13. Earth’s Moon - using radar we find the distance to the Moon to be 384,000 km (this is the length of the orbit’s semi-major axis).

14. Its orbital eccentricity is 0.055, so perigee is 363,000 km and apogee is 405,000 km.

19. Its sidereal orbital period, one sidereal month, (the time it takes the moon to return to the same point in the sky relative to the stars) is 27.3 days.

20. Its synodic orbital period is based on the phases of the moon (from new moon to new moon). A synodic month is 29.5 days.

21. The moon’s orbit is prograde (counterclockwise as viewed from celestial north) and is inclined to the ecliptic only 5.2°. This inclination is the reason there are not solar and lunar eclipses each month.

22. The eccentricity of the moon’s orbit causes the apparent size of the Moon to change by 14% over the course of one month.

26. This difference in distance and apparent size is what makes annular solar eclipses possible. In an annular eclipse, the moon does not completely block out the sun. The sun forms a ring around the moon.

27. The Moon’s radius is about 1700 km (1/4 Earth’s radius).

30. Mass - 7.4 X 10 25 g (1/80 of Earth’s mass). The force of gravity is only 1/6 that of Earth’s. (Approx 1.6 m/s 2 )

31. Density -3.3 g/cm 3. Earth’s is 5.5 g/cm 3. (This means the moon has fewer heavy elements than the Earth.)

32. The Moon’s rotational and revolutionary periods are equal. This is a result of tidal forces from Earth’s gravitational pull on the moon.

33. The Moon’s orbit is said to be tidally locked to the Earth. Such an orbit is called a synchronous orbit.

34. As a result, only one side of the Moon always faces the Earth.

35. The Moon appears to wobble on its north-south axis during one month. This is because the Moon’s rotational speed is constant while its orbital speed varies depending on its distance from the Earth.

36. This rocking is called libration, and it permits 59% of the Moon’s surface to be seen from Earth.

38. The Moon has an equatorial bulge, like the Earth. But it is nearly 4 km, much larger than we would expect to be caused by the Moon’s slow rotation.

39. This is probably because the Moon rotated much faster in the past. The Moon might also have been molten at that time.

40. It is also possible that the Moon was closer to the Earth in the past causing tidal forces to be greater.

41. This would have caused the synchronization of the Moon’s orbit to occur more rapidly; this is supported by the fact that the bulge points directly toward the Earth.

42. Lunar Atmosphere - none. The Moon is 80 times less massive than the Earth, so the speed of gas particles is enough to exceed the escape velocity of 2.4 km/s.

43. Any atmosphere the Moon may have had in the past has been lost long ago.