Synchronous Orbit and Rotation
Date: Summer 2014
How can the moon have the same rotational period as its orbital period?
Good question. The periods are the same because the Moon is experiencing what we call rotational lock. Its period of rotation and its period of revolution about Earth are the same. In fact, Earth's rotation is also slowing down as well. One day in the very distant future-- billions of years hence, its rotation will also be in rotational lock with the Moon.
David H. Levy
There are several important factors we need to consider…. 1) Just as the Moon causes tides on Earth, Earth also causes tides on the Moon - remember that the gravity of the Earth/Moon affect all matter not just water, this means that the land also "bulges". 2) The tides move around the surface of Earth and Moon as they rotate about their axes. This means the tides are constantly shifting matter (the bulge is traveling on the surface) … and that requires much of energy. 3) If you were to draw a straight line between the centers of Earth and the Moon, you would find that the tide on the Moon rests right on the line, but the tide on Earth is a bit "ahead" of that line. This is because the rotation of Earth pushes the tide "ahead". 4) On Earth, this combination of the constantly "traveling" bulge, the tide that is spun ahead of the Earth-Moon line, and the energy required to do so - causes Earth to slow down its rotation … eventually, Earth will take the same day to spin around its axis as to go around the Moon (using the Moon as the point of reference). 5) On the Moon, the same energy requirements (as described in #3 and #4) has already taken effect and basically slowed down the Moon's rotation so that it is now matching with its orbit around Earth (using Earth as a point of reference). 6) If, let us say, the Moon somehow sped up or slowed down its rotation (does not matter how), then the drag resulting from a bulge that is not in the direct line with the Earth-Moon line would cause the Moon "adjust" its rotation so that it would return (eventually) to a rotation-orbit synchronization.
If you want to know more, read up on "tidal locking".
Greg (Roberto Gregorius)
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Update: December 2011