Ch. 30.2 Movements of the Ch. 30.2 Movements of the MoonMoon

From earth, moon appears to orbit From earth, moon appears to orbit the earth, but from space, earth and the earth, but from space, earth and moon orbit each other as they go moon orbit each other as they go around the sun.around the sun.

Balance point of the earth-moon Balance point of the earth-moon system is located system is located within the earth’s within the earth’s interiorinterior, because earth is 80 times as , because earth is 80 times as massive as the moon.massive as the moon.

Lunar Orbit and RotationLunar Orbit and RotationLunar orbit is an Lunar orbit is an ellipseellipse instead of a instead of a circle, so the moon’s distance to the circle, so the moon’s distance to the earth varies.earth varies.ApogeeApogee—when moon is farthest from —when moon is farthest from the earth.the earth.PerigeePerigee—when moon is closest to the —when moon is closest to the earth.earth.Avg. distance from earth to moon is Avg. distance from earth to moon is 340,000 km.340,000 km.

Earth’s rotation causes moon to Earth’s rotation causes moon to appear to rise and set.appear to rise and set.

It rises and sets approximately 50 It rises and sets approximately 50 minutes later each night.minutes later each night.

Moon’s rotation is gravitationally Moon’s rotation is gravitationally locked to earth’s gravity, so earth locked to earth’s gravity, so earth observers always see the same side observers always see the same side of the moon.of the moon.

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

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

Solar EclipsesSolar Eclipses

Occur when the moon is between the Occur when the moon is between the earth and sun, and the moon’s earth and sun, and the moon’s shadow falls on the earth.shadow falls on the earth.

People within the umbra see a People within the umbra see a total total solar eclipse.solar eclipse.

People within the penumbra see a People within the penumbra see a partial solar eclipse.partial solar eclipse.

Moon’s shadow is small, so the Moon’s shadow is small, so the eclipse covers only a small part of eclipse covers only a small part of the earth.the earth.

Earth’s rotation causes the shadow Earth’s rotation causes the shadow to move rapidly across its surface.to move rapidly across its surface.

Max. duration of a total solar eclipse Max. duration of a total solar eclipse is seven minutes at any one location.is seven minutes at any one location.

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

Lunar EclipsesLunar Eclipses

Occur when earth is between sun Occur when earth is between sun and moon, and earth’s shadow and moon, and earth’s shadow blocks out the lighted half of the blocks out the lighted half of the moon.moon.

Total lunar eclipse—moon passes Total lunar eclipse—moon passes completely through earth’s umbra.completely through earth’s umbra.

Partial lunar eclipse—only part of the Partial lunar eclipse—only part of the moon passes through earth’s umbra.moon passes through earth’s umbra.

Penumbral eclipse—moon passes Penumbral eclipse—moon passes through earth’s penumbra only.through earth’s penumbra only.

Lunar eclipses may last several Lunar eclipses may last several hours.hours.

Frequency of EclipsesFrequency of Eclipses

Solar and lunar eclipses occur almost Solar and lunar eclipses occur almost equally often.equally often.

Lunar eclipses can be seen by Lunar eclipses can be seen by everyone on the dark side of the everyone on the dark side of the earth.earth.

Solar eclipses can only be seen by Solar eclipses can only be seen by observers in the small path of the observers in the small path of the earth’s shadow.earth’s shadow.

Orbit of moon is tilted at a 5 degree Orbit of moon is tilted at a 5 degree angle to the plane of the earth and angle to the plane of the earth and sun.sun.

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