowhere near one
another
• They only appear to be
close together because
they are in nearly the
same direction as seen
from Earth

The appearance of the sky changes during the
course of the night and from one night to the next
• Stars appear to rise
in the east, slowly
rotate about the
earth and set in the
west.
• This diurnal or daily
motion of the stars
is actually caused
by the 24-hour
rotation of the earth.

Animation of constellation movement
• To represent what we have just discussed,
follow this animation from the vantage
point of our Californian observer.

Annual Motion
• The stars also appear
to slowly shift in position
throughout the year
• This is due to the orbit
of the earth around the
sun
• If you follow a particular
star on successive
evenings, you will find
that it rises
approximately 4
minutes earlier each
night, or 2 hours earlier
each month

It is convenient to imagine that the stars are
located on a celestial sphere
• The celestial sphere is
an imaginary object
that has no basis in
physical reality
• However it is still a
model that remains a
useful tool of positional
astronomy
• Landmarks on the
celestial sphere are
projections of those on
the Earth

•Celestial equator
divides the sky into
northern and southern
hemispheres
•Celestial poles are
where the Earth’s axis
of rotation would
intersect the celestial
sphere
•Polaris is less than 1°
away from the north
celestial pole, which is
why it is called the
North Star or the Pole
Star.
•Point in the sky directly
overhead an observer
anywhere on Earth is
called that observer’s
zenith.

The Celestial Coordinate System
• Again, let us see what we have just
determined in a more 3-dimension manner.

Positional astronomy plays an important
role in keeping track of time
• Apparent solar time is based on the apparent
motion of the Sun across the celestial sphere,
which varies over the course of the year
• Mean solar time is based on the motion of an
imaginary mean sun along the celestial
equator, which produces a uniform mean
solar day of 24 hours
• Ordinary watches and clocks measure mean
solar time
• Sidereal time is based on the apparent
motion of the celestial sphere

• Local noon is defined to be when the Sun crosses the
upper meridian, which is the half of the meridian
above the horizon

Sidereal and Solar Days
• Appreciating the difference between a solar
day and a sidereal day is a challenging
concept. See if this helps.

Circumpolar
stars

• At any time, an observer can see only half of the celestial sphere
• The other half is below the horizon, hidden by the body of the Earth

Last thought on coordinates
• The Equatorial System of Coordinates is what most
astronomers use when specifying the location of an
object on the Celestial Sphere
• Right Ascension (measured eastwards from the
Vernal Equinox) goes from 0h to 24h
• Declination (measured north or south from the
celestial equator goes from -90° to +90 °.
• The hour angle (HA) of an object is the angle
between the meridian on which the object is situated
and the (observer’s) celestial meridian
• ST = RA + HA

The seasons are caused by the tilt of
Earth’s axis of rotation
• The Earth’s axis of rotation is not
perpendicular to the plane of the Earth’s
orbit
• It is tilted about 23½° away from the
perpendicular & is called the obliquity.
• The Earth maintains this tilt as it orbits
the Sun, with the Earth’s north pole
pointing toward the north celestial pole

The Earth’s orbit

• Seasons do NOT arise from the
distance the Earth is from the Sun but
rather as a result of the Earth’s annual
motion and axial inclination – the tip
of our planet with respect to its orbital
plane. As we move around the Sun,
the orientation of our planet gives us
seasons.

Seasons
• During part of the year the northern hemisphere of
the Earth is tilted toward the Sun
• As the Earth spins on its axis, a point in the
northern hemisphere spends more than 12 hours in
the sunlight
• The days there are long and the nights are short,
and it is summer in the northern hemisphere and
winter in the southern hemisphere
• The summer is hot not only because of the
extended daylight hours but also because the Sun
is high in the northern hemisphere’s sky
• As a result, sunlight strikes the ground at a nearly
perpendicular angle that heats the ground efficiently
• This situation reverses six months later

Sept
21
June
21

Dec
21
March
31

• The Sun appears to
trace out a circular path
called the ecliptic on
the celestial sphere tilted
at 23 ½ degrees to the
equator
• The ecliptic and the
celestial equator
intersect at only two
points
• Each point is called an
equinox
• The point on the ecliptic
farthest north of the
celestial equator that
marks the location of the
Sun at the beginning of
summer in the northern
hemisphere is called the
summer solstice
• At the beginning of the
northern hemisphere’s
winter the Sun is farthest
south of the celestial
equator at a point called
the winter solstice

Landmarks on the Earth’s surface are
marked by the Sun’s position in the sky
throughout the year

The Moon helps to cause precession, a
slow, conical motion of Earth’s axis of
rotation

Precession causes the gradual change of the
star that marks the North Celestial Pole

Astronomical observations led to the
development of the modern calendar
•
•
•
•

The day is based on the Earth’s rotation
The year is based on the Earth’s orbit
The month is based on the lunar cycle
None of these are exactly the same as
nature so astronomers use the average
or mean day and leap years to keep the
calendar and time consistent

The different types of ‘year’.
• The sidereal year (year with respect to
the stars) measured in solar time is
365d 6h 9m 10s (365.2564d) in length.
• The tropical year (successive passages
of the Sun through the Vernal Equinox)
is 365d 5h 48m 46s (365.2422d) in length.
• Due to precession, the tropical year is
20m 24s shorter than the sidereal year.

Calendars
• Caesar introduced the 365.25 days calendar
and thus the Leap Year (February 29)
• However, this is 11m 14s longer than the real
tropical year. This accumulates to 3 days in
4 centuries error.
• To correct, October 4 was followed by
October 15, in 1562 and the century rule was
invoked.


Slide 22

Knowing the Heavens

Chapter Two

Naked-eye astronomy had an important place
in ancient civilizations
• Positional astronomy
– the study of the positions of objects in the sky and
how these positions change

• Naked-eye astronomy
– the sort that requires no equipment but human
vision

• Extends far back in time
–
–
–
–

British Isles Stonehenge
Native American Medicine Wheel
Aztec, Mayan and Incan temples
Egyptian pyramids

Eighty-eight constellations cover the
entire sky
• Ancient peoples looked
at the stars and
imagined groupings
made pictures in the
sky
• We still refer to many of
these groupings
• Astronomers call them
constellations (from
the Latin for “group of
stars”)

Modern Constellations
• On modern star charts,
the entire sky is divided
into 88 regions. Each
is a constellation
• Most stars in a
constellation are
nowhere near one
another
• They only appear to be
close together because
they are in nearly the
same direction as seen
from Earth

The appearance of the sky changes during the
course of the night and from one night to the next
• Stars appear to rise
in the east, slowly
rotate about the
earth and set in the
west.
• This diurnal or daily
motion of the stars
is actually caused
by the 24-hour
rotation of the earth.

Animation of constellation movement
• To represent what we have just discussed,
follow this animation from the vantage
point of our Californian observer.

Annual Motion
• The stars also appear
to slowly shift in position
throughout the year
• This is due to the orbit
of the earth around the
sun
• If you follow a particular
star on successive
evenings, you will find
that it rises
approximately 4
minutes earlier each
night, or 2 hours earlier
each month

It is convenient to imagine that the stars are
located on a celestial sphere
• The celestial sphere is
an imaginary object
that has no basis in
physical reality
• However it is still a
model that remains a
useful tool of positional
astronomy
• Landmarks on the
celestial sphere are
projections of those on
the Earth

•Celestial equator
divides the sky into
northern and southern
hemispheres
•Celestial poles are
where the Earth’s axis
of rotation would
intersect the celestial
sphere
•Polaris is less than 1°
away from the north
celestial pole, which is
why it is called the
North Star or the Pole
Star.
•Point in the sky directly
overhead an observer
anywhere on Earth is
called that observer’s
zenith.

The Celestial Coordinate System
• Again, let us see what we have just
determined in a more 3-dimension manner.

Positional astronomy plays an important
role in keeping track of time
• Apparent solar time is based on the apparent
motion of the Sun across the celestial sphere,
which varies over the course of the year
• Mean solar time is based on the motion of an
imaginary mean sun along the celestial
equator, which produces a uniform mean
solar day of 24 hours
• Ordinary watches and clocks measure mean
solar time
• Sidereal time is based on the apparent
motion of the celestial sphere

• Local noon is defined to be when the Sun crosses the
upper meridian, which is the half of the meridian
above the horizon

Sidereal and Solar Days
• Appreciating the difference between a solar
day and a sidereal day is a challenging
concept. See if this helps.

Circumpolar
stars

• At any time, an observer can see only half of the celestial sphere
• The other half is below the horizon, hidden by the body of the Earth

Last thought on coordinates
• The Equatorial System of Coordinates is what most
astronomers use when specifying the location of an
object on the Celestial Sphere
• Right Ascension (measured eastwards from the
Vernal Equinox) goes from 0h to 24h
• Declination (measured north or south from the
celestial equator goes from -90° to +90 °.
• The hour angle (HA) of an object is the angle
between the meridian on which the object is situated
and the (observer’s) celestial meridian
• ST = RA + HA

The seasons are caused by the tilt of
Earth’s axis of rotation
• The Earth’s axis of rotation is not
perpendicular to the plane of the Earth’s
orbit
• It is tilted about 23½° away from the
perpendicular & is called the obliquity.
• The Earth maintains this tilt as it orbits
the Sun, with the Earth’s north pole
pointing toward the north celestial pole

The Earth’s orbit

• Seasons do NOT arise from the
distance the Earth is from the Sun but
rather as a result of the Earth’s annual
motion and axial inclination – the tip
of our planet with respect to its orbital
plane. As we move around the Sun,
the orientation of our planet gives us
seasons.

Seasons
• During part of the year the northern hemisphere of
the Earth is tilted toward the Sun
• As the Earth spins on its axis, a point in the
northern hemisphere spends more than 12 hours in
the sunlight
• The days there are long and the nights are short,
and it is summer in the northern hemisphere and
winter in the southern hemisphere
• The summer is hot not only because of the
extended daylight hours but also because the Sun
is high in the northern hemisphere’s sky
• As a result, sunlight strikes the ground at a nearly
perpendicular angle that heats the ground efficiently
• This situation reverses six months later

Sept
21
June
21

Dec
21
March
31

• The Sun appears to
trace out a circular path
called the ecliptic on
the celestial sphere tilted
at 23 ½ degrees to the
equator
• The ecliptic and the
celestial equator
intersect at only two
points
• Each point is called an
equinox
• The point on the ecliptic
farthest north of the
celestial equator that
marks the location of the
Sun at the beginning of
summer in the northern
hemisphere is called the
summer solstice
• At the beginning of the
northern hemisphere’s
winter the Sun is farthest
south of the celestial
equator at a point called
the winter solstice

Landmarks on the Earth’s surface are
marked by the Sun’s position in the sky
throughout the year

The Moon helps to cause precession, a
slow, conical motion of Earth’s axis of
rotation

Precession causes the gradual change of the
star that marks the North Celestial Pole

Astronomical observations led to the
development of the modern calendar
•
•
•
•

The day is based on the Earth’s rotation
The year is based on the Earth’s orbit
The month is based on the lunar cycle
None of these are exactly the same as
nature so astronomers use the average
or mean day and leap years to keep the
calendar and time consistent

The different types of ‘year’.
• The sidereal year (year with respect to
the stars) measured in solar time is
365d 6h 9m 10s (365.2564d) in length.
• The tropical year (successive passages
of the Sun through the Vernal Equinox)
is 365d 5h 48m 46s (365.2422d) in length.
• Due to precession, the tropical year is
20m 24s shorter than the sidereal year.

Calendars
• Caesar introduced the 365.25 days calendar
and thus the Leap Year (February 29)
• However, this is 11m 14s longer than the real
tropical year. This accumulates to 3 days in
4 centuries error.
• To correct, October 4 was followed by
October 15, in 1562 and the century rule was
invoked.


Slide 23

Knowing the Heavens

Chapter Two

Naked-eye astronomy had an important place
in ancient civilizations
• Positional astronomy
– the study of the positions of objects in the sky and
how these positions change

• Naked-eye astronomy
– the sort that requires no equipment but human
vision

• Extends far back in time
–
–
–
–

British Isles Stonehenge
Native American Medicine Wheel
Aztec, Mayan and Incan temples
Egyptian pyramids

Eighty-eight constellations cover the
entire sky
• Ancient peoples looked
at the stars and
imagined groupings
made pictures in the
sky
• We still refer to many of
these groupings
• Astronomers call them
constellations (from
the Latin for “group of
stars”)

Modern Constellations
• On modern star charts,
the entire sky is divided
into 88 regions. Each
is a constellation
• Most stars in a
constellation are
nowhere near one
another
• They only appear to be
close together because
they are in nearly the
same direction as seen
from Earth

The appearance of the sky changes during the
course of the night and from one night to the next
• Stars appear to rise
in the east, slowly
rotate about the
earth and set in the
west.
• This diurnal or daily
motion of the stars
is actually caused
by the 24-hour
rotation of the earth.

Animation of constellation movement
• To represent what we have just discussed,
follow this animation from the vantage
point of our Californian observer.

Annual Motion
• The stars also appear
to slowly shift in position
throughout the year
• This is due to the orbit
of the earth around the
sun
• If you follow a particular
star on successive
evenings, you will find
that it rises
approximately 4
minutes earlier each
night, or 2 hours earlier
each month

It is convenient to imagine that the stars are
located on a celestial sphere
• The celestial sphere is
an imaginary object
that has no basis in
physical reality
• However it is still a
model that remains a
useful tool of positional
astronomy
• Landmarks on the
celestial sphere are
projections of those on
the Earth

•Celestial equator
divides the sky into
northern and southern
hemispheres
•Celestial poles are
where the Earth’s axis
of rotation would
intersect the celestial
sphere
•Polaris is less than 1°
away from the north
celestial pole, which is
why it is called the
North Star or the Pole
Star.
•Point in the sky directly
overhead an observer
anywhere on Earth is
called that observer’s
zenith.

The Celestial Coordinate System
• Again, let us see what we have just
determined in a more 3-dimension manner.

