2. Synodic vs. sidereal month

3. If you see the Moon setting and it is full, what did the Moon look like when it was on the eastern horizon 12 hours earlier?

4. Discussion What is the length of the sidereal day on the Moon? Does this change as the speed of the Earth in its orbit changes?

5. Discussion What is the length of the solar day on the Moon?

6. 29.5 Earth days, the Sun rises in the east sets in the west 14.75 days later, followed by 14.75 days of darkness. The Moon’s sidereal day is its actual rotation (and revolution) period of 27.32 days. Mean solar day on the Moon

7. Eclipses If the Sun, Earth and the Moon line up so that the Moon’s shadow falls on the Earth or the Earth’s shadow falls on the Moon, we see and eclipse.

8. Solar eclipse Earth passes through the Moon’s shadow.

9. Lunar eclipse Moon passes through Earth’s shadow.

10. Discussion What phase must the Moon be in for a solar eclipse to occur? What phase must the Moon be in for a lunar eclipse to occur?

12. The Moon’s orbit The Moon’s orbit is tilted 5 degrees relative to the ecliptic. Thus the Moon spends ½ of its orbit above the ecliptic and ½ below. Because the ecliptic is the path of the Sun with respect to the stars, eclipses can only occur when the Moon lies in the ecliptic. Hence, the name ecliptic.

13. Discussion From a latitude of 42 degrees, how high above the southern horizon can the Moon get in the sky?

14. Discussion What time of the year can the full Moon be its highest above the horizon? Explain.

15. Discussion The Moon crosses the ecliptic twice each orbit or twice a month. Why don’t we get an eclipse twice a month?

18. Types of Solar eclipses Total – the Moon completely covers the surface of the Sun. Partial – the Moon covers only part of the Sun, i.e. the Moon and Sun are not perfectly aligned.

19. The parts of a shadow Penumbra – the outer, lighter part of the shadow, corresponds to the Sun’s surface being only partially covered Umbra – the central, darkest part of the shadow, corresponds to the entire disk of the Sun being covered

24. Types of Solar eclipses Annular – the Moon and Sun are perfectly aligned but the Moon is furthest away from Earth in its orbit and appears too small to completely cover the Sun, leaving the rim of the Sun exposed.

27. Lunar Eclipse

28. Discussion During a total lunar eclipse the Moon does not get completely dark. Instead it glows an orangish-red color. Why?

29. Just before sunrise ISS

30. Total Lunar eclipse July 1982 Why do you think the bottom of the Moon is lighter than the top?

31. Moonrise ISS

32. Total Lunar eclipse of July 16, 2000

33. The Motion of the Planets

34. The motions of the Planets Each planet follows the same diurnal motion as the Sun, Moon and stars, rising in the east and setting in the west each day. Like the Sun and Moon, each planet moves west to east with respect to the stars. This is called direct motion. In addition, all the planets stay close to the ecliptic.

35. Path of Mars

36. Discussion Suppose Mars is moving in retrograde motion and will rise at 8:00 PM. Since Mars is moving with retrograde motion, that means that during the night it will be moving west-to-east rather than east-to-west. So at 8 PM it will rise in the west and move across the sky and then set in the east Do you agree or disagree?

37. Two types of planets Superior planets – Mars, Jupiter, and Saturn Superior planets can appear in the sky at any time of the night. Inferior planets – Mercury and Venus Inferior planets always stay close to the Sun.

38. Discussion If we consider the Moon to be a planet, what type of planet would it be?

39. Discussion What is science? What is it scientists do?

40. The Scientific Method 1. Observations 2. Formulate a hypothesis – a conjecture that is used as a model for describing observations. 3. Use hypothesis to make predictions – a hypothesis which does not make testable predictions is of no use in science. 4. Test predictions by further observations

41. Occam’s Razor If two or more models encompass the observations equally well, the simplest model is preferred.

42. Science A collection of explanations (theories) that allow us to make predictions about the future.

43. Fallacy 1 Scientists seek to uncover the truth. The goals of science are actually more limited: to invent theories that make accurate predictions. Example: Creationism may be true, but it is not science!

44. Fallacy 2 Scientists have to always keep an open mind. Scientists almost always work with a particular hypothesis in mind. The question to answer is always do the observations confirm their hypothesis.

45. Fallacy 3 Science has facts which have validity that cannot be questioned. Instead the things we loosely refer to as facts are merely inferences from the observations. Examples: The universe is 13.7 billion years old.

46. Imagine you are in charge of creating the universe What is the simplest possible universe you could come up with, one where predicting the future is the easiest thing to do?

47. The simplest universe Nothing ever Changes! The Sun and Moon do not move in the sky There are no seasons The stars do not move in the sky The planets do not move in the sky

48. Second simplest universe Stars unchanging and fixed to the celestial sphere Celestial sphere rotates about Earth moving east to west once a day carrying the Sun, Moon and planets with it. The Earth is at the center of the universe and does not move, geocentric model

49. What is wrong with our second simplest universe? How can we fix it? Discussion

50. Third Simplest Universe Sun, Moon, and planets are carried east to west along with rotation of celestial sphere Sun, Moon and planets move more slowly at differing speeds west to east against celestial sphere

52. What is wrong with our third simplest universe? Discussion

53. Ptolemy’s explanation for retrograde motion Each planet moves on a small circle called and epicycle. The center of each epicycle moves along a larger circle centered near the Earth called a deferent.

57. Ptolemaic system Very successful at predicting positions of the planets but was not perfect Offered no explanation of why the planets moved on deferents and epicycles There was no relationship between period of revolution and epicycle size

58. Alternatives to Ptolemy’s model Aristarchus proposed a heliocentric model of the Solar System in the 3 rd century B.C.E. Reintroduced in the 16 th century by Copernicus

60. Heliocentric model Sun at the center Diurnal motion explained by rotation of the Earth All the planets including Earth revolve about the Sun in circular orbits with different speeds

61. Disadvantages of the heliocentric model Still required epicycles Was no better at predicting planetary positions No stellar parallax observed

63. Advantages of heliocentric model Provides natural explanation of retrograde motion. Provides natural explanation for the motion of mercury and Venus as inferior planets, i.e. their orbits are interior to that of the Earth. Provides a relationship between distance from Sun and orbital period. Planets further from the Sun took longer to complete an orbit.

64. Retrograde motion

65. Inferior planets

66. How could you explain the the motion of the inferior planets Mercury and Venus with deferents and epicycles in the geocentric model? Discussion

68. Synodic vs. Sidereal Periods of the Planets Sidereal = actual orbital period with respect to the stars Synodic = time to return to same configuration with respect to the Sun

71. Astronomical unit The average distance from the Sun to the Earth is called the astronomical unit (AU).