Transcript Chapter 16 Earth in Space The Shape of the Earth • The Earth is not a perfectly round sphere as believed by the.

Chapter 16
Earth in Space
The Shape of the Earth
• The Earth is not a perfectly round sphere as believed by
the ancient Greeks.
• It is flattened at the poles and has an equatorial bulge,
as do many other planets.
• The distance around is larger around the equator than
around the poles. This is called an oblate shape.
• It is not a perfectly symmetrical oblate shape. The North
Pole is slightly higher and the South Pole is slightly lower
than the average surface.
• It is not perfectly circular around the equator, there is a
lump in the Pacific Ocean and a depression in the Indian
Ocean.
Earth’s motions
1)
2)
3)
Yearly revolution around Sun
Daily rotation on axis
Slow clockwise wobble on axis-Precession
Earth follows the path of a gigantic
helix., moving at a fantastic speed
as it follows the Sun and the galaxy
through space.
Fig. 16.11
A spinning top wobbles as it spins, and the axis of the top traces out a small circle. The
Wobbling of the axis is called precession.
http://www.geocities.com/astrologyages/precession.htm
North Pole
North Pole
Fig. 16.12
The slow, continuous precession of the Earth’s axis results in the North Pole pointing
around a small circle over a period of about 26,000 years.
Revolution around the Sun
• The average distance between the Earth and
the Sun is about 93 million miles.
• The Earth’s movement around the Sun is called
a revolution.
• It takes an average of one year for one complete
circuit.
• Earth’s orbit around the Sun is slightly elliptical,
and it moves with a speed that varies. It moves
fastest when it is closer to the Sun in January
and moves slowest when it is furthest from the
Sun in early July.
Plane of the Ecliptic
• The plane (2-dimensional shape) of the Earth’s orbit
around the Sun is called the Plane of the Ecliptic.
Even though the Earth is about 3 million miles closer to the Sun in January
than in July, and the January Sun is apparently 3% larger than the
July Sun, this effect is not as important as some of the other relationships
that we will examine, so winter occurs in the Northern Hemisphere when
the Earth is closest to the Sun.
Pink-Direction of
motion of Earth’s axis
Rotation or Spinning
• The Earth rotates or spins around an
imaginary line called an axis which goes
through the Earth’s poles.
• This axis is not at a 90o angle
(perpendicular) to the plane of the ecliptic.
It is at a 66.5o angle.
• The axis points towards the Sun for 6
months out of the year and away from the
Sun for the other 6 months.
Effect of the Earth’s Motions
• The inclination and orientation of the axis,
together with the Earth’s rotation and
revolution combine to produce the
following effects:
1. days and nights that vary in length.
2. changing seasons
3. climates that vary with latitude
(north/south)
Effect of the Earth’s Motions
• When the North Pole is pointed towards the Sun it receives sunlight
for a full 24 hours, and the South Pole is in Earth’s shadow for a full
24 hours.
• This is summer in the Northern hemisphere, on June 21 or 22, with
the longest daylight periods and the Sun at its maximum noon height
in the sky.
Effect of the Earth’s Motions
• Six months later, on December 22 or 23, the orientation
is reversed with winter in the Northern Hemisphere, the
shortest daylight periods, and the Sun at its lowest noon
height in the sky.
Solstices
• The summer solstice occurs on or about June 22 in the
Northern Hemisphere.
• The beginning of the summer season.
• The sun has its highest altitude in the sky. (Largest
distance from the Sun)
Summer Solstice
Solstices
• The winter solstice occurs on or about December 22 in
the Northern Hemisphere.
• The beginning of the winter season.
• The sun has its lowest altitude in the sky. (Shortest
distance from the Sun)
Winter Solstice
Equinoxes
• The spring or vernal equinox occurs on or about
March 21.
• It identifies the beginning of the spring season.
Spring Equinox
Equinoxes
• The autumnal equinox occurs on or about September
23.
• It identifies the beginning of the fall season.
Autumnal Equinox
Earth’s Revolution
Spring or Vernal Equinox, all points on
Earth have 12 hours of daylight.
Earth’s Revolution
Autumnal equinox, all points on Earth
have 12 hours of daylight.
The length of daylight during each season is determined by the Earth’s axis and the
Line between the Earth and the Sun.
At the equinoxes, the axis is perpendicular to the line between the Earth and the Sun.
Day and night are of equal length everywhere.
At the summer solstice, the North Pole points towards the Sun and is completely
out of the shadow for a 24 hour day.
