The Effects of Precession What changes do you expect in these things (or phenomena) 13,000 years later when the Earth’s rotation axis is pointed toward Vega? World Atlas (map of Earth)? Star chart (map of sky)? Seasons? Constellations?  Answer

The Night Sky What we observe in the sky during the night… Stars move across the sky during the night… Some move faster Some move very little One doesn’t seem to move at all! The positions of stars with respect to each others seem to be fixed… Star patterns repeat itself about every 24 hours… Star patterns do not change much from night to night. Star patterns in the winter are different from that in the summer. Positions of stars (with respect to the horizon and the zenith) in the sky are different at different locations (latitudes) on Earth. Some ‘stars’ seem to wander around with respect to other stars (you have to be real patient and careful to see this). The motion of the Moon is quite different from the other celestial objects. The appearance and motion of the moon is a subject by itself.

Fixed Star Patterns—The Constellations Star patterns appear fixed, because… Stars are very, very far away from Earth? Earth and the stars are not moving with respect to each other? The fixed star patterns are assigned names… There are a total of 88 constellations—defined by IAU (International Astronomical Union) in 1928. The stars in the same constellation are usually not physically close to each other.  Answer

The Night Sky What we observe in the sky during the night… Stars move across the sky during the night… Some move faster Some move very little One doesn’t seem to move at all! The positions of stars with respect to each others seem to be fixed… Star patterns repeat itself about every 24 hours… Star patterns do not change much from night to night. Star patterns in the winter are different from that in the summer. Positions of stars (with respect to the horizon and the zenith) in the sky are different at different locations (latitudes) on Earth. Some ‘stars’ seem to wander around with respect to other stars (you have to be real patient and careful to see this). The motion of the Moon is quite different from the other celestial objects. The appearance and motion of the moon is a subject by itself.

Why are star patterns in the winter different than those in the summer?

Summer Sky Your horizon Your Location Sun Earth Day Side Night Side

Summer Sky Earth Sun Day Side Night Side Your horizon Your Location---24 hours later Earth Sun Day Side Night Side

Winter Sky Due to Earth’s revolution around the Sun, we look at different part of the sky during different season… Your horizon Your Location Earth Sun Night Side Day Side

If we see different part of the sky during different season, how come we can see the Polaris and nearby region in the sky all night low and all year round? Draw pictures similar to the previous two to show how this can happen…

The Zodiac Constellations As the Earth orbits the Sun, the Sun appears to move eastward along the ecliptic The constellations along the ecliptic are the zodiac constellations The zodiac constellation at any given date is the constellation behind the Sun in the sky, not the one opposite the Sun, or the one you see at midnight Your birthday is August 21st, so your constellation is Leo. Can you go out and find constellation Leo in the night of your birthday? Click on image to start animation

All-Sky View What is the bright band with dark clouds across the sky? Milky Way Galaxy! All-sky image of the southern hemisphere. Photo by Wei-Ho Wang http://www.ifa.hawaii.edu/%7Ewang/ Northern hemisphere all-sky view from Mauna Kea

The Night Sky What we observe in the sky during the night… Stars move across the sky during the night… Some move faster Some move very little One doesn’t seem to move at all! The positions of stars with respect to each others seem to be fixed… Star patterns repeat itself about every 24 hours… Star patterns do not change much from night to night. Star patterns in the winter are different from that in the summer. Positions of stars (with respect to the horizon and the zenith) in the sky are different at different locations (latitudes) on Earth. Some ‘stars’ seem to wander around with respect to other stars (you have to be real patient and careful to see this). The motion of the Moon is quite different from the other celestial objects. The appearance and motion of the moon is a subject by itself.

The Portion of Sky We See Depends on Where We Are On Earth—Northern Hemisphere Your horizon You would not see these stars because they are under your horizon Your Location Day Side Night Side Sun Earth

If You Live Down Under You would not see these stars because they are under your horizon Your horizon Day Side Night Side Your Location Sun Earth Your horizon

Why Do We See Different Sky at Different Latitude?

White Night at North Pole Arctic Circle During arctic summer, the rotation axis of the Earth is tilted toward the Sun, and you can still see the Sun at midnight if you are within the arctic circle—The Sun never set! You cannot see the sun at midnight if you are here

The Night Sky What we observe in the sky during the night… Stars move across the sky during the night… Some move faster Some move very little One doesn’t seem to move at all! The positions of stars with respect to each others seem to be fixed… Star patterns repeat itself about every 24 hours… Star patterns do not change much from night to night. Star patterns in the winter are different from that in the summer. Positions of stars (with respect to the horizon and the zenith) in the sky are different at different locations (latitudes) on Earth. Some ‘stars’ seem to wander around with respect to other stars (you have to be real patient and careful to see this). The motion of the Moon is quite different from the other celestial objects. The appearance and motion of the moon is a subject by itself.

