Discovering The Universe for Yourself

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Transcript Discovering The Universe for Yourself

Discovering The Universe for
Patterns in the Sky
• Constellations: Recognizable patterns in the
sky are called Constellations.
• There are 88 Constellations. 12 are based on
the Zodiac.
– Orion, Leo, Aries, Taurus, etc.
• Asterisms: Recognizable patterns of stars that
are not one of the constellations are called
– Big Dipper, Little Dipper, Summer Triangle
The Celestial Sphere
• Are stars still present in the sky during the
day time?
– Yes, we just can’t see them because their dim
light is overwhelmed by the brightness of the
sun during the day.
The Milky Way
The band of light called the “Milky Way” traces the galactic plane
as it appears from our location in the outskirts of the galaxy.
© Jerry Schad
The Circling Sky
• Earth rotates from
west to east.
• The celestial sphere
appears to rotate east
to west.
Apparent path of a
particular star
The Dome of the Sky:
Definitions in your local
Angular Sizes
• We can measure angles in the sky
• The angular size of an object like the Sun or the
Moon is the angle it appears to span in your field
of view.
• The angular distance between a pair of objects is
the angle that appears to separate them.
Angular size of
moon ~ ½ o
Angular distance between the
“pointer stars” ~ 5o
• Latitude : Measures positions north or south
of the equator. (Note: This is an angle)
– 0o = the equator.
– 90o N= the north pole.
– 90o S = the south pole.
• Longitude : Measures positions east or west
of the Prime Meridian. (This is an angle)
• The Prime meridian is an imaginary line that
passes through Greenwhich, England.
Latitude and
• In the Northern Hemisphere, stars near the
North Celestial Pole are always above the
horizon. Therefore, these stars are called
Circumpolar Stars
• In the Northern Hemisphere, stars near the
south celestial pole remain below the
horizon and so are never seen by a northern
• In the Southern Hemisphere, stars near the
north celestial pole are never visible to the
southern observer.
Different sets of constellations are visible in
northern and southern skies.
Counter-Clockwise Rotation
Northern Hemisphere
Southern Hemisphere
The Altitude of the celestial pole (Polaris) = your latitude
Altitude and Latitude
• For Nacogdoches, Texas:
– Latitude: 31o 45’ 35.3” = 31.7598o
– Longitude: -94o 39’ 40.5” = -94.6613o
• Question: What is the Altitude of Polaris for
• Answer: 31o 45’ 35.3” – the same as our
Seasonal Changes in the Sky
• The night-time constellations change with
the seasons.
• This is due to the Earth’s orbit around the
The Sun’s Path Through The Daytime Sky Changes With Seasons
• The sun traces out a characteristic figure 8 pattern over the course of
a year at the same time of the day. This “Figure 8” is called an
Composite photograph
•Images taken always at same
time of day. (8:30 am. EST)
•Pictures taken at 10 day
intervals over an entire year.
Dennis di Cicco
Path of sunrise for a
particular date.
Seasons- Geocentric View
The apparent path of the
Sun on the celestial
sphere, showing how the
seasons arise.
Figure from Astronomy Today, 3rd ed.,
Chaisson & McMillan, Prentice-Hall, ©1999
Right Ascension and Declination
• Right Ascension (RA): Analogous to
longitude, but on the celestial sphere.
– It is the east-west angle between the vernal
equinox and a location on the celestial sphere.
• Declination (dec): Analogous to latitude,
but on the celestial sphere.
– It is the north-south angle between the celestial
equator and a location on the celestial sphere.
Units of R.A.
360o = 24h
The Moon, Our Constant Companion
• As the Moon moves through the sky, both
its appearance and the time at which it rises
and sets change with the cycle of Lunar
Lunar Phases Demonstration
The Moon
The Earth
Lunar Phases
Note: Look at top view to understand rise and set times.
•We always see the same face of the Moon.
•The Moon rotates as it orbits the Earth.
•The rotation rate is synchronized with the orbital period.
•This is called a “Synchronous Orbit”.
• In order for an Eclipse to occur:
• The nodes of the Moon’s orbit must be
nearly aligned with the Earth and the Sun.
• The phase of the Moon must be either new
or full.
The Ecliptic Plane is represented by the surface of the
Lunar Eclipse
Solar Eclipse
Solar Eclipse
Nodes occur at points where
the orbit crosses the surface
of the pond
Geometry of a Lunar Eclipse
(Sunlight is partially blocked)
(Sunlight is fully blocked)
Three Types of Lunar Eclipses
Three Types of Solar Eclipses
Multiple Exposure Photograph of Progression
of a Solar Eclipse
Totality- lasts only a
few minutes.
The Solar Corona
becomes visible.
How often do Eclipses Occur?
• There are two periods each year when the
nodes of the Moon’s orbit are nearly aligned
with the Sun.
• These are called Eclipse Seasons.
• The combined effect of the changing dates of
eclipse seasons and the 29.5 day lunar cycle,
makes eclipses recur about every 18 years.
• This 18 year cycle is called the Saros Cycle.
Solar Eclipses from
2001 to 2022
Colors represent successive Saros Cycles.
The Ancient Mystery of the Planets
Retrograde Motion
• Apparent Retrograde Motion
• A period during which a planet appears to
move westward relative to the stars is called
a period of Apparent Retrograde Motion.
• Prograde (Direct) Motion
• The usual eastward movement of the planet
is called Prograde or Direct Motion.
Retrograde Motion for Jupiter
Retrograde Motion
Prograde or Direct Motion
What Causes Retrograde
• It is a result of Stellar Parallax.
– Parallax is the apparent shift of the position of a
nearby object with respect to a more distant
background object.
– Do pen/pencil against background wall demo.
• That is, an apparent shift of the planet’s
position relative to the background stars.
Retrograde Motion Demonstration
• Explanation of
apparent retrograde
motion using the
principle of
• Follow the lines of
sight from Earth to
Mars in numerical
Stellar Parallax
More distant
stars are used as
the background
for measuring
End of Section