Positional astronomy plays an important
role in keeping track of time
• Apparent solar time is based on the apparent
motion of the Sun across the celestial sphere,
which varies over the course of the year
• Mean solar time is based on the motion of an
imaginary mean sun along the celestial
equator, which produces a uniform mean
solar day of 24 hours
• Ordinary watches and clocks measure mean
solar time
• Sidereal time is based on the apparent
motion of the celestial sphere

• Local noon is defined to be when the Sun crosses the
upper meridian, which is the half of the meridian
above the horizon

Sidereal and Solar Days
• Appreciating the difference between a solar
day and a sidereal day is a challenging
concept. See if this helps.

Circumpolar
stars

• At any time, an observer can see only half of the celestial sphere
• The other half is below the horizon, hidden by the body of the Earth

Last thought on coordinates
• The Equatorial System of Coordinates is what most
astronomers use when specifying the location of an
object on the Celestial Sphere
• Right Ascension (measured eastwards from the
Vernal Equinox) goes from 0h to 24h
• Declination (measured north or south from the
celestial equator goes from -90° to +90 °.
• The hour angle (HA) of an object is the angle
between the meridian on which the object is situated
and the (observer’s) celestial meridian
• ST = RA + HA

The seasons are caused by the tilt of
Earth’s axis of rotation
• The Earth’s axis of rotation is not
perpendicular to the plane of the Earth’s
orbit
• It is tilted about 23½° away from the
perpendicular & is called the obliquity.
• The Earth maintains this tilt as it orbits
the Sun, with the Earth’s north pole
pointing toward the north celestial pole

The Earth’s orbit

• Seasons do NOT arise from the
distance the Earth is from the Sun but
rather as a result of the Earth’s annual
motion and axial inclination – the tip
of our planet with respect to its orbital
plane. As we move around the Sun,
the orientation of our planet gives us
seasons.

Seasons
• During part of the year the northern hemisphere of
the Earth is tilted toward the Sun
• As the Earth spins on its axis, a point in the
northern hemisphere spends more than 12 hours in
the sunlight
• The days there are long and the nights are short,
and it is summer in the northern hemisphere and
winter in the southern hemisphere
• The summer is hot not only because of the
extended daylight hours but also because the Sun
is high in the northern hemisphere’s sky
• As a result, sunlight strikes the ground at a nearly
perpendicular angle that heats the ground efficiently
• This situation reverses six months later

Sept
21
June
21

Dec
21
March
31

• The Sun appears to
trace out a circular path
called the ecliptic on
the celestial sphere tilted
at 23 ½ degrees to the
equator
• The ecliptic and the
celestial equator
intersect at only two
points
• Each point is called an
equinox
• The point on the ecliptic
farthest north of the
celestial equator that
marks the location of the
Sun at the beginning of
summer in the northern
hemisphere is called the
summer solstice
• At the beginning of the
northern hemisphere’s
winter the Sun is farthest
south of the celestial
equator at a point called
the winter solstice

Landmarks on the Earth’s surface are
marked by the Sun’s position in the sky
throughout the year

The Moon helps to cause precession, a
slow, conical motion of Earth’s axis of
rotation

Precession causes the gradual change of the
star that marks the North Celestial Pole

Astronomical observations led to the
development of the modern calendar
•
•
•
•

The day is based on the Earth’s rotation
The year is based on the Earth’s orbit
The month is based on the lunar cycle
None of these are exactly the same as
nature so astronomers use the average
or mean day and leap years to keep the
calendar and time consistent

The different types of ‘year’.
• The sidereal year (year with respect to
the stars) measured in solar time is
365d 6h 9m 10s (365.2564d) in length.
• The tropical year (successive passages
of the Sun through the Vernal Equinox)
is 365d 5h 48m 46s (365.2422d) in length.
• Due to precession, the tropical year is
20m 24s shorter than the sidereal year.

Calendars
• Caesar introduced the 365.25 days calendar
and thus the Leap Year (February 29)
• However, this is 11m 14s longer than the real
tropical year. This accumulates to 3 days in
4 centuries error.
• To correct, October 4 was followed by
October 15, in 1562 and the century rule was
invoked.


Slide 24

Knowing the Heavens

Chapter Two

Naked-eye astronomy had an important place
in ancient civilizations
• Positional astronomy
– the study of the positions of objects in the sky and
how these positions change

• Naked-eye astronomy
– the sort that requires no equipment but human
vision

• Extends far back in time
–
–
–
–

British Isles Stonehenge
Native American Medicine Wheel
Aztec, Mayan and Incan temples
Egyptian pyramids

Eighty-eight constellations cover the
entire sky
• Ancient peoples looked
at the stars and
imagined groupings
made pictures in the
sky
• We still refer to many of
these groupings
• Astronomers call them
constellations (from
the Latin for “group of
stars”)

Modern Constellations
• On modern star charts,
the entire sky is divided
into 88 regions. Each
is a constellation
• Most stars in a
constellation are
nowhere near one
another
• They only appear to be
close together because
they are in nearly the
same direction as seen
from Earth

The appearance of the sky changes during the
course of the night and from one night to the next
• Stars appear to rise
in the east, slowly
rotate about the
earth and set in the
west.
• This diurnal or daily
motion of the stars
is actually caused
by the 24-hour
rotation of the earth.

Animation of constellation movement
• To represent what we have just discussed,
follow this animation from the vantage
point of our Californian observer.

Annual Motion
• The stars also appear
to slowly shift in position
throughout the year
• This is due to the orbit
of the earth around the
sun
• If you follow a particular
star on successive
evenings, you will find
that it rises
approximately 4
minutes earlier each
night, or 2 hours earlier
each month

It is convenient to imagine that the stars are
located on a celestial sphere
• The celestial sphere is
an imaginary object
that has no basis in
physical reality
• However it is still a
model that remains a
useful tool of positional
astronomy
• Landmarks on the
celestial sphere are
projections of those on
the Earth

•Celestial equator
divides the sky into
northern and southern
hemispheres
•Celestial poles are
where the Earth’s axis
of rotation would
intersect the celestial
sphere
•Polaris is less than 1°
away from the north
celestial pole, which is
why it is called the
North Star or the Pole
Star.
•Point in the sky directly
overhead an observer
anywhere on Earth is
called that observer’s
zenith.

The Celestial Coordinate System
• Again, let us see what we have just
determined in a more 3-dimension manner.

Positional astronomy plays an important
role in keeping track of time
• Apparent solar time is based on the apparent
motion of the Sun across the celestial sphere,
which varies over the course of the year
• Mean solar time is based on the motion of an
imaginary mean sun along the celestial
equator, which produces a uniform mean
solar day of 24 hours
• Ordinary watches and clocks measure mean
solar time
• Sidereal time is based on the apparent
motion of the celestial sphere

• Local noon is defined to be when the Sun crosses the
upper meridian, which is the half of the meridian
above the horizon

Sidereal and Solar Days
• Appreciating the difference between a solar
day and a sidereal day is a challenging
concept. See if this helps.

Circumpolar
stars

• At any time, an observer can see only half of the celestial sphere
• The other half is below the horizon, hidden by the body of the Earth

Last thought on coordinates
• The Equatorial System of Coordinates is what most
astronomers use when specifying the location of an
object on the Celestial Sphere
• Right Ascension (measured eastwards from the
Vernal Equinox) goes from 0h to 24h
• Declination (measured north or south from the
celestial equator goes from -90° to +90 °.
• The hour angle (HA) of an object is the angle
between the meridian on which the object is situated
and the (observer’s) celestial meridian
• ST = RA + HA

The seasons are caused by the tilt of
Earth’s axis of rotation
• The Earth’s axis of rotation is not
perpendicular to the plane of the Earth’s
orbit
• It is tilted about 23½° away from the
perpendicular & is called the obliquity.
• The Earth maintains this tilt as it orbits
the Sun, with the Earth’s north pole
pointing toward the north celestial pole

The Earth’s orbit

• Seasons do NOT arise from the
distance the Earth is from the Sun but
rather as a result of the Earth’s annual
motion and axial inclination – the tip
of our planet with respect to its orbital
plane. As we move around the Sun,
the orientation of our planet gives us
seasons.

Seasons
• During part of the year the northern hemisphere of
the Earth is tilted toward the Sun
• As the Earth spins on its axis, a point in the
northern hemisphere spends more than 12 hours in
the sunlight
• The days there are long and the nights are short,
and it is summer in the northern hemisphere and
winter in the southern hemisphere
• The summer is hot not only because of the
extended daylight hours but also because the Sun
is high in the northern hemisphere’s sky
• As a result, sunlight strikes the ground at a nearly
perpendicular angle that heats the ground efficiently
• This situation reverses six months later

Sept
21
June
21

Dec
21
March
31

• The Sun appears to
trace out a circular path
called the ecliptic on
the celestial sphere tilted
at 23 ½ degrees to the
equator
• The ecliptic and the
celestial equator
intersect at only two
points
• Each point is called an
equinox
• The point on the ecliptic
farthest north of the
celestial equator that
marks the location of the
Sun at the beginning of
summer in the northern
hemisphere is called the
summer solstice
• At the beginning of the
northern hemisphere’s
winter the Sun is farthest
south of the celestial
equator at a point called
the winter solstice

Landmarks on the Earth’s surface are
marked by the Sun’s position in the sky
throughout the year

The Moon helps to cause precession, a
slow, conical motion of Earth’s axis of
rotation

Precession causes the gradual change of the
star that marks the North Celestial Pole

Astronomical observations led to the
development of the modern calendar
•
•
•
•

The day is based on the Earth’s rotation
The year is based on the Earth’s orbit
The month is based on the lunar cycle
None of these are exactly the same as
nature so astronomers use the average
or mean day and leap years to keep the
calendar and time consistent

The different types of ‘year’.
• The sidereal year (year with respect to
the stars) measured in solar time is
365d 6h 9m 10s (365.2564d) in length.
• The tropical year (successive passages
of the Sun through the Vernal Equinox)
is 365d 5h 48m 46s (365.2422d) in length.
• Due to precession, the tropical year is
20m 24s shorter than the sidereal year.

Calendars
• Caesar introduced the 365.25 days calendar
and thus the Leap Year (February 29)
• However, this is 11m 14s longer than the real
tropical year. This accumulates to 3 days in
4 centuries error.
• To correct, October 4 was followed by
October 15, in 1562 and the century rule was
invoked.


Slide 25

Knowing the Heavens

Chapter Two

Naked-eye astronomy had an important place
in ancient civilizations
• Positional astronomy
– the study of the positions of objects in the sky and
how these positions change

• Naked-eye astronomy
– the sort that requires no equipment but human
vision

• Extends far back in time
–
–
–
–

British Isles Stonehenge
Native American Medicine Wheel
Aztec, Mayan and Incan temples
Egyptian pyramids

Eighty-eight constellations cover the
entire sky
• Ancient peoples looked
at the stars and
imagined groupings
made pictures in the
sky
• We still refer to many of
these groupings
• Astronomers call them
constellations (from
the Latin for “group of
stars”)

Modern Constellations
• On modern star charts,
the entire sky is divided
into 88 regions. Each
is a constellation
• Most stars in a
constellation are
nowhere near one
another
• They only appear to be
close together because
they are in nearly the
same direction as seen
from Earth

The appearance of the sky changes during the
course of the night and from one night to the next
• Stars appear to rise
in the east, slowly
rotate about the
earth and set in the
west.
• This diurnal or daily
motion of the stars
is actually caused
by the 24-hour
rotation of the earth.

Animation of constellation movement
• To represent what we have just discussed,
follow this animation from the vantage
point of our Californian observer.

Annual Motion
• The stars also appear
to slowly shift in position
throughout the year
• This is due to the orbit
of the earth around the
sun
• If you follow a particular
star on successive
evenings, you will find
that it rises
approximately 4
minutes earlier each
night, or 2 hours earlier
each month

It is convenient to imagine that the stars are
located on a celestial sphere
• The celestial sphere is
an imaginary object
that has no basis in
physical reality
• However it is still a
model that remains a
useful tool of positional
astronomy
• Landmarks on the
celestial sphere are
projections of those on
the Earth

•Celestial equator
divides the sky into
northern and southern
hemispheres
•Celestial poles are
where the Earth’s axis
of rotation would
intersect the celestial
sphere
•Polaris is less than 1°
away from the north
celestial pole, which is
why it is called the
North Star or the Pole
Star.
•Point in the sky directly
overhead an observer
anywhere on Earth is
called that observer’s
zenith.

The Celestial Coordinate System
• Again, let us see what we have just
determined in a more 3-dimension manner.

Positional astronomy plays an important
role in keeping track of time
• Apparent solar time is based on the apparent
motion of the Sun across the celestial sphere,
which varies over the course of the year
• Mean solar time is based on the motion of an
imaginary mean sun along the celestial
equator, which produces a uniform mean
solar day of 24 hours
• Ordinary watches and clocks measure mean
solar time
• Sidereal time is based on the apparent
motion of the celestial sphere

• Local noon is defined to be when the Sun crosses the
upper meridian, which is the half of the meridian
above the horizon

Sidereal and Solar Days
• Appreciating the difference between a solar
day and a sidereal day is a challenging
concept. See if this helps.

Circumpolar
stars

• At any time, an observer can see only half of the celestial sphere
• The other half is below the horizon, hidden by the body of the Earth

Last thought on coordinates
• The Equatorial System of Coordinates is what most
astronomers use when specifying the location of an
object on the Celestial Sphere
• Right Ascension (measured eastwards from the
Vernal Equinox) goes from 0h to 24h
• Declination (measured north or south from the
celestial equator goes from -90° to +90 °.
• The hour angle (HA) of an object is the angle
between the meridian on which the object is situated
and the (observer’s) celestial meridian
• ST = RA + HA

The seasons are caused by the tilt of
Earth’s axis of rotation
• The Earth’s axis of rotation is not
perpendicular to the plane of the Earth’s
orbit
• It is tilted about 23½° away from the
perpendicular & is called the obliquity.
• The Earth maintains this tilt as it orbits
the Sun, with the Earth’s north pole
pointing toward the north celestial pole

The Earth’s orbit

• Seasons do NOT arise from the
distance the Earth is from the Sun but
rather as a result of the Earth’s annual
motion and axial inclination – the tip
of our planet with respect to its orbital
plane. As we move around the Sun,
the orientation of our planet gives us
seasons.