Do Stars Move? Stellar Parallax Because of Earth’s orbit around the Sun, stars closer to Earth will appear to move in the sky with respect to the stars much further away… *The motions are exaggerated…

Parallax with Your Eyes Put your thumb up in front of you at the arm’s length. Look at your thumb alternatively with your left and right eye. Compare the position of the thumb with respect to the background (that’s sufficiently far away, like more than six feet). You should see the position of your thumb with respect to the background changes as you switch your eyes.

In using parallax method to measure the distance to other stars, we assumed that there are no appreciable relative motion between the Sun and the star we are measuring. However, as we mentioned in the previous chapter, the solar system is always moving with respect to the stars in the solar neighborhood, as well as everything else in the Milky Way galaxy. The apparent motion of stars in the sky can be cause by the relative motion between the stars and the Sun also. How do you distinguish the apparent motion caused by parallax and real motion of the stars? Apparent motion due to stellar parallax is periodical. The closer stars move back and forth with respect to more distant stars. Apparent motion due to real fast relative motion of the stars does not move back and forth in the sky. It will keep going in the same direction after six months.

The Planets We have used two motions of the Earth to explain the motion of the stars in the sky… The rotation of Earth with respect to distance star The revolution of Earth around the Sun We also talked about the precession of Earth’s rotation axis and its implication for the future. However, the motions of some of the brightest objects of the night sky–the planets–do not follow the pattern of most of the stars, and cannot be explained by considering these motions alone.

The Planets—The Wanderers Although the relative position between most of the stars are fixed, some of the brightest celestial objects have peculiar motion in the sky…they wander among the constellations. planets rise in the east and set in the west, just like the ‘regular’ stars. With respect to the stars, planets usually move eastward from night to night relative to the stars. You cannot see this motion on a single night, it is very slow. But sometimes they go westward for a few weeks or months. This is referred to as the apparent retrograde motion Why do the planets do that? Retrograde motion of Mars, June to November, 2003 Retrograde motion of Jupiter, 2004-2005

Ptolemaic Model Circle-upon-Circles Greek philosopher Claudius Ptolemy (A.D. 100-170) proposed a planet model in which the planets move around a small circle which in turns orbits the earth, pretty much like the moon is orbiting the Earth while the Earth is orbiting the Sun. This model can explain the retrograde motion of the planets, and make prediction about the position of the planets with a few degrees accuracy. However, it requires complicated motions (such as even smaller circles upon the small circles) to make the model well.

Retrograde Motion of Planets The retrograde motions of the planets are easily explained when we consider the orbital motion of both the Earth and the other planets—apparent retrograde motion occurs when we pass the other planet in the orbit. This model place Earth and the other planets on two concentric circles with the Sun at the center, and is capable of making precise prediction on the position of the planets. Only two circles are required, which is simpler than Ptolemy circle-upon-circle-upon-circle model. More importantly, this model can make accurate prediction of the motion of the planets. Click on the image to start animation

Summary—Movements of Stars Pattern in the Sky Star pattern repeats itself about every 24 hours… because of the rotation of Earth with respect to the distant stars! Star pattern in the winter is different from that in the summer… because of the revolution of Earth around the Sun! Stars do move back a nd forth (a teeny-tiny bit) in the sky over the course of the year also, due to the revolution of Earth around the Sun (stellar parallax). Note that this motion is not possible to observe without the help of modern telescopes. Position of stars is different at different location (latitude)… because at any given location on Earth, we can only see half of the sky. The retrograde motion of the planets are explained by considering the orbital motion of both the Earth and the other planet. The celestial north pole will be pointed in the direction of Vega 13,000 years later. The star pattern (constellation) will change over the course of hundred-of-thousand years, because the stars are not sitting still in the galaxy.

Fixed Star Patterns—The Constellations Star patterns appear fixed, because… Stars are very, very far away from Earth! Earth and the stars are not moving with respect to each other. Star patterns are assigned names… There are a total of 88 constellations—defined by IAU (International Astronomical Union) in 1928. The stars in the same constellation are usually not physically close to each other.  Back to Presentations

The Effects of Precession What changes do you expect in these things (or phenomena) 13,000 years later when the Earth’s rotation axis is pointed toward Vega? World Atlas (map of Earth)? No change. Star chart (map of sky)? Location of celestial north would be different Seasons? We will still have four seasons, since the tilt of the rotation axis of Earth with respect to the ecliptic plane is not changed. But the time of the season would be changed. Summer would be in January, and winter would be in July. Constellations? The patterns of stars in the sky would not change much…but their position with respect to celestial north would be different). How would the star chart be changed?  Back to Presentation