Motion of the Moon

1. Motion of the Moon

2. Review Question Describe the path the Sun will take across the sky on the day of the autumnal equinox as viewed from the Earth’s equator.

3. Review Question Describe the path the Sun will take across the sky on the day of the autumnal equinox as viewed from the Earth’s north pole.

4. Review Question Describe the path the Sun will take across the sky from a latitude of 40 degrees north on the day of the autumnal equinox.

5. Review Question Describe the path the Sun will take across the sky from a latitude of 40 degrees north on the day of the summer solstice.

6. Review Question Describe the path the Sun will take across the sky from a latitude of 40 degrees north on the day of the winter solstice.

7. Sky as seen from 40 degrees north latitude

9. Review Question What is the latitude of the tropic of Cancer?

10. Review Question What is the latitude of the artic circle?

11. Review Question What is the highest the Sun can get above the southern horizon at a latitude of 30 degrees north?

12. Review Question Why is there a difference between the tropical year and the sidereal year?

13. Review Question What causes the Earth to precess?

14. The Moon As we pass through the month, the appearance of the Moon changes each night. We say the Moon goes through a series of phases.

15. Discussion Many people believe incorrectly that the phases of the Moon are caused by the shadow of the Earth on the Moon. What simple observation of the Moon could you make to prove that they are wrong?

16. First quarter Moon

17. The Earth is round The “shadow” on a first or last quarter Moon is a straight line dividing the face of the Moon in half. The Earth is round and will always cause a curved shadow on the Moon. When the shadow of the Earth does fall on the Moon we call this a lunar eclipse.

18. The real reason The Moon’s phases are actually cause by the changing position of the Moon with respect to the Sun. The side of the Moon facing the Sun is always fully illuminated (except during a lunar eclipse). But because the angle we view the Moon differs from that of the Sun, we see varying degrees of the Moon’s surface facing us illuminated.

20. New Moon Occurs when the Moon is in the same part of the sky as the Sun. Thus the Moon is up during the day, and the illuminated side of the Moon faces away from the Earth. The Moon is not visible when it is new.

21. Waxing crescent When less than ½ of the Moon’s face is illuminated. It is the western side of the Moon’s face that is illuminated and the Moon is visible in the sky before Sunset. Waxing means increasing.

22. First Quarter When exactly ½ of the Moon’s face is illuminated. It is the western side of the Moon’s face that is illuminated and the Moon is up before sunset. Although ½ of the Moon’s face is illuminated it is called a quarter because the Moon is ¼ of the way through it’s cycle and occurs about one week after the new phase. The Moon is 90 degrees away from the Sun in the sky.

23. Waxing Gibbous When more than ½ of the Moon’s western face is illuminated.

24. Full Moon when the Moon’s face is fully illuminated. At this time the Moon is directly opposite the Sun. Occurs about 2 weeks after the new phase.

25. Waning Gibbous when more than ½ of the Moon’s face is illuminated. But now it is the eastern face that is illuminated and waning means decreasing.

26. Last Quarter when ½ of the Moon’s face is illuminated. Again it is the eastern half that is illuminated and the Moon is visible after sunrise. Occurs about 3 weeks after the new phase.

27. Waning Crescent When less than ½ of the eastern half of the Moon’s face is illuminated.

28. Discussion If the Moon is in its full phase on March 21st and the Sun sets at 6:00 P.M. what time does the Moon rise? What time does the Moon reach the meridian? What time does the Moon set?

29. The full Moon is exactly opposite the Sun or 180 degrees away in the sky. Astronomers would measure this angle in hours and say it is 12 hours from the Sun. If sunset is at 6:00 P.M. the full Moon rises at 6:00 P.M., crosses the meridian at midnight and sets at sunrise at 6:00 A.M., 12 hours different from the Sun.

30. Discussion If the Moon is in its last quarter phase on March 21st and the Sun sets at 6:00 P.M. what time does the Moon rise? What time does the Moon reach the meridian? What time does the Moon set?

31. The last quarter Moon is 90 degrees to the west of the Sun in the sky, or 6 hours before the Sun. If sunset is at 6:00 P.M. and sunrise is at 6:00 A.M. (we are at the equinox) the last quarter Moon rises at midnight, crosses the meridian at 6:00 A.M and sets at noon.

32. Discussion Suppose it is the first day of autumn in the northern hemisphere. What phase is the Moon if the Moon is located at the position of the vernal equinox?

33. Discussion Suppose it is the first day of autumn in the northern hemisphere. What phase is the Moon if the Moon is located at the position of the vernal equinox? If the Moon is at the position of the vernal equinox it must be 180 degrees from the Sun and therefore full.

34. Discussion Suppose it is the first day of autumn in the northern hemisphere. What phase is the Moon if the Moon is located at the position of the winter solstice? Explain.

35. Discussion Suppose it is the first day of autumn in the northern hemisphere. What phase is the Moon if the Moon is located at the position of the summer solstice? Explain.

36. Discussion Suppose it is the first day of autumn in the northern hemisphere. What phase is the Moon if the Moon is located at the position of the summer solstice? Explain. The Moon is 90 degrees behind the Sun. Because the Sun is moving eastward relative to the stars, the Moon is 90 degrees to the west of the Sun and therefore in 3rd or last quarter phase.

37. What time is it in this drawing?

38. What time is it in this drawing?

39. Discussion The Moon goes through its phases in 29.53 days. This is called the synodic month, i.e. the month relative to the Sun. The actual orbital period of the Moon is 27.32 days, the sidereal month. Why is there a difference in the synodic and sidereal months?

40. Synodic and Sidereal Month’s

41. Discussion The Moon always keeps the same side facing the Earth. Does this mean the Moon does not spin on its axis? Explain?

44. Synchronous rotation Orbital period is the same as the rotational period

45. Discussion What is the mean solar day on the Moon? What is the sidereal day on the Moon?

46. Mean solar day on the Moon 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.

47. 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 an eclipse.

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

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

50. 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?