Seasons
• During part of the year the northern hemisphere of
the Earth is tilted toward the Sun
• As the Earth spins on its axis, a point in the
northern hemisphere spends more than 12 hours in
the sunlight
• The days there are long and the nights are short,
and it is summer in the northern hemisphere and
winter in the southern hemisphere
• The summer is hot not only because of the
extended daylight hours but also because the Sun
is high in the northern hemisphere’s sky
• As a result, sunlight strikes the ground at a nearly
perpendicular angle that heats the ground efficiently
• This situation reverses six months later

Sept
21
June
21

Dec
21
March
31

• The Sun appears to
trace out a circular path
called the ecliptic on
the celestial sphere tilted
at 23 ½ degrees to the
equator
• The ecliptic and the
celestial equator
intersect at only two
points
• Each point is called an
equinox
• The point on the ecliptic
farthest north of the
celestial equator that
marks the location of the
Sun at the beginning of
summer in the northern
hemisphere is called the
summer solstice
• At the beginning of the
northern hemisphere’s
winter the Sun is farthest
south of the celestial
equator at a point called
the winter solstice

Landmarks on the Earth’s surface are
marked by the Sun’s position in the sky
throughout the year

The Moon helps to cause precession, a
slow, conical motion of Earth’s axis of
rotation

Precession causes the gradual change of the
star that marks the North Celestial Pole

Astronomical observations led to the
development of the modern calendar
•
•
•
•

The day is based on the Earth’s rotation
The year is based on the Earth’s orbit
The month is based on the lunar cycle
None of these are exactly the same as
nature so astronomers use the average
or mean day and leap years to keep the
calendar and time consistent

The different types of ‘year’.
• The sidereal year (year with respect to
the stars) measured in solar time is
365d 6h 9m 10s (365.2564d) in length.
• The tropical year (successive passages
of the Sun through the Vernal Equinox)
is 365d 5h 48m 46s (365.2422d) in length.
• Due to precession, the tropical year is
20m 24s shorter than the sidereal year.

Calendars
• Caesar introduced the 365.25 days calendar
and thus the Leap Year (February 29)
• However, this is 11m 14s longer than the real
tropical year. This accumulates to 3 days in
4 centuries error.
• To correct, October 4 was followed by
October 15, in 1562 and the century rule was
invoked.


Slide 26

Knowing the Heavens

Chapter Two

Naked-eye astronomy had an important place
in ancient civilizations
• Positional astronomy
– the study of the positions of objects in the sky and
how these positions change

• Naked-eye astronomy
– the sort that requires no equipment but human
vision

• Extends far back in time
–
–
–
–

British Isles Stonehenge
Native American Medicine Wheel
Aztec, Mayan and Incan temples
Egyptian pyramids

Eighty-eight constellations cover the
entire sky
• Ancient peoples looked
at the stars and
imagined groupings
made pictures in the
sky
• We still refer to many of
these groupings
• Astronomers call them
constellations (from
the Latin for “group of
stars”)

Modern Constellations
• On modern star charts,
the entire sky is divided
into 88 regions. Each
is a constellation
• Most stars in a
constellation are
nowhere near one
another
• They only appear to be
close together because
they are in nearly the
same direction as seen
from Earth

The appearance of the sky changes during the
course of the night and from one night to the next
• Stars appear to rise
in the east, slowly
rotate about the
earth and set in the
west.
• This diurnal or daily
motion of the stars
is actually caused
by the 24-hour
rotation of the earth.

Animation of constellation movement
• To represent what we have just discussed,
follow this animation from the vantage
point of our Californian observer.

Annual Motion
• The stars also appear
to slowly shift in position
throughout the year
• This is due to the orbit
of the earth around the
sun
• If you follow a particular
star on successive
evenings, you will find
that it rises
approximately 4
minutes earlier each
night, or 2 hours earlier
each month

It is convenient to imagine that the stars are
located on a celestial sphere
• The celestial sphere is
an imaginary object
that has no basis in
physical reality
• However it is still a
model that remains a
useful tool of positional
astronomy
• Landmarks on the
celestial sphere are
projections of those on
the Earth

•Celestial equator
divides the sky into
northern and southern
hemispheres
•Celestial poles are
where the Earth’s axis
of rotation would
intersect the celestial
sphere
•Polaris is less than 1°
away from the north
celestial pole, which is
why it is called the
North Star or the Pole
Star.
•Point in the sky directly
overhead an observer
anywhere on Earth is
called that observer’s
zenith.

The Celestial Coordinate System
• Again, let us see what we have just
determined in a more 3-dimension manner.

Positional astronomy plays an important
role in keeping track of time
• Apparent solar time is based on the apparent
motion of the Sun across the celestial sphere,
which varies over the course of the year
• Mean solar time is based on the motion of an
imaginary mean sun along the celestial
equator, which produces a uniform mean
solar day of 24 hours
• Ordinary watches and clocks measure mean
solar time
• Sidereal time is based on the apparent
motion of the celestial sphere

• Local noon is defined to be when the Sun crosses the
upper meridian, which is the half of the meridian
above the horizon

Sidereal and Solar Days
• Appreciating the difference between a solar
day and a sidereal day is a challenging
concept. See if this helps.

Circumpolar
stars

• At any time, an observer can see only half of the celestial sphere
• The other half is below the horizon, hidden by the body of the Earth

Last thought on coordinates
• The Equatorial System of Coordinates is what most
astronomers use when specifying the location of an
object on the Celestial Sphere
• Right Ascension (measured eastwards from the
Vernal Equinox) goes from 0h to 24h
• Declination (measured north or south from the
celestial equator goes from -90° to +90 °.
• The hour angle (HA) of an object is the angle
between the meridian on which the object is situated
and the (observer’s) celestial meridian
• ST = RA + HA

The seasons are caused by the tilt of
Earth’s axis of rotation
• The Earth’s axis of rotation is not
perpendicular to the plane of the Earth’s
orbit
• It is tilted about 23½° away from the
perpendicular & is called the obliquity.
• The Earth maintains this tilt as it orbits
the Sun, with the Earth’s north pole
pointing toward the north celestial pole

The Earth’s orbit

• Seasons do NOT arise from the
distance the Earth is from the Sun but
rather as a result of the Earth’s annual
motion and axial inclination – the tip
of our planet with respect to its orbital
plane. As we move around the Sun,
the orientation of our planet gives us
seasons.

Seasons
• During part of the year the northern hemisphere of
the Earth is tilted toward the Sun
• As the Earth spins on its axis, a point in the
northern hemisphere spends more than 12 hours in
the sunlight
• The days there are long and the nights are short,
and it is summer in the northern hemisphere and
winter in the southern hemisphere
• The summer is hot not only because of the
extended daylight hours but also because the Sun
is high in the northern hemisphere’s sky
• As a result, sunlight strikes the ground at a nearly
perpendicular angle that heats the ground efficiently
• This situation reverses six months later

Sept
21
June
21

Dec
21
March
31

• The Sun appears to
trace out a circular path
called the ecliptic on
the celestial sphere tilted
at 23 ½ degrees to the
equator
• The ecliptic and the
celestial equator
intersect at only two
points
• Each point is called an
equinox
• The point on the ecliptic
farthest north of the
celestial equator that
marks the location of the
Sun at the beginning of
summer in the northern
hemisphere is called the
summer solstice
• At the beginning of the
northern hemisphere’s
winter the Sun is farthest
south of the celestial
equator at a point called
the winter solstice

Landmarks on the Earth’s surface are
marked by the Sun’s position in the sky
throughout the year

The Moon helps to cause precession, a
slow, conical motion of Earth’s axis of
rotation

Precession causes the gradual change of the
star that marks the North Celestial Pole

Astronomical observations led to the
development of the modern calendar
•
•
•
•

The day is based on the Earth’s rotation
The year is based on the Earth’s orbit
The month is based on the lunar cycle
None of these are exactly the same as
nature so astronomers use the average
or mean day and leap years to keep the
calendar and time consistent

The different types of ‘year’.
• The sidereal year (year with respect to
the stars) measured in solar time is
365d 6h 9m 10s (365.2564d) in length.
• The tropical year (successive passages
of the Sun through the Vernal Equinox)
is 365d 5h 48m 46s (365.2422d) in length.
• Due to precession, the tropical year is
20m 24s shorter than the sidereal year.

Calendars
• Caesar introduced the 365.25 days calendar
and thus the Leap Year (February 29)
• However, this is 11m 14s longer than the real
tropical year. This accumulates to 3 days in
4 centuries error.
• To correct, October 4 was followed by
October 15, in 1562 and the century rule was
invoked.


Slide 27

Knowing the Heavens

Chapter Two

Naked-eye astronomy had an important place
in ancient civilizations
• Positional astronomy
– the study of the positions of objects in the sky and
how these positions change

• Naked-eye astronomy
– the sort that requires no equipment but human
vision

• Extends far back in time
–
–
–
–

British Isles Stonehenge
Native American Medicine Wheel
Aztec, Mayan and Incan temples
Egyptian pyramids

Eighty-eight constellations cover the
entire sky
• Ancient peoples looked
at the stars and
imagined groupings
made pictures in the
sky
• We still refer to many of
these groupings
• Astronomers call them
constellations (from
the Latin for “group of
stars”)

Modern Constellations
• On modern star charts,
the entire sky is divided
into 88 regions. Each
is a constellation
• Most stars in a
constellation are
nowhere near one
another
• They only appear to be
close together because
they are in nearly the
same direction as seen
from Earth

The appearance of the sky changes during the
course of the night and from one night to the next
• Stars appear to rise
in the east, slowly
rotate about the
earth and set in the
west.
• This diurnal or daily
motion of the stars
is actually caused
by the 24-hour
rotation of the earth.

Animation of constellation movement
• To represent what we have just discussed,
follow this animation from the vantage
point of our Californian observer.

Annual Motion
• The stars also appear
to slowly shift in position
throughout the year
• This is due to the orbit
of the earth around the
sun
• If you follow a particular
star on successive
evenings, you will find
that it rises
approximately 4
minutes earlier each
night, or 2 hours earlier
each month

It is convenient to imagine that the stars are
located on a celestial sphere
• The celestial sphere is
an imaginary object
that has no basis in
physical reality
• However it is still a
model that remains a
useful tool of positional
astronomy
• Landmarks on the
celestial sphere are
projections of those on
the Earth

•Celestial equator
divides the sky into
northern and southern
hemispheres
•Celestial poles are
where the Earth’s axis
of rotation would
intersect the celestial
sphere
•Polaris is less than 1°
away from the north
celestial pole, which is
why it is called the
North Star or the Pole
Star.
•Point in the sky directly
overhead an observer
anywhere on Earth is
called that observer’s
zenith.

The Celestial Coordinate System
• Again, let us see what we have just
determined in a more 3-dimension manner.

Positional astronomy plays an important
role in keeping track of time
• Apparent solar time is based on the apparent
motion of the Sun across the celestial sphere,
which varies over the course of the year
• Mean solar time is based on the motion of an
imaginary mean sun along the celestial
equator, which produces a uniform mean
solar day of 24 hours
• Ordinary watches and clocks measure mean
solar time
• Sidereal time is based on the apparent
motion of the celestial sphere

• Local noon is defined to be when the Sun crosses the
upper meridian, which is the half of the meridian
above the horizon

Sidereal and Solar Days
• Appreciating the difference between a solar
day and a sidereal day is a challenging
concept. See if this helps.

Circumpolar
stars

• At any time, an observer can see only half of the celestial sphere
• The other half is below the horizon, hidden by the body of the Earth

Last thought on coordinates
• The Equatorial System of Coordinates is what most
astronomers use when specifying the location of an
object on the Celestial Sphere
• Right Ascension (measured eastwards from the
Vernal Equinox) goes from 0h to 24h
• Declination (measured north or south from the
celestial equator goes from -90° to +90 °.
• The hour angle (HA) of an object is the angle
between the meridian on which the object is situated
and the (observer’s) celestial meridian
• ST = RA + HA

The seasons are caused by the tilt of
Earth’s axis of rotation
• The Earth’s axis of rotation is not
perpendicular to the plane of the Earth’s
orbit
• It is tilted about 23½° away from the
perpendicular & is called the obliquity.
• The Earth maintains this tilt as it orbits
the Sun, with the Earth’s north pole
pointing toward the north celestial pole

The Earth’s orbit

• Seasons do NOT arise from the
distance the Earth is from the Sun but
rather as a result of the Earth’s annual
motion and axial inclination – the tip
of our planet with respect to its orbital
plane. As we move around the Sun,
the orientation of our planet gives us
seasons.

Seasons
• During part of the year the northern hemisphere of
the Earth is tilted toward the Sun
• As the Earth spins on its axis, a point in the
northern hemisphere spends more than 12 hours in
the sunlight
• The days there are long and the nights are short,
and it is summer in the northern hemisphere and
winter in the southern hemisphere
• The summer is hot not only because of the
extended daylight hours but also because the Sun
is high in the northern hemisphere’s sky
• As a result, sunlight strikes the ground at a nearly
perpendicular angle that heats the ground efficiently
• This situation reverses six months later

Sept
21
June
21

Dec
21
March
31

• The Sun appears to
trace out a circular path
called the ecliptic on
the celestial sphere tilted
at 23 ½ degrees to the
equator
• The ecliptic and the
celestial equator
intersect at only two
points
• Each point is called an
equinox
• The point on the ecliptic
farthest north of the
celestial equator that
marks the location of the
Sun at the beginning of
summer in the northern
hemisphere is called the
summer solstice
• At the beginning of the
northern hemisphere’s
winter the Sun is farthest
south of the celestial
equator at a point called
the winter solstice

Landmarks on the Earth’s surface are
marked by the Sun’s position in the sky
throughout the year

The Moon helps to cause precession, a
slow, conical motion of Earth’s axis of
rotation

Precession causes the gradual change of the
star that marks the North Celestial Pole

Astronomical observations led to the
development of the modern calendar
•
•
•
•

The day is based on the Earth’s rotation
The year is based on the Earth’s orbit
The month is based on the lunar cycle
None of these are exactly the same as
nature so astronomers use the average
or mean day and leap years to keep the
calendar and time consistent

The different types of ‘year’.
• The sidereal year (year with respect to
the stars) measured in solar time is
365d 6h 9m 10s (365.2564d) in length.
• The tropical year (successive passages
of the Sun through the Vernal Equinox)
is 365d 5h 48m 46s (365.2422d) in length.
• Due to precession, the tropical year is
20m 24s shorter than the sidereal year.

