Patterns in the Sky

1. Patterns in the Sky Solar System Astronomy Chapter 2

2. Motions in the Sky • The Sun and stars show regular patterns of motion which • reflect the motion of the Earth. • are clues to how the Solar System works. • Our focus: • Daily rotation of the Earth • Annual orbit of the Earth • Monthly orbit of the Moon

3. Frames of Reference • An observer sees things from a frame of reference. • On the Earth, different observers see • different parts of the sky. • different motions of the stars. • This shows the Earth is a sphere and rotates daily.

4. Constellations • Constellations are arbitrary patterns of stars in the sky. • During the year, the Sun passes in front of some constellations. • The annual path of the Sun is called the ecliptic. • The constellations along the ecliptic are called the zodiac.

5. Constellations & such… Sagittarius Scorpius Pisces Aries Pegasus Boötes Libra Andromeda Leo Orion Ursa Major Canis Major Taurus SummerTriangle Argo Crux Centaurus

6. The Celestial Sphere • It helps to think of the sky as the celestial sphere. • The stars, Sun, and planets are actually at different distances. • The celestial sphere rotates around the north and south celestial poles each day. • Midway between the poles is the celestial equator.

7. Celestial Sphere and Ecliptic

8. Celestial Sphere, Ecliptic, &c

9. At the North Pole • An observer would see half the celestial sphere. • The north celestial pole is overhead. • The stars rotate counterclockwise in about 24 hours. • No star rises or sets: all are circumpolar.

10. At the Equator • All stars rise and set. • The celestial poles are on the northern and southern horizons. • Observers can see the whole celestial sphere as it rotates.

11. In Between (like Radford) • One pole is above the horizon. • The angle to the horizon equals the latitude. • Some stars are circumpolar; others rise and set. • Some of the celestial sphere is never visible.

12. Rotation of the Earth • The motion of the celestial sphere reflects the daily spin of the Earth. • The rotation of the Earth sets the spin of hurricanes (Coriolis effect). • Storms spin counterclockwise in northern hemisphere, clockwise in the southern.

13. Annual Motion of the Sky • The changing position of the Sun reflects the Earth’s orbit around the Sun. • As the Earth’s position changes, the Sun is seen against different constellations.

14. The Earth’s Axis & Rotation • The Earth’s axis is not perpendicular to the ecliptic plane. • 23.5˚ • Sun is alternately north and south of the celestial equator during the year. • Produces seasons • receive more direct sunlight in the summer, and Sun is up longer

15. The Earth’s Axis & Rotation

16. Special Days of the Year • Apparent Motion of the Sun • Summer solstice: Sun farthest north. • Autumnal equinox: Sun on the equator, moving southward. • Winter solstice: Sun farthest south. • Spring (vernal) equinox: Sun on the equator, moving northward.

17. The Earth Spins and Revolves

18. Precession • Gyroscopic Effect • The location of the celestial poles slowly shifts. • Currently the north celestial pole is near the bright star Polaris. • This reflects a 26,000-year wobble in the Earth’s spin axis.

19. Precession • Gyroscopic Effect • The location of the celestial poles slowly shifts. • Currently the north celestial pole is near the bright star Polaris. • This reflects a 26,000-year wobble in the Earth’s spin axis.

20. the poor, dull-witted astrologers… • Astrology developed in Babylonia • ~2500 years ago • Precession of the equinoxes unknown • Aries if born March 21 to April 20 • Sun is in Aries April 19 to May 13

21. Moon’s Orbit: Eclipses & Phases

22. Orbital Period of the Moon

23. Orbital Period of the Moon • Moon takes 27.3 days to orbit once with respect to the stars • sidereal period • Lunar phases repeat every 29.5 days • synodic period • The Moon’s rotation period equals its sidereal period • As a result, we always see the same hemisphere of the Moon • Or do we??

24. Libration

25. Eclipses • Types: • Solar eclipse: Moon passes in front of Sun. • Lunar eclipse: Moon enters Earth’s shadow. • Solar eclipses happen at new moon. • Lunar eclipses happen at full moon. • We don’t get eclipses every month: Moon’s orbital plane is at an angle to the ecliptic plane.

26. Eclipses

27. Lunar Eclipse

28. Lunar Eclipse

29. Lunar Eclipse • Can last up to 1 hour 40 min • Red?

30. Lunar Eclipse • Typically 1-2 each year • Fairly common

31. Solar Eclipse

32. Solar Eclipse • Sun & Moon are same angular size…

33. Solar Eclipse

34. Solar Eclipse • About 1/year – rare. Applause

35. Last look at lunar cycle