Ch Movements of the Moon

From earth, moon appears to orbit the earth, but from space, earth and moon orbit each other as they go around the sun. Balance point of the earth-moon system is located within the earth’s interior, because earth is 80 times as massive as the moon.

Lunar Orbit and Rotation Lunar orbit is an ellipse instead of a circle, so the moon’s distance to the earth varies. Apogee—when moon is farthest from the earth. Perigee—when moon is closest to the earth. Avg. distance from earth to moon is 340,000 km.

Earth’s rotation causes moon to appear to rise and set. It rises and sets approximately 50 minutes later each night. Moon’s rotation is gravitationally locked to earth’s gravity, so earth observers always see the same side of the moon.

Moon wobbles slightly on its axis, so earth observers can see about 59% of moon’s surface over a period of time.

Eclipses Occur when one planetary body passes through the shadow of another. Two parts to a shadow, the inner umbra, in which the sunlight is completely blocked, and the penumbra, the outer part of the shadow in which sunlight is only partially blocked.

Solar Eclipses Occur when the moon is between the earth and sun, and the moon’s shadow falls on the earth. People within the umbra see a total solar eclipse. People within the penumbra see a partial solar eclipse.

Moon’s shadow is small, so the eclipse covers only a small part of the earth. Earth’s rotation causes the shadow to move rapidly across its surface. Max. duration of a total solar eclipse is seven minutes at any one location.

If umbra is too short to reach the earth (at or near apogee), a ring- shaped annular eclipse occurs, in which the sunlight isn’t completely blocked out, and a thin ring of sunlight is visible.

Lunar Eclipses Occur when earth is between sun and moon, and earth’s shadow blocks out the lighted half of the moon. Total lunar eclipse—moon passes completely through earth’s umbra. Partial lunar eclipse—only part of the moon passes through earth’s umbra.

Penumbral eclipse—moon passes through earth’s penumbra only. Lunar eclipses may last several hours.

Frequency of Eclipses Solar and lunar eclipses occur almost equally often. Lunar eclipses can be seen by everyone on the dark side of the earth. Solar eclipses can only be seen by observers in the small path of the earth’s shadow.

Orbit of moon is tilted at a 5 degree angle to the plane of the earth and sun. Eclipses can only occur when the moon crosses the plane of earth’s orbit, but the earth, moon and sun also have to be correctly lined up at this time.