Welcome to You Ask Andy

Jean Carlson, age 13, of Gilbert, Minnesota, for her question:

Why do we never see the back of the moon?

When you trip through a merry square dance, there comes a time to swing your partner. The two of you grasp hands and spin around each other    face to face. This is no time to study the back of your part¬ner's head. In a way, the earth and the moon act like a couple of hoedown dancers. But though the moon always faces the earth, the earth turns different sides to face the moon.

The moon orbits around the earth and together the two of them orbit around the sun. This heavenly hoedown goes on day and night, year after year. It has been going on for at least four billion years and is expected to continue indefinitely. The dizzy whirl is powered by the force of gravity    which makes the two heavenly bodies pull at each other.

Since the earth is more massive than the moon, its gravity is much stronger. If it had its way, this stronger gravity would pull the less massive moon crashing down to the earth. But the force of gravity has an opponent, which pulls against it. The moon's orbiting speed counter¬acts the pull of the earth's stronger gravity. So, instead of crashing down, it swings in a curved orbital path around the earth.

However, this is not the whole story. Both the earth and the moon rotate like spinning tops. Their rotation speeds are related to the pull of gravity    which is related to the distance between them. At the present distance of about 239,000 miles, the earth rotates once every 24 hours. But if the moon were closer, the effects of its gravity would be stronger. Then the earth would be forced to rotate faster    and our day and night period would be shorter.

Meantime the earth's gravity influences the rotation of the moon, only more so. It allows the moon to rotate, but only very, very slowly. As a matter of fact, the moon rotates on its axis only once during its orbital period around the earth. It acts like one member of a swing your partner team    spinning around while keeping the same side facing the earth. This is why we never see the other side of the moon.

The lunar month is the time it takes to orbit the earth. The lunarday is the time it takes to rotate on its axis. And the two time periods are more or less equal. During this period, first one side then another side of the moon rotates to face the sun. At any point, the lunar day lasts about two earth weeks    and so does the lunar night.

Meantime the earth does not co operate fully with this swing your¬partner routine. It merely wobbles a little and spins around to face the moon with all sides of its globe, once every 24 hours. The moon sees four earth phases every 24 hours, though it takes a whole month to show us its four lunar phases.

 

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