Motion of the Moon

1. Motion of the Moon

2. Discussion What is the ecliptic?

3. Discussion You are in Cleveland and facing north, describe how the stars move through the course of a night?

4. Discussion You are in Cleveland and facing south, describe how the stars move through the course of a night?

5. Discussion You are on the equator and facing north, describe how the stars move through the course of a night?

6. Discussion You are on the equator and facing west, describe how the stars move through the course of a night?

7. Which planet has most the extreme seasons?

8. Discussion If the Earth’s rotation axis were exactly perpendicular to the ecliptic, would we still experience seasons?

9. Discussion What are the two reasons most directly responsible for it being warmer in the summer than in the winter?

10. Discussion Another student tells you that the seasons are caused the Earth being closer to the Sun in the summer and farther from the Sun in the winter. What evidence could you use to refute this claim?

11. The arctic circle The arctic circle is the northern latitude at which on the summer solstice the Sun never sets and on the winter solstice the Sun never rises above the horizon.

13. Discussion The tropics mark the farthest points north and south where the Sun can appear at the zenith. What is the latitude of the tropics?

14. Discussion What is the latitude of the arctic circle?

15. Discussion From Cleveland on the vernal equinox, describe how the sun moves through the sky during the course of a day. (Where does it rise, where does it cross the meridian, and where does it set.)

16. Discussion From Cleveland on the first day of summer, describe how the sun moves through the sky during the course of a day.

17. Discussion From Cleveland on the first day of winter, describe how the sun moves through the sky during the course of a day.

19. Precession: a complication The Earth’s rotation axis is not fixed in space over long periods of time

21. The Earth’s rotation rate is nearly constant, its speed is not The speed at which points on the Earth’s equator are moving is larger than points on the Earth at higher latitudes. At the equator you would be moving at 1,650 km/hr, while at the north pole you would not be moving at all, just rotating around a point.

22. Discussion Why is Kennedy Space Center in Florida and not in Maine?

23. Florida is closer to the equator and a space shuttle just sitting on the launch pad is moving about 1,550 km/hr. If we moved the launch pad to Maine, the space shuttle sitting on the launch pad is only moving 1,275 km/hr or about 275 km/hr less than in Florida. To launch the space shuttle in Maine would require more fuel to accelerate the shuttle the extra 275 km/hr.

24. Equatorial Bulge The extra velocity at the equator pushes the matter out at the equator. The same thing happens on a merry-go-round. The closer you are to the edge, the faster you will be moving, and the greater the force pushing you off.

25. The Earth is not a perfect sphere The Earth bulges out at its equator, that is, its diameter measured along its equator is 43 km larger than its diameter from the north to the south pole.

26. Oblateness

27. Precession

28. The Sun and Moon apply a torque to the Earth Because the Earth is not a perfect sphere and its rotation axis is tilted 23.5 degrees from its orbital plane, the Sun and the Moon pull on the extra mass in Earth’s equatorial bulge and try to straighten out the tilt.

29. Precession This off axis force, or torque on the Earth causes Earth’s rotation axis to vary slightly, or precess, over long periods of time. Thus, the celestial poles trace out a circle against the stars over a period 26,000 years.

31. Discussion The Great Pyramid at Giza has a tunnel which points toward the north celestial pole. At the time the pyramid was built, around 2600 BCE, toward which star did it point?

32. Precession of the Equinoxes If the celestial poles change with time, so to must the celestial equator, which is 90 degrees away from the poles. If the celestial equator changes with time, than the intersection of ecliptic and the celestial equator will also change with time.

34. Age of Aquarius? The “age” is defined by the constellation the Sun is in on the vernal equinox. Currently, it is the age of Pisces.

35. Discussion Why is the changing of the equinox position important?

36. Tropical year The length of our calendar year is defined by the time it takes between two vernal equinoxes. This is called the tropical year and is 365 days 5 hours 48 minutes and 46 seconds long.

37. Sidereal year However, because the equinox positions slowly changes, this is not the Earth’s actual orbital period around the Sun. The actual orbital period is called the sidereal year and is 365 days, 6 hours, 9 minutes and 10 seconds, or 20 minutes 24 seconds longer.

38. Leap Years Notice that our tropical year is not an even number of days. The extra 5 hours, 48 minutes and 46 seconds is almost, but not quite ¼ of a day. To make up for this, every 4 years we have to add an extra day to our calendar.

39. What is the difference? If we used the sidereal year for our calendar year the seasons would slowly migrate through the different months over time. Spring would come one day earlier every 70 years.

40. Astronomical coordinates Because of precession and the changing position of the celestial poles and the vernal equinox, celestial coordinates of astronomical objects change from year to year.

42. Astrology Precession breaks the cause and effect relationship between the position of the stars and seasonal changes.

44. The Moon’s motion Diurnal motion – rises in the east crosses the meridian and sets in the west every day. Monthly motion – moves eastward with respect to stars moving its diameter in about an hour.

45. 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.

46. 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?

47. 1st Quarter Moon

48. 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.

49. 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.