Calendars
• Caesar introduced the 365.25 days calendar
and thus the Leap Year (February 29)
• However, this is 11m 14s longer than the real
tropical year. This accumulates to 3 days in
4 centuries error.
• To correct, October 4 was followed by
October 15, in 1562 and the century rule was
invoked.


Slide 28

Knowing the Heavens

Chapter Two

Naked-eye astronomy had an important place
in ancient civilizations
• Positional astronomy
– the study of the positions of objects in the sky and
how these positions change

• Naked-eye astronomy
– the sort that requires no equipment but human
vision

• Extends far back in time
–
–
–
–

British Isles Stonehenge
Native American Medicine Wheel
Aztec, Mayan and Incan temples
Egyptian pyramids

Eighty-eight constellations cover the
entire sky
• Ancient peoples looked
at the stars and
imagined groupings
made pictures in the
sky
• We still refer to many of
these groupings
• Astronomers call them
constellations (from
the Latin for “group of
stars”)

Modern Constellations
• On modern star charts,
the entire sky is divided
into 88 regions. Each
is a constellation
• Most stars in a
constellation are
nowhere near one
another
• They only appear to be
close together because
they are in nearly the
same direction as seen
from Earth

The appearance of the sky changes during the
course of the night and from one night to the next
• Stars appear to rise
in the east, slowly
rotate about the
earth and set in the
west.
• This diurnal or daily
motion of the stars
is actually caused
by the 24-hour
rotation of the earth.

Animation of constellation movement
• To represent what we have just discussed,
follow this animation from the vantage
point of our Californian observer.

Annual Motion
• The stars also appear
to slowly shift in position
throughout the year
• This is due to the orbit
of the earth around the
sun
• If you follow a particular
star on successive
evenings, you will find
that it rises
approximately 4
minutes earlier each
night, or 2 hours earlier
each month

It is convenient to imagine that the stars are
located on a celestial sphere
• The celestial sphere is
an imaginary object
that has no basis in
physical reality
• However it is still a
model that remains a
useful tool of positional
astronomy
• Landmarks on the
celestial sphere are
projections of those on
the Earth

•Celestial equator
divides the sky into
northern and southern
hemispheres
•Celestial poles are
where the Earth’s axis
of rotation would
intersect the celestial
sphere
•Polaris is less than 1°
away from the north
celestial pole, which is
why it is called the
North Star or the Pole
Star.
•Point in the sky directly
overhead an observer
anywhere on Earth is
called that observer’s
zenith.

The Celestial Coordinate System
• Again, let us see what we have just
determined in a more 3-dimension manner.

Positional astronomy plays an important
role in keeping track of time
• Apparent solar time is based on the apparent
motion of the Sun across the celestial sphere,
which varies over the course of the year
• Mean solar time is based on the motion of an
imaginary mean sun along the celestial
equator, which produces a uniform mean
solar day of 24 hours
• Ordinary watches and clocks measure mean
solar time
• Sidereal time is based on the apparent
motion of the celestial sphere

• Local noon is defined to be when the Sun crosses the
upper meridian, which is the half of the meridian
above the horizon

Sidereal and Solar Days
• Appreciating the difference between a solar
day and a sidereal day is a challenging
concept. See if this helps.

Circumpolar
stars

• At any time, an observer can see only half of the celestial sphere
• The other half is below the horizon, hidden by the body of the Earth

Last thought on coordinates
• The Equatorial System of Coordinates is what most
astronomers use when specifying the location of an
object on the Celestial Sphere
• Right Ascension (measured eastwards from the
Vernal Equinox) goes from 0h to 24h
• Declination (measured north or south from the
celestial equator goes from -90° to +90 °.
• The hour angle (HA) of an object is the angle
between the meridian on which the object is situated
and the (observer’s) celestial meridian
• ST = RA + HA

The seasons are caused by the tilt of
Earth’s axis of rotation
• The Earth’s axis of rotation is not
perpendicular to the plane of the Earth’s
orbit
• It is tilted about 23½° away from the
perpendicular & is called the obliquity.
• The Earth maintains this tilt as it orbits
the Sun, with the Earth’s north pole
pointing toward the north celestial pole

The Earth’s orbit

• Seasons do NOT arise from the
distance the Earth is from the Sun but
rather as a result of the Earth’s annual
motion and axial inclination – the tip
of our planet with respect to its orbital
plane. As we move around the Sun,
the orientation of our planet gives us
seasons.

Seasons
• During part of the year the northern hemisphere of
the Earth is tilted toward the Sun
• As the Earth spins on its axis, a point in the
northern hemisphere spends more than 12 hours in
the sunlight
• The days there are long and the nights are short,
and it is summer in the northern hemisphere and
winter in the southern hemisphere
• The summer is hot not only because of the
extended daylight hours but also because the Sun
is high in the northern hemisphere’s sky
• As a result, sunlight strikes the ground at a nearly
perpendicular angle that heats the ground efficiently
• This situation reverses six months later

Sept
21
June
21

Dec
21
March
31

• The Sun appears to
trace out a circular path
called the ecliptic on
the celestial sphere tilted
at 23 ½ degrees to the
equator
• The ecliptic and the
celestial equator
intersect at only two
points
• Each point is called an
equinox
• The point on the ecliptic
farthest north of the
celestial equator that
marks the location of the
Sun at the beginning of
summer in the northern
hemisphere is called the
summer solstice
• At the beginning of the
northern hemisphere’s
winter the Sun is farthest
south of the celestial
equator at a point called
the winter solstice

Landmarks on the Earth’s surface are
marked by the Sun’s position in the sky
throughout the year

The Moon helps to cause precession, a
slow, conical motion of Earth’s axis of
rotation

Precession causes the gradual change of the
star that marks the North Celestial Pole

Astronomical observations led to the
development of the modern calendar
•
•
•
•

The day is based on the Earth’s rotation
The year is based on the Earth’s orbit
The month is based on the lunar cycle
None of these are exactly the same as
nature so astronomers use the average
or mean day and leap years to keep the
calendar and time consistent

The different types of ‘year’.
• The sidereal year (year with respect to
the stars) measured in solar time is
365d 6h 9m 10s (365.2564d) in length.
• The tropical year (successive passages
of the Sun through the Vernal Equinox)
is 365d 5h 48m 46s (365.2422d) in length.
• Due to precession, the tropical year is
20m 24s shorter than the sidereal year.

Calendars
• Caesar introduced the 365.25 days calendar
and thus the Leap Year (February 29)
• However, this is 11m 14s longer than the real
tropical year. This accumulates to 3 days in
4 centuries error.
• To correct, October 4 was followed by
October 15, in 1562 and the century rule was
invoked.


Slide 29

Knowing the Heavens

Chapter Two

Naked-eye astronomy had an important place
in ancient civilizations
• Positional astronomy
– the study of the positions of objects in the sky and
how these positions change

• Naked-eye astronomy
– the sort that requires no equipment but human
vision

• Extends far back in time
–
–
–
–

British Isles Stonehenge
Native American Medicine Wheel
Aztec, Mayan and Incan temples
Egyptian pyramids

Eighty-eight constellations cover the
entire sky
• Ancient peoples looked
at the stars and
imagined groupings
made pictures in the
sky
• We still refer to many of
these groupings
• Astronomers call them
constellations (from
the Latin for “group of
stars”)

Modern Constellations
• On modern star charts,
the entire sky is divided
into 88 regions. Each
is a constellation
• Most stars in a
constellation are
nowhere near one
another
• They only appear to be
close together because
they are in nearly the
same direction as seen
from Earth

The appearance of the sky changes during the
course of the night and from one night to the next
• Stars appear to rise
in the east, slowly
rotate about the
earth and set in the
west.
• This diurnal or daily
motion of the stars
is actually caused
by the 24-hour
rotation of the earth.

Animation of constellation movement
• To represent what we have just discussed,
follow this animation from the vantage
point of our Californian observer.

Annual Motion
• The stars also appear
to slowly shift in position
throughout the year
• This is due to the orbit
of the earth around the
sun
• If you follow a particular
star on successive
evenings, you will find
that it rises
approximately 4
minutes earlier each
night, or 2 hours earlier
each month

It is convenient to imagine that the stars are
located on a celestial sphere
• The celestial sphere is
an imaginary object
that has no basis in
physical reality
• However it is still a
model that remains a
useful tool of positional
astronomy
• Landmarks on the
celestial sphere are
projections of those on
the Earth

•Celestial equator
divides the sky into
northern and southern
hemispheres
•Celestial poles are
where the Earth’s axis
of rotation would
intersect the celestial
sphere
•Polaris is less than 1°
away from the north
celestial pole, which is
why it is called the
North Star or the Pole
Star.
•Point in the sky directly
overhead an observer
anywhere on Earth is
called that observer’s
zenith.

The Celestial Coordinate System
• Again, let us see what we have just
determined in a more 3-dimension manner.

Positional astronomy plays an important
role in keeping track of time
• Apparent solar time is based on the apparent
motion of the Sun across the celestial sphere,
which varies over the course of the year
• Mean solar time is based on the motion of an
imaginary mean sun along the celestial
equator, which produces a uniform mean
solar day of 24 hours
• Ordinary watches and clocks measure mean
solar time
• Sidereal time is based on the apparent
motion of the celestial sphere

• Local noon is defined to be when the Sun crosses the
upper meridian, which is the half of the meridian
above the horizon

Sidereal and Solar Days
• Appreciating the difference between a solar
day and a sidereal day is a challenging
concept. See if this helps.

Circumpolar
stars

• At any time, an observer can see only half of the celestial sphere
• The other half is below the horizon, hidden by the body of the Earth

Last thought on coordinates
• The Equatorial System of Coordinates is what most
astronomers use when specifying the location of an
object on the Celestial Sphere
• Right Ascension (measured eastwards from the
Vernal Equinox) goes from 0h to 24h
• Declination (measured north or south from the
celestial equator goes from -90° to +90 °.
• The hour angle (HA) of an object is the angle
between the meridian on which the object is situated
and the (observer’s) celestial meridian
• ST = RA + HA

The seasons are caused by the tilt of
Earth’s axis of rotation
• The Earth’s axis of rotation is not
perpendicular to the plane of the Earth’s
orbit
• It is tilted about 23½° away from the
perpendicular & is called the obliquity.
• The Earth maintains this tilt as it orbits
the Sun, with the Earth’s north pole
pointing toward the north celestial pole

The Earth’s orbit

• Seasons do NOT arise from the
distance the Earth is from the Sun but
rather as a result of the Earth’s annual
motion and axial inclination – the tip
of our planet with respect to its orbital
plane. As we move around the Sun,
the orientation of our planet gives us
seasons.

Seasons
• During part of the year the northern hemisphere of
the Earth is tilted toward the Sun
• As the Earth spins on its axis, a point in the
northern hemisphere spends more than 12 hours in
the sunlight
• The days there are long and the nights are short,
and it is summer in the northern hemisphere and
winter in the southern hemisphere
• The summer is hot not only because of the
extended daylight hours but also because the Sun
is high in the northern hemisphere’s sky
• As a result, sunlight strikes the ground at a nearly
perpendicular angle that heats the ground efficiently
• This situation reverses six months later

Sept
21
June
21

Dec
21
March
31

• The Sun appears to
trace out a circular path
called the ecliptic on
the celestial sphere tilted
at 23 ½ degrees to the
equator
• The ecliptic and the
celestial equator
intersect at only two
points
• Each point is called an
equinox
• The point on the ecliptic
farthest north of the
celestial equator that
marks the location of the
Sun at the beginning of
summer in the northern
hemisphere is called the
summer solstice
• At the beginning of the
northern hemisphere’s
winter the Sun is farthest
south of the celestial
equator at a point called
the winter solstice

Landmarks on the Earth’s surface are
marked by the Sun’s position in the sky
throughout the year

The Moon helps to cause precession, a
slow, conical motion of Earth’s axis of
rotation

Precession causes the gradual change of the
star that marks the North Celestial Pole

Astronomical observations led to the
development of the modern calendar
•
•
•
•

The day is based on the Earth’s rotation
The year is based on the Earth’s orbit
The month is based on the lunar cycle
None of these are exactly the same as
nature so astronomers use the average
or mean day and leap years to keep the
calendar and time consistent

The different types of ‘year’.
• The sidereal year (year with respect to
the stars) measured in solar time is
365d 6h 9m 10s (365.2564d) in length.
• The tropical year (successive passages
of the Sun through the Vernal Equinox)
is 365d 5h 48m 46s (365.2422d) in length.
• Due to precession, the tropical year is
20m 24s shorter than the sidereal year.

Calendars
• Caesar introduced the 365.25 days calendar
and thus the Leap Year (February 29)
• However, this is 11m 14s longer than the real
tropical year. This accumulates to 3 days in
4 centuries error.
• To correct, October 4 was followed by
October 15, in 1562 and the century rule was
invoked.


