The Planets Prof. Geoff Marcy Prof. Michael Manga

1. The Seasons Phases of the Moon Eclipses Constellations The “Rotating Sky” The Planets Prof. Geoff Marcy Prof. Michael Manga © 2005 Pearson Education Inc., publishing as Addison-Wesley

2. Announcements • Read Chapters 1 & 2 . • Discussion Sections meet this week. • Homework Assignment: Posted on Web. • 5 Problems / Questions • Due: Friday, Feb 1 • Turn in: • Basement of Campbell Hall or Discussion Section. © 2005 Pearson Education Inc., publishing as Addison-Wesley

3. Telescope Observations Project Two Parts 1. Make Telescope Observations of two objects Suggestions: Saturn, Mars, Orion Nebula Telescope Hours: Tue & Thu 7-8 pm, 7th floor of Campbell Hall Sketch both objects on 1/2 sheet of paper. Note Date and Time. Mark the position of Mars with a dot, at three times during the Semester, early, middle, late. (Use either map.) Note date of each observation. © 2005 Pearson Education Inc., publishing as Addison-Wesley

4. Facing South (West is to your Right) 7-9pm Saturn . Mars . Sirius Brightest Star © 2005 Pearson Education Inc., publishing as Addison-Wesley

5. Pleiades . Mars Taurus . . Orion Belt South © 2005 Pearson Education Inc., publishing as Addison-Wesley

6. Astronomical ObservationsYou’ve Already Made • Sun rises in east, goes overhead, sets in west • Moon and Stars also rise in east, set in west • In Summer: Sun rises higher & Stays up longer. • Seasons occur regularly, every year • Moon goes through phases (new, crescent, quarter, gibbous, full, ) • Tides are sometimes high, sometimes low • Separate Phenomena? • Is there a simple theory that connects these observations ? © 2005 Pearson Education Inc., publishing as Addison-Wesley

7. Constellations: • A Pattern or grouping of stars. • Official borders set by the IAU*. • *International Astronomical Union © 2005 Pearson Education Inc., publishing as Addison-Wesley

8. Constellations • No physical significance! Stars that appear close together may lie at very different distances. . • Useful only as • rough landmarks. • Names come from • Ancient Arabs and Greeks. • Some southern hemisphere • constellations were named by • European explorers in the 17th & 18th centuries. . © 2005 Pearson Education Inc., publishing as Addison-Wesley

9. The Celestial Sphere North Celestial Pole . • The sky above looks like a dome…a hemisphere. • Imaginary “celestial sphere” • This a 2-dimensional representation of the sky (missing distances !) • Earth at center of this sphere. Celestial Equator North Celestial Pole Celestial Equator © 2005 Pearson Education Inc., publishing as Addison-Wesley

10. The Celestial Sphere North Celestial Pole Near North Star North & South celestial poles: The points in the sky directly above the Earth’s North and South poles. . Celestial equator: The extension of the Earth’s equator onto the celestial sphere Ecliptic: Sun’s path against the star background Orbital plane of Earth © 2005 Pearson Education Inc., publishing as Addison-Wesley

11. Earth’s Annual Motion • Earth orbits the Sun. Sun appears to move eastward with respect to the stars. • The Sun appears to circle the celestial sphere once every year. Sun appears in Aquarius on Feb 21 . . Earth © 2005 Pearson Education Inc., publishing as Addison-Wesley

12. The Milky Way Band of light across the sky. What are we actually seeing? © 2005 Pearson Education Inc., publishing as Addison-Wesley

13. The Milky Way © 2005 Pearson Education Inc., publishing as Addison-Wesley

14. The Milky Way • Our Galaxy is shaped like a disk • (with spiral arms). • Our solar system is in that disk. • Band of Light: • Looking along that disk. © 2005 Pearson Education Inc., publishing as Addison-Wesley

15. Specify any location in the sky Specify two Angles: Altitude and Direction Angle (N, E, S, W)along the horizon © 2005 Pearson Education Inc., publishing as Addison-Wesley

16. The Daily Motion • Earth rotates: the sky appears to rotate in the opposite direction. • The sky appears to rotate around the N (or S) celestial poles. • If you are standing at the poles, nothing rises or sets. • If you are standing at the equator, everything rises & sets 90 to the horizon. • Quiz: Sun rises in East. Earth’s rotation carries you toward the east or the west ? East © 2005 Pearson Education Inc., publishing as Addison-Wesley

17. The Daily Motion • The altitude of the celestial pole = [your latitude: 37o]. • All stars located at an angle less than your latitude away from: • your celestial pole never set. (circumpolar) • the other celestial pole is never seen by you. • Other stars, (& Sun, Moon, planets) rise in East and set in West You are Here. © 2005 Pearson Education Inc., publishing as Addison-Wesley

