1. © 2010 Pearson Education, Inc. Discovering the Universe for Yourself

2. © 2010 Pearson Education, Inc. What does the universe look like from Earth? With the naked eye, we can see more than 2,000 stars as well as the Milky Way.

3. © 2010 Pearson Education, Inc. Apparent motion = Sun, Moon, stars appear to move counter-clockwise East to West Rise in East Set in West Rotation - Apparent 3

4. © 2010 Pearson Education, Inc. Why do stars rise and set? Earth rotates west to east, so stars appear to circle from east to west. 4

5. © 2010 Pearson Education, Inc. Earth orbits the Sun (revolves) once every year: at an average distance of 1 AU ≈ 150 million km. with Earth’s axis tilted by 23.5º (pointing to Polaris) and rotating in the same direction it orbits, counter- clockwise as viewed from above the North Pole.

6. © 2010 Pearson Education, Inc. Constellations A constellation is a region of the sky. 88 constellations fill the entire sky. ~12 constellations along the ecliptic (Zodiac, and technically 13)

7. © 2010 Pearson Education, Inc. Annual Motion – Apparent As the Earth orbits the Sun, the Sun appears to move eastward along the ecliptic. At midnight, the stars overhead are opposite the Sun in the sky.

8. © 2010 Pearson Education, Inc. The Milky Way A band of light making a circle around the celestial sphere. What is it? Our view into the plane of our galaxy.

9. © 2010 Pearson Education, Inc. The Milky Way

10. © 2010 Pearson Education, Inc. The Local Sky An object’s altitude (above horizon) and direction (along horizon) specifies its location in your local sky

11. © 2010 Pearson Education, Inc. We measure the sky using angles Full circle = 360º 1º = 60 (arcminutes) 1º = 60 (arcminutes) 1 = 60  (arcseconds) 1 = 60  (arcseconds)

12. © 2010 Pearson Education, Inc. The Reason for the Seasons

13. © 2010 Pearson Education, Inc. CLOSER means MORE right?

14. © 2010 Pearson Education, Inc. CLOSER means MORE right? Heat –The closer you are the hotter it is

15. © 2010 Pearson Education, Inc. CLOSER means MORE right? Heat –The closer you are the hotter it is Sound –The closer you get, the louder it is

16. © 2010 Pearson Education, Inc. CLOSER means MORE right? Heat –The closer you are the hotter it is Sound –The closer you get, the louder it is Light –The closer you get, the brighter it is

17. © 2010 Pearson Education, Inc. Complete this statement! When the Sun is ______ it is summer, and when the Sun is _______ it is winter.

18. © 2010 Pearson Education, Inc. Complete this statement! When the Sun is CLOSER it is summer, and when the Sun is FARTHER it is winter.

19. © 2010 Pearson Education, Inc. When the Sun is high in the sky, the amount of direct sunlight received is greater. This results in SUMMER

20. © 2010 Pearson Education, Inc. When the Sun is low in the sky, the amount of direct sunlight received is less. This results in WINTER

21. © 2010 Pearson Education, Inc. When the Sun is low in the sky, the amount of direct sunlight received is less. This results in WINTER When the Sun is high in the sky, the amount of direct sunlight received is greater. This results in SUMMER

22. © 2010 Pearson Education, Inc. What causes the seasons?

23. © 2010 Pearson Education, Inc. Summary: The Real Reason for Seasons Earth’s axis points in the same direction (to Polaris) all year round, so its orientation relative to the Sun changes as Earth orbits the Sun. Summer occurs in your hemisphere when sunlight hits it more directly; winter occurs when the sunlight is less direct. AXIS TILT is the key to the seasons; without it, we would not have seasons on Earth.

24. © 2010 Pearson Education, Inc. Why doesn’t distance matter? Variation of Earth-Sun distance is small — about 3%; this small variation is overwhelmed by the effects of axis tilt.

25. © 2010 Pearson Education, Inc. How do we mark the progression of the seasons? We define four special points: summer solstice winter solstice spring (vernal) equinox fall (autumnal) equinox

26. © 2010 Pearson Education, Inc. We can recognize solstices and equinoxes by Sun’s path across sky: Summer solstice: Highest path, rise and set at most extreme north of due east. Winter solstice: Lowest path, rise and set at most extreme south of due east. Equinoxes: Sun rises precisely due east and sets precisely due west.

27. © 2010 Pearson Education, Inc. Seasonal changes are more extreme at high latitudes Path of the Sun on the summer solstice at the Arctic Circle

28. © 2010 Pearson Education, Inc. The changing phases of the Moon inspired the concept of the month

29. © 2010 Pearson Education, Inc. Phases of the Moon: 29.5-day cycle Waxing Moon visible in afternoon/evening Gets “fuller” and rises later each day Waning Moon visible in late night/morning Gets “less full” and sets later each day

30. © 2010 Pearson Education, Inc. Although the Moon is always ½ lit by the Sun, we see different amounts of the lit portion from Earth depending on where the Moon is located in its orbit.

31. © 2010 Pearson Education, Inc. Moon is illuminated (always ½) by Sun We see a changing combination of the bright and dark faces as Moon orbits the Earth Animations at links from our website

32. © 2010 Pearson Education, Inc. We see only one side of Moon Synchronous rotation: the Moon rotates exactly once with each orbit That is why only one side is visible from Earth

33. © 2010 Pearson Education, Inc.

34. What causes eclipses? The Earth and Moon cast shadows. When either passes through the other’s shadow, we have an eclipse.

35. © 2010 Pearson Education, Inc. When can eclipses occur? Lunar eclipses can occur only at full moon. Lunar eclipses can be penumbral, partial, or total.

36. © 2010 Pearson Education, Inc. Solar eclipses can occur only at new moon. Solar eclipses can be partial, total, or annular.

37. © 2010 Pearson Education, Inc. Why don’t we have an eclipse at every new and full moon? –The Moon’s orbit is tilted 5° to ecliptic plane… –So we have about two eclipse seasons each year, with a lunar eclipse at new moon and solar eclipse at full moon.

38. © 2010 Pearson Education, Inc. Summary: Two conditions must be met to have an eclipse: 1.It must be full moon (for a lunar eclipse) or new moon (for a solar eclipse). AND 2. The Moon must be at or near one of the two points in its orbit where it crosses the ecliptic plane (its nodes).

39. © 2010 Pearson Education, Inc. Predicting Eclipses Eclipses recur with the 18 yr, 11 1/3 day saros cycle, but type (e.g., partial, total) and location may vary.

40. © 2010 Pearson Education, Inc. Planets Known in Ancient Times Mercury –difficult to see; always close to Sun in sky Venus –very bright when visible; morning or evening “star” Mars –noticeably red Jupiter –very bright Saturn –moderately bright