Slide 30

Knowing the Heavens

Chapter Two

Naked-eye astronomy had an important place
in ancient civilizations
• Positional astronomy
– the study of the positions of objects in the sky and
how these positions change

• Naked-eye astronomy
– the sort that requires no equipment but human
vision

• Extends far back in time
–
–
–
–

British Isles Stonehenge
Native American Medicine Wheel
Aztec, Mayan and Incan temples
Egyptian pyramids

Eighty-eight constellations cover the
entire sky
• Ancient peoples looked
at the stars and
imagined groupings
made pictures in the
sky
• We still refer to many of
these groupings
• Astronomers call them
constellations (from
the Latin for “group of
stars”)

Modern Constellations
• On modern star charts,
the entire sky is divided
into 88 regions. Each
is a constellation
• Most stars in a
constellation are
nowhere near one
another
• They only appear to be
close together because
they are in nearly the
same direction as seen
from Earth

The appearance of the sky changes during the
course of the night and from one night to the next
• Stars appear to rise
in the east, slowly
rotate about the
earth and set in the
west.
• This diurnal or daily
motion of the stars
is actually caused
by the 24-hour
rotation of the earth.

Animation of constellation movement
• To represent what we have just discussed,
follow this animation from the vantage
point of our Californian observer.

Annual Motion
• The stars also appear
to slowly shift in position
throughout the year
• This is due to the orbit
of the earth around the
sun
• If you follow a particular
star on successive
evenings, you will find
that it rises
approximately 4
minutes earlier each
night, or 2 hours earlier
each month

It is convenient to imagine that the stars are
located on a celestial sphere
• The celestial sphere is
an imaginary object
that has no basis in
physical reality
• However it is still a
model that remains a
useful tool of positional
astronomy
• Landmarks on the
celestial sphere are
projections of those on
the Earth

•Celestial equator
divides the sky into
northern and southern
hemispheres
•Celestial poles are
where the Earth’s axis
of rotation would
intersect the celestial
sphere
•Polaris is less than 1°
away from the north
celestial pole, which is
why it is called the
North Star or the Pole
Star.
•Point in the sky directly
overhead an observer
anywhere on Earth is
called that observer’s
zenith.

The Celestial Coordinate System
• Again, let us see what we have just
determined in a more 3-dimension manner.

Positional astronomy plays an important
role in keeping track of time
• Apparent solar time is based on the apparent
motion of the Sun across the celestial sphere,
which varies over the course of the year
• Mean solar time is based on the motion of an
imaginary mean sun along the celestial
equator, which produces a uniform mean
solar day of 24 hours
• Ordinary watches and clocks measure mean
solar time
• Sidereal time is based on the apparent
motion of the celestial sphere

• Local noon is defined to be when the Sun crosses the
upper meridian, which is the half of the meridian
above the horizon

Sidereal and Solar Days
• Appreciating the difference between a solar
day and a sidereal day is a challenging
concept. See if this helps.

Circumpolar
stars

• At any time, an observer can see only half of the celestial sphere
• The other half is below the horizon, hidden by the body of the Earth

Last thought on coordinates
• The Equatorial System of Coordinates is what most
astronomers use when specifying the location of an
object on the Celestial Sphere
• Right Ascension (measured eastwards from the
Vernal Equinox) goes from 0h to 24h
• Declination (measured north or south from the
celestial equator goes from -90° to +90 °.
• The hour angle (HA) of an object is the angle
between the meridian on which the object is situated
and the (observer’s) celestial meridian
• ST = RA + HA

The seasons are caused by the tilt of
Earth’s axis of rotation
• The Earth’s axis of rotation is not
perpendicular to the plane of the Earth’s
orbit
• It is tilted about 23½° away from the
perpendicular & is called the obliquity.
• The Earth maintains this tilt as it orbits
the Sun, with the Earth’s north pole
pointing toward the north celestial pole

The Earth’s orbit

• Seasons do NOT arise from the
distance the Earth is from the Sun but
rather as a result of the Earth’s annual
motion and axial inclination – the tip
of our planet with respect to its orbital
plane. As we move around the Sun,
the orientation of our planet gives us
seasons.

Seasons
• During part of the year the northern hemisphere of
the Earth is tilted toward the Sun
• As the Earth spins on its axis, a point in the
northern hemisphere spends more than 12 hours in
the sunlight
• The days there are long and the nights are short,
and it is summer in the northern hemisphere and
winter in the southern hemisphere
• The summer is hot not only because of the
extended daylight hours but also because the Sun
is high in the northern hemisphere’s sky
• As a result, sunlight strikes the ground at a nearly
perpendicular angle that heats the ground efficiently
• This situation reverses six months later

Sept
21
June
21

Dec
21
March
31

• The Sun appears to
trace out a circular path
called the ecliptic on
the celestial sphere tilted
at 23 ½ degrees to the
equator
• The ecliptic and the
celestial equator
intersect at only two
points
• Each point is called an
equinox
• The point on the ecliptic
farthest north of the
celestial equator that
marks the location of the
Sun at the beginning of
summer in the northern
hemisphere is called the
summer solstice
• At the beginning of the
northern hemisphere’s
winter the Sun is farthest
south of the celestial
equator at a point called
the winter solstice

Landmarks on the Earth’s surface are
marked by the Sun’s position in the sky
throughout the year

The Moon helps to cause precession, a
slow, conical motion of Earth’s axis of
rotation

Precession causes the gradual change of the
star that marks the North Celestial Pole

Astronomical observations led to the
development of the modern calendar
•
•
•
•

The day is based on the Earth’s rotation
The year is based on the Earth’s orbit
The month is based on the lunar cycle
None of these are exactly the same as
nature so astronomers use the average
or mean day and leap years to keep the
calendar and time consistent

The different types of ‘year’.
• The sidereal year (year with respect to
the stars) measured in solar time is
365d 6h 9m 10s (365.2564d) in length.
• The tropical year (successive passages
of the Sun through the Vernal Equinox)
is 365d 5h 48m 46s (365.2422d) in length.
• Due to precession, the tropical year is
20m 24s shorter than the sidereal year.

Calendars
• Caesar introduced the 365.25 days calendar
and thus the Leap Year (February 29)
• However, this is 11m 14s longer than the real
tropical year. This accumulates to 3 days in
4 centuries error.
• To correct, October 4 was followed by
October 15, in 1562 and the century rule was
invoked.


Slide 31

Knowing the Heavens

Chapter Two

Naked-eye astronomy had an important place
in ancient civilizations
• Positional astronomy
– the study of the positions of objects in the sky and
how these positions change

• Naked-eye astronomy
– the sort that requires no equipment but human
vision

• Extends far back in time
–
–
–
–

British Isles Stonehenge
Native American Medicine Wheel
Aztec, Mayan and Incan temples
Egyptian pyramids

Eighty-eight constellations cover the
entire sky
• Ancient peoples looked
at the stars and
imagined groupings
made pictures in the
sky
• We still refer to many of
these groupings
• Astronomers call them
constellations (from
the Latin for “group of
stars”)

Modern Constellations
• On modern star charts,
the entire sky is divided
into 88 regions. Each
is a constellation
• Most stars in a
constellation are
nowhere near one
another
• They only appear to be
close together because
they are in nearly the
same direction as seen
from Earth

The appearance of the sky changes during the
course of the night and from one night to the next
• Stars appear to rise
in the east, slowly
rotate about the
earth and set in the
west.
• This diurnal or daily
motion of the stars
is actually caused
by the 24-hour
rotation of the earth.

Animation of constellation movement
• To represent what we have just discussed,
follow this animation from the vantage
point of our Californian observer.

Annual Motion
• The stars also appear
to slowly shift in position
throughout the year
• This is due to the orbit
of the earth around the
sun
• If you follow a particular
star on successive
evenings, you will find
that it rises
approximately 4
minutes earlier each
night, or 2 hours earlier
each month

It is convenient to imagine that the stars are
located on a celestial sphere
• The celestial sphere is
an imaginary object
that has no basis in
physical reality
• However it is still a
model that remains a
useful tool of positional
astronomy
• Landmarks on the
celestial sphere are
projections of those on
the Earth

•Celestial equator
divides the sky into
northern and southern
hemispheres
•Celestial poles are
where the Earth’s axis
of rotation would
intersect the celestial
sphere
•Polaris is less than 1°
away from the north
celestial pole, which is
why it is called the
North Star or the Pole
Star.
•Point in the sky directly
overhead an observer
anywhere on Earth is
called that observer’s
zenith.

The Celestial Coordinate System
• Again, let us see what we have just
determined in a more 3-dimension manner.

Positional astronomy plays an important
role in keeping track of time
• Apparent solar time is based on the apparent
motion of the Sun across the celestial sphere,
which varies over the course of the year
• Mean solar time is based on the motion of an
imaginary mean sun along the celestial
equator, which produces a uniform mean
solar day of 24 hours
• Ordinary watches and clocks measure mean
solar time
• Sidereal time is based on the apparent
motion of the celestial sphere

• Local noon is defined to be when the Sun crosses the
upper meridian, which is the half of the meridian
above the horizon

Sidereal and Solar Days
• Appreciating the difference between a solar
day and a sidereal day is a challenging
concept. See if this helps.

Circumpolar
stars

• At any time, an observer can see only half of the celestial sphere
• The other half is below the horizon, hidden by the body of the Earth

Last thought on coordinates
• The Equatorial System of Coordinates is what most
astronomers use when specifying the location of an
object on the Celestial Sphere
• Right Ascension (measured eastwards from the
Vernal Equinox) goes from 0h to 24h
• Declination (measured north or south from the
celestial equator goes from -90° to +90 °.
• The hour angle (HA) of an object is the angle
between the meridian on which the object is situated
and the (observer’s) celestial meridian
• ST = RA + HA

The seasons are caused by the tilt of
Earth’s axis of rotation
• The Earth’s axis of rotation is not
perpendicular to the plane of the Earth’s
orbit
• It is tilted about 23½° away from the
perpendicular & is called the obliquity.
• The Earth maintains this tilt as it orbits
the Sun, with the Earth’s north pole
pointing toward the north celestial pole

The Earth’s orbit

• Seasons do NOT arise from the
distance the Earth is from the Sun but
rather as a result of the Earth’s annual
motion and axial inclination – the tip
of our planet with respect to its orbital
plane. As we move around the Sun,
the orientation of our planet gives us
seasons.

Seasons
• During part of the year the northern hemisphere of
the Earth is tilted toward the Sun
• As the Earth spins on its axis, a point in the
northern hemisphere spends more than 12 hours in
the sunlight
• The days there are long and the nights are short,
and it is summer in the northern hemisphere and
winter in the southern hemisphere
• The summer is hot not only because of the
extended daylight hours but also because the Sun
is high in the northern hemisphere’s sky
• As a result, sunlight strikes the ground at a nearly
perpendicular angle that heats the ground efficiently
• This situation reverses six months later

Sept
21
June
21

Dec
21
March
31

• The Sun appears to
trace out a circular path
called the ecliptic on
the celestial sphere tilted
at 23 ½ degrees to the
equator
• The ecliptic and the
celestial equator
intersect at only two
points
• Each point is called an
equinox
• The point on the ecliptic
farthest north of the
celestial equator that
marks the location of the
Sun at the beginning of
summer in the northern
hemisphere is called the
summer solstice
• At the beginning of the
northern hemisphere’s
winter the Sun is farthest
south of the celestial
equator at a point called
the winter solstice

Landmarks on the Earth’s surface are
marked by the Sun’s position in the sky
throughout the year

The Moon helps to cause precession, a
slow, conical motion of Earth’s axis of
rotation

Precession causes the gradual change of the
star that marks the North Celestial Pole

Astronomical observations led to the
development of the modern calendar
•
•
•
•

The day is based on the Earth’s rotation
The year is based on the Earth’s orbit
The month is based on the lunar cycle
None of these are exactly the same as
nature so astronomers use the average
or mean day and leap years to keep the
calendar and time consistent

The different types of ‘year’.
• The sidereal year (year with respect to
the stars) measured in solar time is
365d 6h 9m 10s (365.2564d) in length.
• The tropical year (successive passages
of the Sun through the Vernal Equinox)
is 365d 5h 48m 46s (365.2422d) in length.
• Due to precession, the tropical year is
20m 24s shorter than the sidereal year.

Calendars
• Caesar introduced the 365.25 days calendar
and thus the Leap Year (February 29)
• However, this is 11m 14s longer than the real
tropical year. This accumulates to 3 days in
4 centuries error.
• To correct, October 4 was followed by
October 15, in 1562 and the century rule was
invoked.


Slide 32

Knowing the Heavens

Chapter Two

Naked-eye astronomy had an important place
in ancient civilizations
• Positional astronomy
– the study of the positions of objects in the sky and
how these positions change

• Naked-eye astronomy
– the sort that requires no equipment but human
vision

• Extends far back in time
–
–
–
–

British Isles Stonehenge
Native American Medicine Wheel
Aztec, Mayan and Incan temples
Egyptian pyramids

Eighty-eight constellations cover the
entire sky
• Ancient peoples looked
at the stars and
imagined groupings
made pictures in the
sky
• We still refer to many of
these groupings
• Astronomers call them
constellations (from
the Latin for “group of
stars”)

Modern Constellations
• On modern star charts,
the entire sky is divided
into 88 regions. Each
is a constellation
• Most stars in a
constellation are
nowhere near one
another
• They only appear to be
close together because
they are in nearly the
same direction as seen
from Earth

The appearance of the sky changes during the
course of the night and from one night to the next
• Stars appear to rise
in the east, slowly
rotate about the
earth and set in the
west.
• This diurnal or daily
motion of the stars
is actually caused
by the 24-hour
rotation of the earth.

Animation of constellation movement
• To represent what we have just discussed,
follow this animation from the vantage
point of our Californian observer.

Annual Motion
• The stars also appear
to slowly shift in position
throughout the year
• This is due to the orbit
of the earth around the
sun
• If you follow a particular
star on successive
evenings, you will find
that it rises
approximately 4
minutes earlier each
night, or 2 hours earlier
each month

It is convenient to imagine that the stars are
located on a celestial sphere
• The celestial sphere is
an imaginary object
that has no basis in
physical reality
• However it is still a
model that remains a
useful tool of positional
astronomy
• Landmarks on the
celestial sphere are
projections of those on
the Earth

•Celestial equator
divides the sky into
northern and southern
hemispheres
•Celestial poles are
where the Earth’s axis
of rotation would
intersect the celestial
sphere
•Polaris is less than 1°
away from the north
celestial pole, which is
why it is called the
North Star or the Pole
Star.
•Point in the sky directly
overhead an observer
anywhere on Earth is
called that observer’s
zenith.