The situation is reversed for the South Pole. At the winter solstice the North Pole is in
the shadow for a 24 hour night
http://daphne.palomar.edu/jthorngren/tutorial.htm
http://www.astro.uiuc.edu/~kaler/celsph.html
Identifying Place
• A system of 2 straight lines can be used to
identify a point, or position, on a flat, 2
dimensional surface:
Identifying Place
• A system of curved lines can also be used
to identify place on the surface of the
earth.
• The earth’s axis provides the north-south
reference point.
• The equator is a big circle around the
earth that is exactly halfway between the
two ends, or poles, of the rotational axis.
A circle that is parallel to the equator is used to specify a position
north or south of the equator. A few possible ones are shown here.
These lines are called parallels. The same degree numbers are repeated
going south from the equator.
Identifying Place
• Each parallel is the same distance
between the equator and one of the poles
all the way around the earth.
• The distance from the equator to a point
on a parallel is called the latitude of that
point.
• Latitude tells us how far north or south a
point is from the equator by telling you the
parallel the point is located on.
If you could see to the earth’ center, you would see that latitudes run from 0o at
the equator north to 90 o at the North Pole (or to 90o at the South Pole).
Identifying Place
• To identify a location you need another line, this time one
that runs pole to pole and perpendicular to the parallels.
• These north-south running arcs that intersect at both
poles are called meridians.
• There is not a naturally occurring, identifiable meridian
that can be used as a point of reference such as the
equator serves for parallels, so one is identified as the
referent by international agreement.
• The referent meridian is the one that passes through the
Greenwich Observatory near London, England. This
meridian is called the prime meridian.
• The distance from the prime meridian east or west is
called the longitude.
The degrees of longitude of a point on a parallel are measured to the east or to
The west from the prime meridian up to 180o.
If you could see inside the earth you would see 360o around the equator and 180o
of longitude east and west of the prime meridian.
Identifying Place
• New Orleans has a latitude of about 30oN of the
equator and a longitude of about 90oW of the
prime meridian. The location of New Orleans is
therefore described as 30oN, 90oW.
• Locations are more precisely identified by
dividing each degree of latitude into subdivisions
of 60 minutes (60’) per degree and each minute
into 60 seconds (60’’).
• Latitudes near the equator are referred to as low
latitudes and near the poles as high latitudes.
Identifying Place
• In addition to the equator (0o) and the poles (90o), the
latitudes of 23.5oN and 23.5oS from the equator are
important references for climatic considerations.
• 23.5oN is called the tropic of Cancer and 23.5oS is
called the tropic of Capricorn. These two parallels
identify the limits towards the poles within which the Sun
appears directly overhead during the course of a year.
• The parallel of 66.5oN is called the Arctic circle, and
66.5oS is called the Antarctic circle. These two parallels
identify the limits toward the equator within which the
Sun appears above the horizon all day during the
summer. This starts with six months of daylight every day
at the pole, then decreases as you get fewer days of full
light until reaching the limit of one day of 24-hour
daylight at the 66.5o limit.
At the summer solstice, the noon Sun appears directly overhead at the tropic
of Cancer (23.5oN) and 24 hrs. of daylight occurs north of the Arctic circle (66.5oN).
At the winter solstice, the noon Sun appears overhead at the tropic of Capricorn
(23.5oS) and 24 hrs of daylight occurs south of the Antarctic circle (66.5oS).
23.5oN is called the tropic of Cancer and 23.5oS is called the tropic of Capricorn.
The parallel of 66.5oN is called the Arctic circle, and 66.5oS is called the
Antarctic circle.
Measuring Time
• The Babylonians originated the technique of using
astronomical motions for keeping time about four
thousand years ago.
• They marked the yearly journey of the Sun against the
background of the stars, and it was divided into 12
periods, or months, after the signs of the zodiac.
• It was divided also into 360 days and they invented the
week and divided the day into hours, minutes, and
seconds.
• The week was identified as a group of 7 days, each
based on one of the 7 heavenly bodies that were known
at the time.
• The hours, minutes, and seconds of a day were
determined from the movement of the shadow around a
straight vertical rod.
Measuring Time
• As seen above the North Pole, the Earth
rotates counterclockwise, turning toward
the east.
• On the earth, this motion causes the Sun
to appear to rise in the east, travel across
the sky, and set in the west.
• A day is defined as the length of time
required for the earth to rotate once on its
axis.
Measuring Time
• Imagine the Earth as the ancient Greek
did, fixed in space as the Sun circled
around it.
• Then you can picture the Sun as going
east to west around the Earth.
The standard time zones. Hawaii and most of Alaska are two hours
Earlier than Pacific Standard Time.
Going Westward, you gain a day
Going Eastward, you lose a day
The international date line follows the 180o meridian in a way that land areas
and island chains have the same date.
Length of a Year
• A year is generally defined as the interval of time
required for Earth to make one complete revolution in its
orbit.