18. The Daily Motion Daily circles of Stars --- CCW looking north, W N E Looking North Looking South © 2005 Pearson Education Inc., publishing as Addison-Wesley

19. Time Exposure Photograph Exposure Time: 1 min 30 min 2 hr 4 hr 2. Latitude on Earth = ? 20 deg 40 60 80 Which direction did stars move ? © 2005 Pearson Education Inc., publishing as Addison-Wesley

20. Seasons What is the cause of the seasons on Earth? • Earth’s axis is tilted • Earth’s distance from Sun varies during the year • Sun’s brightness varies during the year • The Greenhouse effect varies during the year Observation: When summer in northern hemisphere, it’s winter in southern hemisphere. © 2005 Pearson Education Inc., publishing as Addison-Wesley

21. The Earth orbits around the Sun once every year. Elliptical Orbit (not circle) The Earth’s axis is tilted by 23.5º! © 2005 Pearson Education Inc., publishing as Addison-Wesley

22. Annual Motion • The Earth’s axis is tilted 23.5° from being perpendicular to the ecliptic plane (Earth’s orbital plane). • So, the celestial equator is tilted 23.5° to the ecliptic. • As seen from Earth, the Sun spends 6 months north of the celestial equator and 6 months south of the celestial equator. • Seasons are caused by the Earth’s axis tilt, not the distance from the Earth to the Sun! © 2005 Pearson Education Inc., publishing as Addison-Wesley

23. The Cause of the Seasons © 2005 Pearson Education Inc., publishing as Addison-Wesley

24. Axis tilt causes uneven heating by sunlight throughout the year. © 2005 Pearson Education Inc., publishing as Addison-Wesley

25. Seasonal Change in Sun’s Altitude • The “Figure 8” shows Sun at same time each day over a year. © 2005 Pearson Education Inc., publishing as Addison-Wesley

26. When is summer? • Summer solstice occurs June 21: The first day of summer. • It takes time for the more direct sunlight to heat up the land and water. • Therefore, July & August are typically hotter than June. © 2005 Pearson Education Inc., publishing as Addison-Wesley

27. Why distance to the Sun Doesn’t Matter for Earth : • Small variation for Earth — about 3% • Distance does matter for some other planets, notably Mars, Mercury, and Pluto). © 2005 Pearson Education Inc., publishing as Addison-Wesley

28. Precession of Earth’s Axis • The Earth’s axis precesses (wobbles) like a top, once about every 26,000 years. • Precession changes the positions in the sky of the celestial poles and the equinoxes. • Polaris won't always be the north star. • The spring equinox, seen by ancient Greeks in Aries, moves westward and is now in Pisces! © 2005 Pearson Education Inc., publishing as Addison-Wesley

29. The Moon • Why do we see phases of the Moon? • What conditions are necessary for an eclipse? © 2005 Pearson Education Inc., publishing as Addison-Wesley

30. Phases of the Moon: 29.5 day cycle • new • crescent • first quarter • gibbous • full • gibbous • last quarter • crescent Waxing (getting bigger) Full Waning: (Getting Smaller) © 2005 Pearson Education Inc., publishing as Addison-Wesley

31. Why do we see phases? • Half the Moon illuminated by Sun and half dark • We see some combination of the bright and dark faces © 2005 Pearson Education Inc., publishing as Addison-Wesley

32. Phases of the Moon © 2005 Pearson Education Inc., publishing as Addison-Wesley

33. Eclipses Solar Eclipse: Moon blocks the Sun • Sun is behind the moon: • Only at new moon You are in the Moon’s shadow. within umbra: total solar eclipse within penumbra: partial solar eclipse Lunar eclipse: Earth blocks Sunlight to Moon • Only at full moon © 2005 Pearson Education Inc., publishing as Addison-Wesley

34. Solar Eclipse © 2005 Pearson Education Inc., publishing as Addison-Wesley

35. Lunar Eclipse © 2005 Pearson Education Inc., publishing as Addison-Wesley

36. Eclipses • The Earth & Moon cast shadows. • When either passes through the other’s shadow, we have an eclipse. • Why don’t we have an eclipse every full & new Moon? Sun © 2005 Pearson Education Inc., publishing as Addison-Wesley

37. Eclipse Predictions • Eclipses recur in the approx. 18 yr, 11 1/3 day saros cycle • But even then, eclipse location and type (e.g., partial, total) may vary © 2005 Pearson Education Inc., publishing as Addison-Wesley

38. Mysterious Backward Motion of Planets • Why do planets sometimes seem to move backwards relative to the stars? © 2005 Pearson Education Inc., publishing as Addison-Wesley