The Celestial Coordinate System
• Again, let us see what we have just
determined in a more 3-dimension manner.

Positional astronomy plays an important
role in keeping track of time
• Apparent solar time is based on the apparent
motion of the Sun across the celestial sphere,
which varies over the course of the year
• Mean solar time is based on the motion of an
imaginary mean sun along the celestial
equator, which produces a uniform mean
solar day of 24 hours
• Ordinary watches and clocks measure mean
solar time
• Sidereal time is based on the apparent
motion of the celestial sphere

• Local noon is defined to be when the Sun crosses the
upper meridian, which is the half of the meridian
above the horizon

Sidereal and Solar Days
• Appreciating the difference between a solar
day and a sidereal day is a challenging
concept. See if this helps.

Circumpolar
stars

• At any time, an observer can see only half of the celestial sphere
• The other half is below the horizon, hidden by the body of the Earth

Last thought on coordinates
• The Equatorial System of Coordinates is what most
astronomers use when specifying the location of an
object on the Celestial Sphere
• Right Ascension (measured eastwards from the
Vernal Equinox) goes from 0h to 24h
• Declination (measured north or south from the
celestial equator goes from -90° to +90 °.
• The hour angle (HA) of an object is the angle
between the meridian on which the object is situated
and the (observer’s) celestial meridian
• ST = RA + HA

The seasons are caused by the tilt of
Earth’s axis of rotation
• The Earth’s axis of rotation is not
perpendicular to the plane of the Earth’s
orbit
• It is tilted about 23½° away from the
perpendicular & is called the obliquity.
• The Earth maintains this tilt as it orbits
the Sun, with the Earth’s north pole
pointing toward the north celestial pole

The Earth’s orbit

• Seasons do NOT arise from the
distance the Earth is from the Sun but
rather as a result of the Earth’s annual
motion and axial inclination – the tip
of our planet with respect to its orbital
plane. As we move around the Sun,
the orientation of our planet gives us
seasons.

Seasons
• During part of the year the northern hemisphere of
the Earth is tilted toward the Sun
• As the Earth spins on its axis, a point in the
northern hemisphere spends more than 12 hours in
the sunlight
• The days there are long and the nights are short,
and it is summer in the northern hemisphere and
winter in the southern hemisphere
• The summer is hot not only because of the
extended daylight hours but also because the Sun
is high in the northern hemisphere’s sky
• As a result, sunlight strikes the ground at a nearly
perpendicular angle that heats the ground efficiently
• This situation reverses six months later

Sept
21
June
21

Dec
21
March
31

• The Sun appears to
trace out a circular path
called the ecliptic on
the celestial sphere tilted
at 23 ½ degrees to the
equator
• The ecliptic and the
celestial equator
intersect at only two
points
• Each point is called an
equinox
• The point on the ecliptic
farthest north of the
celestial equator that
marks the location of the
Sun at the beginning of
summer in the northern
hemisphere is called the
summer solstice
• At the beginning of the
northern hemisphere’s
winter the Sun is farthest
south of the celestial
equator at a point called
the winter solstice

Landmarks on the Earth’s surface are
marked by the Sun’s position in the sky
throughout the year

The Moon helps to cause precession, a
slow, conical motion of Earth’s axis of
rotation

Precession causes the gradual change of the
star that marks the North Celestial Pole

Astronomical observations led to the
development of the modern calendar
•
•
•
•

The day is based on the Earth’s rotation
The year is based on the Earth’s orbit
The month is based on the lunar cycle
None of these are exactly the same as
nature so astronomers use the average
or mean day and leap years to keep the
calendar and time consistent

The different types of ‘year’.
• The sidereal year (year with respect to
the stars) measured in solar time is
365d 6h 9m 10s (365.2564d) in length.
• The tropical year (successive passages
of the Sun through the Vernal Equinox)
is 365d 5h 48m 46s (365.2422d) in length.
• Due to precession, the tropical year is
20m 24s shorter than the sidereal year.

Calendars
• Caesar introduced the 365.25 days calendar
and thus the Leap Year (February 29)
• However, this is 11m 14s longer than the real
tropical year. This accumulates to 3 days in
4 centuries error.
• To correct, October 4 was followed by
October 15, in 1562 and the century rule was
invoked.


Slide 33

Knowing the Heavens

Chapter Two

Naked-eye astronomy had an important place
in ancient civilizations
• Positional astronomy
– the study of the positions of objects in the sky and
how these positions change

• Naked-eye astronomy
– the sort that requires no equipment but human
vision

• Extends far back in time
–
–
–
–

British Isles Stonehenge
Native American Medicine Wheel
Aztec, Mayan and Incan temples
Egyptian pyramids

Eighty-eight constellations cover the
entire sky
• Ancient peoples looked
at the stars and
imagined groupings
made pictures in the
sky
• We still refer to many of
these groupings
• Astronomers call them
constellations (from
the Latin for “group of
stars”)

Modern Constellations
• On modern star charts,
the entire sky is divided
into 88 regions. Each
is a constellation
• Most stars in a
constellation are
nowhere near one
another
• They only appear to be
close together because
they are in nearly the
same direction as seen
from Earth

The appearance of the sky changes during the
course of the night and from one night to the next
• Stars appear to rise
in the east, slowly
rotate about the
earth and set in the
west.
• This diurnal or daily
motion of the stars
is actually caused
by the 24-hour
rotation of the earth.

Animation of constellation movement
• To represent what we have just discussed,
follow this animation from the vantage
point of our Californian observer.

Annual Motion
• The stars also appear
to slowly shift in position
throughout the year
• This is due to the orbit
of the earth around the
sun
• If you follow a particular
star on successive
evenings, you will find
that it rises
approximately 4
minutes earlier each
night, or 2 hours earlier
each month

It is convenient to imagine that the stars are
located on a celestial sphere
• The celestial sphere is
an imaginary object
that has no basis in
physical reality
• However it is still a
model that remains a
useful tool of positional
astronomy
• Landmarks on the
celestial sphere are
projections of those on
the Earth

•Celestial equator
divides the sky into
northern and southern
hemispheres
•Celestial poles are
where the Earth’s axis
of rotation would
intersect the celestial
sphere
•Polaris is less than 1°
away from the north
celestial pole, which is
why it is called the
North Star or the Pole
Star.
•Point in the sky directly
overhead an observer
anywhere on Earth is
called that observer’s
zenith.

The Celestial Coordinate System
• Again, let us see what we have just
determined in a more 3-dimension manner.

Positional astronomy plays an important
role in keeping track of time
• Apparent solar time is based on the apparent
motion of the Sun across the celestial sphere,
which varies over the course of the year
• Mean solar time is based on the motion of an
imaginary mean sun along the celestial
equator, which produces a uniform mean
solar day of 24 hours
• Ordinary watches and clocks measure mean
solar time
• Sidereal time is based on the apparent
motion of the celestial sphere

• Local noon is defined to be when the Sun crosses the
upper meridian, which is the half of the meridian
above the horizon

Sidereal and Solar Days
• Appreciating the difference between a solar
day and a sidereal day is a challenging
concept. See if this helps.

Circumpolar
stars

• At any time, an observer can see only half of the celestial sphere
• The other half is below the horizon, hidden by the body of the Earth

Last thought on coordinates
• The Equatorial System of Coordinates is what most
astronomers use when specifying the location of an
object on the Celestial Sphere
• Right Ascension (measured eastwards from the
Vernal Equinox) goes from 0h to 24h
• Declination (measured north or south from the
celestial equator goes from -90° to +90 °.
• The hour angle (HA) of an object is the angle
between the meridian on which the object is situated
and the (observer’s) celestial meridian
• ST = RA + HA

The seasons are caused by the tilt of
Earth’s axis of rotation
• The Earth’s axis of rotation is not
perpendicular to the plane of the Earth’s
orbit
• It is tilted about 23½° away from the
perpendicular & is called the obliquity.
• The Earth maintains this tilt as it orbits
the Sun, with the Earth’s north pole
pointing toward the north celestial pole

The Earth’s orbit

• Seasons do NOT arise from the
distance the Earth is from the Sun but
rather as a result of the Earth’s annual
motion and axial inclination – the tip
of our planet with respect to its orbital
plane. As we move around the Sun,
the orientation of our planet gives us
seasons.

Seasons
• During part of the year the northern hemisphere of
the Earth is tilted toward the Sun
• As the Earth spins on its axis, a point in the
northern hemisphere spends more than 12 hours in
the sunlight
• The days there are long and the nights are short,
and it is summer in the northern hemisphere and
winter in the southern hemisphere
• The summer is hot not only because of the
extended daylight hours but also because the Sun
is high in the northern hemisphere’s sky
• As a result, sunlight strikes the ground at a nearly
perpendicular angle that heats the ground efficiently
• This situation reverses six months later

Sept
21
June
21

Dec
21
March
31

• The Sun appears to
trace out a circular path
called the ecliptic on
the celestial sphere tilted
at 23 ½ degrees to the
equator
• The ecliptic and the
celestial equator
intersect at only two
points
• Each point is called an
equinox
• The point on the ecliptic
farthest north of the
celestial equator that
marks the location of the
Sun at the beginning of
summer in the northern
hemisphere is called the
summer solstice
• At the beginning of the
northern hemisphere’s
winter the Sun is farthest
south of the celestial
equator at a point called
the winter solstice

Landmarks on the Earth’s surface are
marked by the Sun’s position in the sky
throughout the year

The Moon helps to cause precession, a
slow, conical motion of Earth’s axis of
rotation

Precession causes the gradual change of the
star that marks the North Celestial Pole

Astronomical observations led to the
development of the modern calendar
•
•
•
•

The day is based on the Earth’s rotation
The year is based on the Earth’s orbit
The month is based on the lunar cycle
None of these are exactly the same as
nature so astronomers use the average
or mean day and leap years to keep the
calendar and time consistent

The different types of ‘year’.
• The sidereal year (year with respect to
the stars) measured in solar time is
365d 6h 9m 10s (365.2564d) in length.
• The tropical year (successive passages
of the Sun through the Vernal Equinox)
is 365d 5h 48m 46s (365.2422d) in length.
• Due to precession, the tropical year is
20m 24s shorter than the sidereal year.

Calendars
• Caesar introduced the 365.25 days calendar
and thus the Leap Year (February 29)
• However, this is 11m 14s longer than the real
tropical year. This accumulates to 3 days in
4 centuries error.
• To correct, October 4 was followed by
October 15, in 1562 and the century rule was
invoked.


Slide 34

Knowing the Heavens

Chapter Two

Naked-eye astronomy had an important place
in ancient civilizations
• Positional astronomy
– the study of the positions of objects in the sky and
how these positions change

• Naked-eye astronomy
– the sort that requires no equipment but human
vision

• Extends far back in time
–
–
–
–

British Isles Stonehenge
Native American Medicine Wheel
Aztec, Mayan and Incan temples
Egyptian pyramids

Eighty-eight constellations cover the
entire sky
• Ancient peoples looked
at the stars and
imagined groupings
made pictures in the
sky
• We still refer to many of
these groupings
• Astronomers call them
constellations (from
the Latin for “group of
stars”)

Modern Constellations
• On modern star charts,
the entire sky is divided
into 88 regions. Each
is a constellation
• Most stars in a
constellation are
nowhere near one
another
• They only appear to be
close together because
they are in nearly the
same direction as seen
from Earth

The appearance of the sky changes during the
course of the night and from one night to the next
• Stars appear to rise
in the east, slowly
rotate about the
earth and set in the
west.
• This diurnal or daily
motion of the stars
is actually caused
by the 24-hour
rotation of the earth.

Animation of constellation movement
• To represent what we have just discussed,
follow this animation from the vantage
point of our Californian observer.

Annual Motion
• The stars also appear
to slowly shift in position
throughout the year
• This is due to the orbit
of the earth around the
sun
• If you follow a particular
star on successive
evenings, you will find
that it rises
approximately 4
minutes earlier each
night, or 2 hours earlier
each month

It is convenient to imagine that the stars are
located on a celestial sphere
• The celestial sphere is
an imaginary object
that has no basis in
physical reality
• However it is still a
model that remains a
useful tool of positional
astronomy
• Landmarks on the
celestial sphere are
projections of those on
the Earth

•Celestial equator
divides the sky into
northern and southern
hemispheres
•Celestial poles are
where the Earth’s axis
of rotation would
intersect the celestial
sphere
•Polaris is less than 1°
away from the north
celestial pole, which is
why it is called the
North Star or the Pole
Star.
•Point in the sky directly
overhead an observer
anywhere on Earth is
called that observer’s
zenith.

The Celestial Coordinate System
• Again, let us see what we have just
determined in a more 3-dimension manner.

Positional astronomy plays an important
role in keeping track of time
• Apparent solar time is based on the apparent
motion of the Sun across the celestial sphere,
which varies over the course of the year
• Mean solar time is based on the motion of an
imaginary mean sun along the celestial
equator, which produces a uniform mean
solar day of 24 hours
• Ordinary watches and clocks measure mean
solar time
• Sidereal time is based on the apparent
motion of the celestial sphere

• Local noon is defined to be when the Sun crosses the
upper meridian, which is the half of the meridian
above the horizon

Sidereal and Solar Days
• Appreciating the difference between a solar
day and a sidereal day is a challenging
concept. See if this helps.

