The Ever-Changing Sky

1. The sky is constantly changing. We experience the day-night cycle every day. Night after night, the pattern of the stars seems identical, yet it changes with the seasons. The motions of some of the celestial objects don’t seem to follow that of the others… Why? How do we explain the motion of the celestial objects? The Ever-Changing Sky Check out the all-sky images at http://nightskylive.net/

2. Understanding What’s Going On Around Us In Chapter I., we have discussed in very broad terms what our universe looks like, what is in it, and how things are moving around, as if we can elevate ourselves to a vintage point outside Earth. In this chapter, we will move ourselves back to Earth. Knowing how Earth is moving in the universe, with respect to the distant stars, and with respect to the Sun, allows us to explain what we see on Earth in our sky, and how the environment (e.g., the four seasons) on Earth changes through time. In the next couple of classes, we will be talking about: Night Sky—Stars, Planets The Four Seasons Moon, its appearances Lunar and Solar Eclipses

3. Where are the stars during the day? • There are no other stars around the sky near the Sun. • The stars near the Sun are all too dim to be seen by either our eyes or the telescopes. • The Sun is too bright compared with distant stars. If we block Sun light, then we can see the stars. • The sky during the day is much brighter than the stars.  Answers

4. Why Can’t We See the Stars During the Day? • The daytime sky is bright because Earth’s atmosphere is illuminated by direct sunlight. The brightness of the stars are much less than the daytime sky. • If we go into space where there is no atmosphere, then we can see stars around the Sun, if the Sun is blocked out. Your horizon Your Location Sun Earth Day Side Night Side

5. Stars During the Day • The reason that we cannot see stars during the day is because of the existence of the atmosphere, which brightens up when illuminated by direct sunlight. • If we go into space, then we can see stars right next to the Sun (as long as you block sunlight), because there is no atmosphere in space. Movie from SoHO LASCO Instrument. Light from the Sun is blocked by the occulter. The bright radial structures are the

6. There are four seasons on Earth because… • The distance between the Earth and the Sun changes through the year. • Earth’s self-rotation axis is tilted with respect to the Earth’s rotation axis around the Sun. • The temperature of the Sun changes slightly through the year.  Answers

7. 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. Can we explain these phenomena?

8. Think about • Motion • Position • Orientation

9. The Celestial Sphere • The celestial sphere is an imaginary sphere on which all the celestial objects are located. However, the location of a celestial object on the celestial sphere only tells us about the orientation of the objects, but not the distant. Two stars located very close together in the sky may in fact very far apart because their distance to Earth are very different.

10. If you are at the north pole If you are at 45º latitude If you are on the equator The Dome of the Sky At any given location on Earth, we can always see half of the sky... • Where is the celestial north?

11. 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.

12. Earth’s Rotation and Revolution Around the Sun • The day-night cycle, and the seasonal change of the night sky pattern can be explained by • the rotation of the Earth, and • the revolution of the Earth around the Sun. • Here is a good animation of the Earth’s rotation and its revolution around the Sun on the web.

13. Sidereal Day — The Earth is rotating with respect to the distant stars with a 23 hours and 56 minutes period. Solar Day — The average time (24 hours) it takes the Sun to make one circle in the sky. Solar day is longer than sidereal day because of the revolution of the Earth around the Sun.. It takes a little extra rotation by Earth to catch up… The 24-hour Cycle

14. North Celestial Pole Trails of stars in the sky over Utah

15. Mauna Kea Sky North Celestial Pole Photo by Wei-Ho Wang http://www.ifa.hawaii.edu/%7Ewang/ Click on image to start animation

16. Precession of Earth’s Rotation Axis • Precession: The rotation of the rotation axis of Earth (with respect to distant stars) with respect to the rotation axis of Earth around the Sun. • The tilt of the Earth’s rotation axis with respect to the axis of Earth’s rotation around the Sun does not change because of precession. • The period of precession is ~ 26,000 years • About 13,000 year from now, Vega would be our north star. Click on the image to start animation

17. Where are the stars during the day? • There are no other stars around the sky near the Sun. • The stars near the Sun are all too dim to be seen by either our eyes or the telescopes. • The Sun is too bright compared with distant stars. If we block Sun light, then we can see the stars---this is true only in space. • The sky during the day is much brighter than the stars.  Back to Presentations

18. There are four seasons on Earth because… • The distance between the Earth and the Sun changes through the year. • Earth’s self-rotation axis is tilted with respect to the Earth’s rotation axis around the Sun. • The temperature of the Sun changes slightly through the year.  Back to Presentation