Ever wonder why the same side of the moon always faces the earth? It wasn't always so. The same forces that caused this to happen are also slowing down the rotation of the earth on its axis. Every century, the day is longer by a few seconds. What causes this slowing? A phenomenon known as tidal locking ...

Most people know that the tides are caused by the moon's gravitation pull on the ocean. At the point where the moon is above a particular region of the earth, the oceans get pulled in that direction, causing a high tide. In fact, the earth's crust does the same thing. These solid tides are much smaller, but do occur. You can imagine the moon pulling on the earth, giving it a slightly ellipsoid shape as a result of this pulling. However, because the earth is rotating, the pointy end of this ellipsoid is always slightly ahead of the moon. Since the pointy end is slightly closer to the moon than the flattened end, the gravitational pull on this part of the earth is stronger. While the earth is rotating, a small force is pulling against the direction of rotation, causing the gradual slowing.

The same process in reverse is occuring with the moon. The solid tide created by the earth slowed down the rotation of the moon with respect to the earth's position until it stopped rotating completely. This happened much faster because the moon is a smaller body than the earth.

Tidal locking affects other bodies in the solar system, too. Mercury, which has a highly elliptical orbit, is in a similar relationship with the sun where it turns 1.5 times every revolution which brings the bulging side facing the sun when Mercury is closest in its orbit to the sun.

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