Circumpolar
stars

• At any time, an observer can see only half of the celestial sphere
• The other half is below the horizon, hidden by the body of the Earth

Last thought on coordinates
• The Equatorial System of Coordinates is what most
astronomers use when specifying the location of an
object on the Celestial Sphere
• Right Ascension (measured eastwards from the
Vernal Equinox) goes from 0h to 24h
• Declination (measured north or south from the
celestial equator goes from -90° to +90 °.
• The hour angle (HA) of an object is the angle
between the meridian on which the object is situated
and the (observer’s) celestial meridian
• ST = RA + HA

The seasons are caused by the tilt of
Earth’s axis of rotation
• The Earth’s axis of rotation is not
perpendicular to the plane of the Earth’s
orbit
• It is tilted about 23½° away from the
perpendicular & is called the obliquity.
• The Earth maintains this tilt as it orbits
the Sun, with the Earth’s north pole
pointing toward the north celestial pole

The Earth’s orbit

• Seasons do NOT arise from the
distance the Earth is from the Sun but
rather as a result of the Earth’s annual
motion and axial inclination – the tip
of our planet with respect to its orbital
plane. As we move around the Sun,
the orientation of our planet gives us
seasons.

Seasons
• During part of the year the northern hemisphere of
the Earth is tilted toward the Sun
• As the Earth spins on its axis, a point in the
northern hemisphere spends more than 12 hours in
the sunlight
• The days there are long and the nights are short,
and it is summer in the northern hemisphere and
winter in the southern hemisphere
• The summer is hot not only because of the
extended daylight hours but also because the Sun
is high in the northern hemisphere’s sky
• As a result, sunlight strikes the ground at a nearly
perpendicular angle that heats the ground efficiently
• This situation reverses six months later

Sept
21
June
21

Dec
21
March
31

• The Sun appears to
trace out a circular path
called the ecliptic on
the celestial sphere tilted
at 23 ½ degrees to the
equator
• The ecliptic and the
celestial equator
intersect at only two
points
• Each point is called an
equinox
• The point on the ecliptic
farthest north of the
celestial equator that
marks the location of the
Sun at the beginning of
summer in the northern
hemisphere is called the
summer solstice
• At the beginning of the
northern hemisphere’s
winter the Sun is farthest
south of the celestial
equator at a point called
the winter solstice

Landmarks on the Earth’s surface are
marked by the Sun’s position in the sky
throughout the year

The Moon helps to cause precession, a
slow, conical motion of Earth’s axis of
rotation

Precession causes the gradual change of the
star that marks the North Celestial Pole

Astronomical observations led to the
development of the modern calendar
•
•
•
•

The day is based on the Earth’s rotation
The year is based on the Earth’s orbit
The month is based on the lunar cycle
None of these are exactly the same as
nature so astronomers use the average
or mean day and leap years to keep the
calendar and time consistent

The different types of ‘year’.
• The sidereal year (year with respect to
the stars) measured in solar time is
365d 6h 9m 10s (365.2564d) in length.
• The tropical year (successive passages
of the Sun through the Vernal Equinox)
is 365d 5h 48m 46s (365.2422d) in length.
• Due to precession, the tropical year is
20m 24s shorter than the sidereal year.

Calendars
• Caesar introduced the 365.25 days calendar
and thus the Leap Year (February 29)
• However, this is 11m 14s longer than the real
tropical year. This accumulates to 3 days in
4 centuries error.
• To correct, October 4 was followed by
October 15, in 1562 and the century rule was
invoked.


Slide 35

Knowing the Heavens

Chapter Two

Naked-eye astronomy had an important place
in ancient civilizations
• Positional astronomy
– the study of the positions of objects in the sky and
how these positions change

• Naked-eye astronomy
– the sort that requires no equipment but human
vision

• Extends far back in time
–
–
–
–

British Isles Stonehenge
Native American Medicine Wheel
Aztec, Mayan and Incan temples
Egyptian pyramids

Eighty-eight constellations cover the
entire sky
• Ancient peoples looked
at the stars and
imagined groupings
made pictures in the
sky
• We still refer to many of
these groupings
• Astronomers call them
constellations (from
the Latin for “group of
stars”)

Modern Constellations
• On modern star charts,
the entire sky is divided
into 88 regions. Each
is a constellation
• Most stars in a
constellation are
nowhere near one
another
• They only appear to be
close together because
they are in nearly the
same direction as seen
from Earth

The appearance of the sky changes during the
course of the night and from one night to the next
• Stars appear to rise
in the east, slowly
rotate about the
earth and set in the
west.
• This diurnal or daily
motion of the stars
is actually caused
by the 24-hour
rotation of the earth.

Animation of constellation movement
• To represent what we have just discussed,
follow this animation from the vantage
point of our Californian observer.

Annual Motion
• The stars also appear
to slowly shift in position
throughout the year
• This is due to the orbit
of the earth around the
sun
• If you follow a particular
star on successive
evenings, you will find
that it rises
approximately 4
minutes earlier each
night, or 2 hours earlier
each month

It is convenient to imagine that the stars are
located on a celestial sphere
• The celestial sphere is
an imaginary object
that has no basis in
physical reality
• However it is still a
model that remains a
useful tool of positional
astronomy
• Landmarks on the
celestial sphere are
projections of those on
the Earth

•Celestial equator
divides the sky into
northern and southern
hemispheres
•Celestial poles are
where the Earth’s axis
of rotation would
intersect the celestial
sphere
•Polaris is less than 1°
away from the north
celestial pole, which is
why it is called the
North Star or the Pole
Star.
•Point in the sky directly
overhead an observer
anywhere on Earth is
called that observer’s
zenith.

The Celestial Coordinate System
• Again, let us see what we have just
determined in a more 3-dimension manner.

Positional astronomy plays an important
role in keeping track of time
• Apparent solar time is based on the apparent
motion of the Sun across the celestial sphere,
which varies over the course of the year
• Mean solar time is based on the motion of an
imaginary mean sun along the celestial
equator, which produces a uniform mean
solar day of 24 hours
• Ordinary watches and clocks measure mean
solar time
• Sidereal time is based on the apparent
motion of the celestial sphere

• Local noon is defined to be when the Sun crosses the
upper meridian, which is the half of the meridian
above the horizon

Sidereal and Solar Days
• Appreciating the difference between a solar
day and a sidereal day is a challenging
concept. See if this helps.

Circumpolar
stars

• At any time, an observer can see only half of the celestial sphere
• The other half is below the horizon, hidden by the body of the Earth

Last thought on coordinates
• The Equatorial System of Coordinates is what most
astronomers use when specifying the location of an
object on the Celestial Sphere
• Right Ascension (measured eastwards from the
Vernal Equinox) goes from 0h to 24h
• Declination (measured north or south from the
celestial equator goes from -90° to +90 °.
• The hour angle (HA) of an object is the angle
between the meridian on which the object is situated
and the (observer’s) celestial meridian
• ST = RA + HA

The seasons are caused by the tilt of
Earth’s axis of rotation
• The Earth’s axis of rotation is not
perpendicular to the plane of the Earth’s
orbit
• It is tilted about 23½° away from the
perpendicular & is called the obliquity.
• The Earth maintains this tilt as it orbits
the Sun, with the Earth’s north pole
pointing toward the north celestial pole

The Earth’s orbit

• Seasons do NOT arise from the
distance the Earth is from the Sun but
rather as a result of the Earth’s annual
motion and axial inclination – the tip
of our planet with respect to its orbital
plane. As we move around the Sun,
the orientation of our planet gives us
seasons.

Seasons
• During part of the year the northern hemisphere of
the Earth is tilted toward the Sun
• As the Earth spins on its axis, a point in the
northern hemisphere spends more than 12 hours in
the sunlight
• The days there are long and the nights are short,
and it is summer in the northern hemisphere and
winter in the southern hemisphere
• The summer is hot not only because of the
extended daylight hours but also because the Sun
is high in the northern hemisphere’s sky
• As a result, sunlight strikes the ground at a nearly
perpendicular angle that heats the ground efficiently
• This situation reverses six months later

Sept
21
June
21

Dec
21
March
31

• The Sun appears to
trace out a circular path
called the ecliptic on
the celestial sphere tilted
at 23 ½ degrees to the
equator
• The ecliptic and the
celestial equator
intersect at only two
points
• Each point is called an
equinox
• The point on the ecliptic
farthest north of the
celestial equator that
marks the location of the
Sun at the beginning of
summer in the northern
hemisphere is called the
summer solstice
• At the beginning of the
northern hemisphere’s
winter the Sun is farthest
south of the celestial
equator at a point called
the winter solstice

Landmarks on the Earth’s surface are
marked by the Sun’s position in the sky
throughout the year

The Moon helps to cause precession, a
slow, conical motion of Earth’s axis of
rotation

Precession causes the gradual change of the
star that marks the North Celestial Pole

Astronomical observations led to the
development of the modern calendar
•
•
•
•

The day is based on the Earth’s rotation
The year is based on the Earth’s orbit
The month is based on the lunar cycle
None of these are exactly the same as
nature so astronomers use the average
or mean day and leap years to keep the
calendar and time consistent

The different types of ‘year’.
• The sidereal year (year with respect to
the stars) measured in solar time is
365d 6h 9m 10s (365.2564d) in length.
• The tropical year (successive passages
of the Sun through the Vernal Equinox)
is 365d 5h 48m 46s (365.2422d) in length.
• Due to precession, the tropical year is
20m 24s shorter than the sidereal year.

Calendars
• Caesar introduced the 365.25 days calendar
and thus the Leap Year (February 29)
• However, this is 11m 14s longer than the real
tropical year. This accumulates to 3 days in
4 centuries error.
• To correct, October 4 was followed by
October 15, in 1562 and the century rule was
invoked.


Slide 36

Knowing the Heavens

Chapter Two

Naked-eye astronomy had an important place
in ancient civilizations
• Positional astronomy
– the study of the positions of objects in the sky and
how these positions change

• Naked-eye astronomy
– the sort that requires no equipment but human
vision

• Extends far back in time
–
–
–
–

British Isles Stonehenge
Native American Medicine Wheel
Aztec, Mayan and Incan temples
Egyptian pyramids

Eighty-eight constellations cover the
entire sky
• Ancient peoples looked
at the stars and
imagined groupings
made pictures in the
sky
• We still refer to many of
these groupings
• Astronomers call them
constellations (from
the Latin for “group of
stars”)

Modern Constellations
• On modern star charts,
the entire sky is divided
into 88 regions. Each
is a constellation
• Most stars in a
constellation are
nowhere near one
another
• They only appear to be
close together because
they are in nearly the
same direction as seen
from Earth

The appearance of the sky changes during the
course of the night and from one night to the next
• Stars appear to rise
in the east, slowly
rotate about the
earth and set in the
west.
• This diurnal or daily
motion of the stars
is actually caused
by the 24-hour
rotation of the earth.

Animation of constellation movement
• To represent what we have just discussed,
follow this animation from the vantage
point of our Californian observer.

Annual Motion
• The stars also appear
to slowly shift in position
throughout the year
• This is due to the orbit
of the earth around the
sun
• If you follow a particular
star on successive
evenings, you will find
that it rises
approximately 4
minutes earlier each
night, or 2 hours earlier
each month

It is convenient to imagine that the stars are
located on a celestial sphere
• The celestial sphere is
an imaginary object
that has no basis in
physical reality
• However it is still a
model that remains a
useful tool of positional
astronomy
• Landmarks on the
celestial sphere are
projections of those on
the Earth

•Celestial equator
divides the sky into
northern and southern
hemispheres
•Celestial poles are
where the Earth’s axis
of rotation would
intersect the celestial
sphere
•Polaris is less than 1°
away from the north
celestial pole, which is
why it is called the
North Star or the Pole
Star.
•Point in the sky directly
overhead an observer
anywhere on Earth is
called that observer’s
zenith.

The Celestial Coordinate System
• Again, let us see what we have just
determined in a more 3-dimension manner.

Positional astronomy plays an important
role in keeping track of time
• Apparent solar time is based on the apparent
motion of the Sun across the celestial sphere,
which varies over the course of the year
• Mean solar time is based on the motion of an
imaginary mean sun along the celestial
equator, which produces a uniform mean
solar day of 24 hours
• Ordinary watches and clocks measure mean
solar time
• Sidereal time is based on the apparent
motion of the celestial sphere

• Local noon is defined to be when the Sun crosses the
upper meridian, which is the half of the meridian
above the horizon

Sidereal and Solar Days
• Appreciating the difference between a solar
day and a sidereal day is a challenging
concept. See if this helps.

Circumpolar
stars

• At any time, an observer can see only half of the celestial sphere
• The other half is below the horizon, hidden by the body of the Earth

Last thought on coordinates
• The Equatorial System of Coordinates is what most
astronomers use when specifying the location of an
object on the Celestial Sphere
• Right Ascension (measured eastwards from the
Vernal Equinox) goes from 0h to 24h
• Declination (measured north or south from the
celestial equator goes from -90° to +90 °.
• The hour angle (HA) of an object is the angle
between the meridian on which the object is situated
and the (observer’s) celestial meridian
• ST = RA + HA

The seasons are caused by the tilt of
Earth’s axis of rotation
• The Earth’s axis of rotation is not
perpendicular to the plane of the Earth’s
orbit
• It is tilted about 23½° away from the
perpendicular & is called the obliquity.
• The Earth maintains this tilt as it orbits
the Sun, with the Earth’s north pole
pointing toward the north celestial pole

The Earth’s orbit

• Seasons do NOT arise from the
distance the Earth is from the Sun but
rather as a result of the Earth’s annual
motion and axial inclination – the tip
of our planet with respect to its orbital
plane. As we move around the Sun,
the orientation of our planet gives us
seasons.