• There are different definitions of what is meant by a year.
The most common definition of a year is the interval
between two consecutive spring equinoxes. This is
known as a tropical year and it is 365.24220 mean solar
days.
• The sidereal year is defined as the interval of time
required for Earth to move around its orbit so the Sun id
again in the same position relative to the stars. It is
slightly longer than the tropical year, by 20 minutes.
Length of a Year
• It is the tropical year that is used as a standard time
interval to determine the calendar year. In this time
period Earth does not complete an exact number of turns
on its axis while completing one trip around the Sun so it
is necessary of periodically adjust the calendar so it
stays in step with the seasons.
• The calendar system initially designed to stay in step
with the seasons was devised by the ancient Romans.
• Julius Caesar reformed the calendar to have 365 day
years with a 366 year (leap year) every fourth year. This
is called the Julian calendar and it takes into account
that a tropical year, the name given to the calendar year,
actually has 365.24220 days, very close to 365 ¼ days.
Length of a Year
• The Julian calendar was very close to the one used now,
except the year began in March, the month of the spring
equinox.
• The month of July was named in honor of Julius Caesar
and the following month was later named after Augustus
Caesar.
• The Julian calendar year was longer than the tropical
year by .0078 days per year (11minutes, 14 seconds), so
in 1582 Pope Gregory XIII revised the calendar and it
was corrected for 10 days even though the error was on
14 days. This resulted in the Gregorian calendar, the
system used today.
• The accumulated error is almost .75 days per century, so
every four centuries there will be an error of 3 days. As a
result the leap year is dropped every three out of four
century years. 2000 was a leap year but 2100, 2200 and
2300 will not be leap years.
The sun
Illuminates
the not visible
side of the
Moon.
The Phases of
the Moon
The sun
Illuminates
the visible
side of the
Moon.
Half of the Moon is always lighted by the Sun and half is always in the shadow.
The Moon phases result from the view of the lighted and dark parts as the Moon
revolves around the Earth. The period of time between two consecutive phases,
such as new moon to new moon, is the synodic month, or about 29.5 days.
Eclipses
• The event of Earth’s or Moon’s shadow falling on
the other body is called and eclipse.
• Most of the time eclipses don’t occur because
the plane of the Moon’s orbit is inclined to the
Earth’s orbit (about 5o). The shadow from the
Moon or the shadow from Earth usually falls
above or below the other body, too high or too
low to produce an eclipse.
• An eclipse occurs only when the Sun, Moon, and
Earth are in a line with each other.
The plane of the Moon’s orbit is inclined to the plane of the Earth’s orbit by about
5o. An eclipse occurs only when the two planes meet intersect and the Earth, Moon,
and Sun are in a line.
Solar Eclipse
People in a location where the tip of the umbra falls on the surface of the Earth
see a total eclipse.
People in locations where the penumbra falls on the Earth’s surface see a partial
solar eclipse.
Solar Eclipse-Occurs once every several hundred years
And then lasting for less than 7 minutes.
Lunar Eclipse occurs when there is a Full Moon.
A lunar eclipse is visible to everyone on the dark side of the Earth.
These eclipses last for hours.
Review Exercises
• Applying the Concepts p. 420-421:
# 1, 3, 4, 5, 6, 7, 8, 11, 12, 17, 18, 20, 21
New Book: p. 460-463 # 1, 2, 3, 4, 5, 6, 9,
10, 18, 21, 23, 24, 25, 26, 27, 28, 31, 32,
37, 38, 41, 42, 46, 47.
Review Chapter 16
• Shape of the Earth-Oblate
• 3 types of Earth’s motions:
revolution, rotation, wobble
(precession).
• Revolution of Earth around the
Sun-Slowest and further from the
Sun in July, faster and closer to the
Sun in January.
• Plane of the ecliptic.
• The Earth’s axis is not at a 90o
angle. It points towards the Sun
only 6 months out of the year.
• Effects of the inclination and
orientation of the axis and the
rotation of the Earth: days and
nights that vary in length, changing
seasons, climates that vary with
latitude (north/south).
• Solstices and equinoxes-length of
days and nights.
• Equator, parallels and latitudes.
• Meridians, prime meridian,
longitudes.
• Identifying place with latitude and
longitude.
• Significance of tropic of cancer
(23.5oN), tropic of capricorn
(23.5oS), arctic circle (66.5oN) and
antarctic circle (66.5oS).
• Earth rotates counterclockwise,
sun rises east and sets west. 24
hrs for a full rotation.
• International date line (180o
meridian) and time zones. You gain
time going west.
• Phases of the Moon and why they
are.
• Solar eclipses and lunar eclipses.