Why the Same Side of the Moon Always Faces the Earth

This actually has a lot to do with the tides.  Please first read "How the Moon Creates Ocean Tides".

Okay, now that you have the stretching concept down, all you really have to understand is that the moon's constant falling toward the Earth has stretched it into a slight egg shape (with the pointy parts pointing toward and away from the Earth).  If the moon turns so that the points are no longer pointing toward and away from the Earth, the stretching action (in addition to mildly reshaping the moon) will pull the points back in line with the Earth.  Thus the pointy side of the moon remains pointing toward the Earth.

copyright © 2000 Sean Barton, all rights reserved

a more detailed and technical explanation of the same phenomenon courtesy of Michael Rothschild:

The tidal forces the Earth exerts on the moon cause material movement and or distortions on the moon (especially when the moon was still in fluid state, but even when it is "solid").  These movements and distortions result in friction, which creates heat.  This heat energy is taken from the rotational energy of the moon and cannot be recovered.  This loss of energy causes the moon to gradually slow its rotation with respect to the Earth.  When in magmatic (fluid) state, the rotation rate will decrease indefinitely.  If, at some stage the moon solidifies, it will reach a stage (since it is not perfectly round and the distribution of matter in it is not homogeneous) where it does not have enough energy to complete a whole rotation and it starts oscillating.  The loss of energy continues and the oscillations gradually decrease too.  Then Homo sapiens appears and starts asking questions.  When aware of this explanation, one expects to find many other moons which face their planet with a fixed side.  And rightfully so, some of Jupiter's moons behave this way.  This argument also explains why the day on Mercury is so long.

copyright © 2001 Michael Rothschild, all rights reserved

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