Seasons
• During part of the year the northern hemisphere of
the Earth is tilted toward the Sun
• As the Earth spins on its axis, a point in the
northern hemisphere spends more than 12 hours in
the sunlight
• The days there are long and the nights are short,
and it is summer in the northern hemisphere and
winter in the southern hemisphere
• The summer is hot not only because of the
extended daylight hours but also because the Sun
is high in the northern hemisphere’s sky
• As a result, sunlight strikes the ground at a nearly
perpendicular angle that heats the ground efficiently
• This situation reverses six months later

Sept
21
June
21

Dec
21
March
31

• The Sun appears to
trace out a circular path
called the ecliptic on
the celestial sphere tilted
at 23 ½ degrees to the
equator
• The ecliptic and the
celestial equator
intersect at only two
points
• Each point is called an
equinox
• The point on the ecliptic
farthest north of the
celestial equator that
marks the location of the
Sun at the beginning of
summer in the northern
hemisphere is called the
summer solstice
• At the beginning of the
northern hemisphere’s
winter the Sun is farthest
south of the celestial
equator at a point called
the winter solstice

Landmarks on the Earth’s surface are
marked by the Sun’s position in the sky
throughout the year

The Moon helps to cause precession, a
slow, conical motion of Earth’s axis of
rotation

Precession causes the gradual change of the
star that marks the North Celestial Pole

Astronomical observations led to the
development of the modern calendar
•
•
•
•

The day is based on the Earth’s rotation
The year is based on the Earth’s orbit
The month is based on the lunar cycle
None of these are exactly the same as
nature so astronomers use the average
or mean day and leap years to keep the
calendar and time consistent

The different types of ‘year’.
• The sidereal year (year with respect to
the stars) measured in solar time is
365d 6h 9m 10s (365.2564d) in length.
• The tropical year (successive passages
of the Sun through the Vernal Equinox)
is 365d 5h 48m 46s (365.2422d) in length.
• Due to precession, the tropical year is
20m 24s shorter than the sidereal year.

Calendars
• Caesar introduced the 365.25 days calendar
and thus the Leap Year (February 29)
• However, this is 11m 14s longer than the real
tropical year. This accumulates to 3 days in
4 centuries error.
• To correct, October 4 was followed by
October 15, in 1562 and the century rule was
invoked.


Slide 37

Knowing the Heavens

Chapter Two

Naked-eye astronomy had an important place
in ancient civilizations
• Positional astronomy
– the study of the positions of objects in the sky and
how these positions change

• Naked-eye astronomy
– the sort that requires no equipment but human
vision

• Extends far back in time
–
–
–
–

British Isles Stonehenge
Native American Medicine Wheel
Aztec, Mayan and Incan temples
Egyptian pyramids

Eighty-eight constellations cover the
entire sky
• Ancient peoples looked
at the stars and
imagined groupings
made pictures in the
sky
• We still refer to many of
these groupings
• Astronomers call them
constellations (from
the Latin for “group of
stars”)

Modern Constellations
• On modern star charts,
the entire sky is divided
into 88 regions. Each
is a constellation
• Most stars in a
constellation are
nowhere near one
another
• They only appear to be
close together because
they are in nearly the
same direction as seen
from Earth

The appearance of the sky changes during the
course of the night and from one night to the next
• Stars appear to rise
in the east, slowly
rotate about the
earth and set in the
west.
• This diurnal or daily
motion of the stars
is actually caused
by the 24-hour
rotation of the earth.

Animation of constellation movement
• To represent what we have just discussed,
follow this animation from the vantage
point of our Californian observer.

Annual Motion
• The stars also appear
to slowly shift in position
throughout the year
• This is due to the orbit
of the earth around the
sun
• If you follow a particular
star on successive
evenings, you will find
that it rises
approximately 4
minutes earlier each
night, or 2 hours earlier
each month

It is convenient to imagine that the stars are
located on a celestial sphere
• The celestial sphere is
an imaginary object
that has no basis in
physical reality
• However it is still a
model that remains a
useful tool of positional
astronomy
• Landmarks on the
celestial sphere are
projections of those on
the Earth

•Celestial equator
divides the sky into
northern and southern
hemispheres
•Celestial poles are
where the Earth’s axis
of rotation would
intersect the celestial
sphere
•Polaris is less than 1°
away from the north
celestial pole, which is
why it is called the
North Star or the Pole
Star.
•Point in the sky directly
overhead an observer
anywhere on Earth is
called that observer’s
zenith.

The Celestial Coordinate System
• Again, let us see what we have just
determined in a more 3-dimension manner.

Positional astronomy plays an important
role in keeping track of time
• Apparent solar time is based on the apparent
motion of the Sun across the celestial sphere,
which varies over the course of the year
• Mean solar time is based on the motion of an
imaginary mean sun along the celestial
equator, which produces a uniform mean
solar day of 24 hours
• Ordinary watches and clocks measure mean
solar time
• Sidereal time is based on the apparent
motion of the celestial sphere

• Local noon is defined to be when the Sun crosses the
upper meridian, which is the half of the meridian
above the horizon

Sidereal and Solar Days
• Appreciating the difference between a solar
day and a sidereal day is a challenging
concept. See if this helps.

Circumpolar
stars

• At any time, an observer can see only half of the celestial sphere
• The other half is below the horizon, hidden by the body of the Earth

Last thought on coordinates
• The Equatorial System of Coordinates is what most
astronomers use when specifying the location of an
object on the Celestial Sphere
• Right Ascension (measured eastwards from the
Vernal Equinox) goes from 0h to 24h
• Declination (measured north or south from the
celestial equator goes from -90° to +90 °.
• The hour angle (HA) of an object is the angle
between the meridian on which the object is situated
and the (observer’s) celestial meridian
• ST = RA + HA

The seasons are caused by the tilt of
Earth’s axis of rotation
• The Earth’s axis of rotation is not
perpendicular to the plane of the Earth’s
orbit
• It is tilted about 23½° away from the
perpendicular & is called the obliquity.
• The Earth maintains this tilt as it orbits
the Sun, with the Earth’s north pole
pointing toward the north celestial pole

The Earth’s orbit

• Seasons do NOT arise from the
distance the Earth is from the Sun but
rather as a result of the Earth’s annual
motion and axial inclination – the tip
of our planet with respect to its orbital
plane. As we move around the Sun,
the orientation of our planet gives us
seasons.

Seasons
• During part of the year the northern hemisphere of
the Earth is tilted toward the Sun
• As the Earth spins on its axis, a point in the
northern hemisphere spends more than 12 hours in
the sunlight
• The days there are long and the nights are short,
and it is summer in the northern hemisphere and
winter in the southern hemisphere
• The summer is hot not only because of the
extended daylight hours but also because the Sun
is high in the northern hemisphere’s sky
• As a result, sunlight strikes the ground at a nearly
perpendicular angle that heats the ground efficiently
• This situation reverses six months later

Sept
21
June
21

Dec
21
March
31

• The Sun appears to
trace out a circular path
called the ecliptic on
the celestial sphere tilted
at 23 ½ degrees to the
equator
• The ecliptic and the
celestial equator
intersect at only two
points
• Each point is called an
equinox
• The point on the ecliptic
farthest north of the
celestial equator that
marks the location of the
Sun at the beginning of
summer in the northern
hemisphere is called the
summer solstice
• At the beginning of the
northern hemisphere’s
winter the Sun is farthest
south of the celestial
equator at a point called
the winter solstice

Landmarks on the Earth’s surface are
marked by the Sun’s position in the sky
throughout the year

The Moon helps to cause precession, a
slow, conical motion of Earth’s axis of
rotation

Precession causes the gradual change of the
star that marks the North Celestial Pole

Astronomical observations led to the
development of the modern calendar
•
•
•
•

The day is based on the Earth’s rotation
The year is based on the Earth’s orbit
The month is based on the lunar cycle
None of these are exactly the same as
nature so astronomers use the average
or mean day and leap years to keep the
calendar and time consistent

The different types of ‘year’.
• The sidereal year (year with respect to
the stars) measured in solar time is
365d 6h 9m 10s (365.2564d) in length.
• The tropical year (successive passages
of the Sun through the Vernal Equinox)
is 365d 5h 48m 46s (365.2422d) in length.
• Due to precession, the tropical year is
20m 24s shorter than the sidereal year.

Calendars
• Caesar introduced the 365.25 days calendar
and thus the Leap Year (February 29)
• However, this is 11m 14s longer than the real
tropical year. This accumulates to 3 days in
4 centuries error.
• To correct, October 4 was followed by
October 15, in 1562 and the century rule was
invoked.


Slide 38

Knowing the Heavens

Chapter Two

Naked-eye astronomy had an important place
in ancient civilizations
• Positional astronomy
– the study of the positions of objects in the sky and
how these positions change

• Naked-eye astronomy
– the sort that requires no equipment but human
vision

• Extends far back in time
–
–
–
–

British Isles Stonehenge
Native American Medicine Wheel
Aztec, Mayan and Incan temples
Egyptian pyramids

Eighty-eight constellations cover the
entire sky
• Ancient peoples looked
at the stars and
imagined groupings
made pictures in the
sky
• We still refer to many of
these groupings
• Astronomers call them
constellations (from
the Latin for “group of
stars”)

Modern Constellations
• On modern star charts,
the entire sky is divided
into 88 regions. Each
is a constellation
• Most stars in a
constellation are
nowhere near one
another
• They only appear to be
close together because
they are in nearly the
same direction as seen
from Earth

The appearance of the sky changes during the
course of the night and from one night to the next
• Stars appear to rise
in the east, slowly
rotate about the
earth and set in the
west.
• This diurnal or daily
motion of the stars
is actually caused
by the 24-hour
rotation of the earth.

Animation of constellation movement
• To represent what we have just discussed,
follow this animation from the vantage
point of our Californian observer.

Annual Motion
• The stars also appear
to slowly shift in position
throughout the year
• This is due to the orbit
of the earth around the
sun
• If you follow a particular
star on successive
evenings, you will find
that it rises
approximately 4
minutes earlier each
night, or 2 hours earlier
each month

It is convenient to imagine that the stars are
located on a celestial sphere
• The celestial sphere is
an imaginary object
that has no basis in
physical reality
• However it is still a
model that remains a
useful tool of positional
astronomy
• Landmarks on the
celestial sphere are
projections of those on
the Earth

•Celestial equator
divides the sky into
northern and southern
hemispheres
•Celestial poles are
where the Earth’s axis
of rotation would
intersect the celestial
sphere
•Polaris is less than 1°
away from the north
celestial pole, which is
why it is called the
North Star or the Pole
Star.
•Point in the sky directly
overhead an observer
anywhere on Earth is
called that observer’s
zenith.

The Celestial Coordinate System
• Again, let us see what we have just
determined in a more 3-dimension manner.

Positional astronomy plays an important
role in keeping track of time
• Apparent solar time is based on the apparent
motion of the Sun across the celestial sphere,
which varies over the course of the year
• Mean solar time is based on the motion of an
imaginary mean sun along the celestial
equator, which produces a uniform mean
solar day of 24 hours
• Ordinary watches and clocks measure mean
solar time
• Sidereal time is based on the apparent
motion of the celestial sphere

• Local noon is defined to be when the Sun crosses the
upper meridian, which is the half of the meridian
above the horizon

Sidereal and Solar Days
• Appreciating the difference between a solar
day and a sidereal day is a challenging
concept. See if this helps.

Circumpolar
stars

• At any time, an observer can see only half of the celestial sphere
• The other half is below the horizon, hidden by the body of the Earth

Last thought on coordinates
• The Equatorial System of Coordinates is what most
astronomers use when specifying the location of an
object on the Celestial Sphere
• Right Ascension (measured eastwards from the
Vernal Equinox) goes from 0h to 24h
• Declination (measured north or south from the
celestial equator goes from -90° to +90 °.
• The hour angle (HA) of an object is the angle
between the meridian on which the object is situated
and the (observer’s) celestial meridian
• ST = RA + HA

The seasons are caused by the tilt of
Earth’s axis of rotation
• The Earth’s axis of rotation is not
perpendicular to the plane of the Earth’s
orbit
• It is tilted about 23½° away from the
perpendicular & is called the obliquity.
• The Earth maintains this tilt as it orbits
the Sun, with the Earth’s north pole
pointing toward the north celestial pole

The Earth’s orbit

• Seasons do NOT arise from the
distance the Earth is from the Sun but
rather as a result of the Earth’s annual
motion and axial inclination – the tip
of our planet with respect to its orbital
plane. As we move around the Sun,
the orientation of our planet gives us
seasons.

Seasons
• During part of the year the northern hemisphere of
the Earth is tilted toward the Sun
• As the Earth spins on its axis, a point in the
northern hemisphere spends more than 12 hours in
the sunlight
• The days there are long and the nights are short,
and it is summer in the northern hemisphere and
winter in the southern hemisphere
• The summer is hot not only because of the
extended daylight hours but also because the Sun
is high in the northern hemisphere’s sky
• As a result, sunlight strikes the ground at a nearly
perpendicular angle that heats the ground efficiently
• This situation reverses six months later

Sept
21
June
21

Dec
21
March
31

• The Sun appears to
trace out a circular path
called the ecliptic on
the celestial sphere tilted
at 23 ½ degrees to the
equator
• The ecliptic and the
celestial equator
intersect at only two
points
• Each point is called an
equinox
• The point on the ecliptic
farthest north of the
celestial equator that
marks the location of the
Sun at the beginning of
summer in the northern
hemisphere is called the
summer solstice
• At the beginning of the
northern hemisphere’s
winter the Sun is farthest
south of the celestial
equator at a point called
the winter solstice

Landmarks on the Earth’s surface are
marked by the Sun’s position in the sky
throughout the year

The Moon helps to cause precession, a
slow, conical motion of Earth’s axis of
rotation

Precession causes the gradual change of the
star that marks the North Celestial Pole

Astronomical observations led to the
development of the modern calendar
•
•
•
•

The day is based on the Earth’s rotation
The year is based on the Earth’s orbit
The month is based on the lunar cycle
None of these are exactly the same as
nature so astronomers use the average
or mean day and leap years to keep the
calendar and time consistent

The different types of ‘year’.
• The sidereal year (year with respect to
the stars) measured in solar time is
365d 6h 9m 10s (365.2564d) in length.
• The tropical year (successive passages
of the Sun through the Vernal Equinox)
is 365d 5h 48m 46s (365.2422d) in length.
• Due to precession, the tropical year is
20m 24s shorter than the sidereal year.

Calendars
• Caesar introduced the 365.25 days calendar
and thus the Leap Year (February 29)
• However, this is 11m 14s longer than the real
tropical year. This accumulates to 3 days in
4 centuries error.
• To correct, October 4 was followed by
October 15, in 1562 and the century rule was
invoked.