39. 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 © 2005 Pearson Education Inc., publishing as Addison-Wesley

40. Apparent retrograde motion — try it yourself! © 2005 Pearson Education Inc., publishing as Addison-Wesley

41. Retrograde Motion • Like the Sun, planets usually appear to move eastward relative to the stars. • But as we pass them by in our orbit, they move west relative to the stars for a few weeks or months. • Noticeable over many nights; on a single night, a planet rises in east and sets in west… © 2005 Pearson Education Inc., publishing as Addison-Wesley

42. Parallax Angle Apparent shift of a star’s position due to the Earth’s orbiting of the Sun The nearest stars are much farther away than the Greeks thought. So the parallax angles of the star are so small, that you need a telescope to observe them. © 2005 Pearson Education Inc., publishing as Addison-Wesley

43. Patterns and Motions in the Sky • The Circling Night Sky Overhead • the rotation of the Earth about its axis • The Reason for Seasons • Tilt of Earth’s axis as Earth orbits around Sun • “Precession” of the Earth’s Axis • the wobbling of Earth’s axis • The Moon • Orbits around the Earth • Phases of the Moon: We see it illuminated by Sunlight > Eclipses of the Moon and Sun day year month moonth © 2005 Pearson Education Inc., publishing as Addison-Wesley

44. Possible reasons why stellar parallax was undetectable: • Stars are so far away that stellar parallax is too small for naked eye to notice • Earth does not orbit Sun; it is the center of the universe Unfortunately, with notable exceptions like Aristarchus, the Greeks did not think the stars could be that far away, and therefore rejected the correct explanation (1)… Thus setting the stage for the long, historical showdown between Earth-centered and Sun-centered systems. © 2005 Pearson Education Inc., publishing as Addison-Wesley

45. A Universe in motion • Contrary to our perception, we are not “sitting still.” • We are moving with the Earth. • and not just in one direction The Earth rotates around it’s axis once every day! © 2005 Pearson Education Inc., publishing as Addison-Wesley

46. What have we learned? • What is a constellation? • A constellation is a region of the sky. The sky is divided into 88 official constellations. • What is the celestial sphere? • An imaginary sphere surrounding the Earth upon which the stars, Sun, Moon, and planets appear to reside. • Why do we see a band of light called the MilkyWay in our sky? • It traces the Galactic plane as it appears from our location in the Milky Way Galaxy. © 2005 Pearson Education Inc., publishing as Addison-Wesley

47. What have we learned? • Describe the basic features of the local sky. • The horizon is the boundary between Earth and sky. The meridian traces a half circle from due south on your horizon, through the zenith (the point directly overhead), to due north on your horizon. Any point in the sky can be located by its altitude and direction. • How does the sky vary with latitude? • As the celestial sphere appears to rotate around us each day, we see different portions of the paths of stars from different latitudes. The altitude of the celestial pole (north or south) is the same as your latitude (north or south). © 2005 Pearson Education Inc., publishing as Addison-Wesley

48. What have we learned? • Why are some stars above the horizon at all times? • All stars appear to make a daily circle. Circumpolar stars are those for which their entire daily circles are above the horizon, which depends on latitude. • What is the cause of the seasons on Earth? • As the Earth orbits the sun, the tilt of the axis causes different portions of the Earth to receive more or less direct sunlight at different times of year. The two hemispheres have opposite seasons. The summer solstice is the time when the northern hemisphere gets its most direct sunlight; the winter solstice is the time when the southern hemisphere gets its most direct sunlight. The two hemispheres get equally direct sunlight on the spring and fall equinoxes. © 2005 Pearson Education Inc., publishing as Addison-Wesley

49. What have we learned? • Why are the warmest days typically a month after the beginning of summer? • The summer solstice is usually considered the first day of summer, but the warmest days come later because it takes time for the more direct sunlight to heat up the ground and oceans from the winter cold. • How does the night sky change through the year? • The visible constellations at a particular time of night depend on where the Earth is located in its orbit around the Sun. • What is the Earth’s cycle of precession? • A roughly 26,000 year cycle over which the earth’s axis traces a cone as it gradually points to different places in space. © 2005 Pearson Education Inc., publishing as Addison-Wesley

50. What have we learned? • Why do we see phases of the Moon? • At any time, half the Moon is illuminated by the Sun and half is in darkness. The face of the Moon that we see is some combination of these two portions, determined by the relative locations of the Sun, Earth, and Moon. • What conditions are necessary for an eclipse? • An eclipse can occur only when the nodes of the Moon’s orbit are nearly aligned with the Earth and the Sun. When this condition is met, we can get a solar eclipse at new moon and a lunar eclipse at full moon. © 2005 Pearson Education Inc., publishing as Addison-Wesley