Why does the Moon face Earth with the same side?

Question

I know that the rotation period of the moon equals its revolution period. It's just so astonishing that these 2 values have such a small difference. I mean, what is the probability of these 2 values to be practically the same? I don't believe this to be a coincidence. It's just too much for a coincidence. What could have caused this?

Answer 1

This is a gravitational phenomenon known as tidal lock. It is closely related to the phenomenon of tides on Earth, hence the name.

Tidal locking is an effect caused by the gravitational gradient from the near side to the far side of the moon. (That is, the continuous variation of the gravitational field strength across the Moon.) The end result is that the Moon rotates around its own axis with the same period as which it rotates around the Earth, causing the face of one hemisphere always to point towards the Earth.

Answer 2

Begin by imagining that the moon isn't quite a perfect sphere. One side is just a little bigger than the other. As the moon rotates, the heavier face will swing around towards the earth a little faster, and it will swing away from the earth a little slower, since it feels a stronger gravitational attraction via its larger mass.

Since gravity is a conservative force, you might think that this continues forever - but it doesn't! The moon isn't totally rigid; rocks can slide around both on the surface and even inside the moon. The heavier lump actually slides through the moon to try to stay facing the earth, which causes friction inside the moon. That friction heats up the rocks, but the heat is slowly lost into space.

Now we have a conservation of energy problem: the rotational energy of the moon is being converted to thermal energy in the rocks, and that thermal energy is being slowly leaked out of the system. The only resolution is that the rotation slowly stops over many millions of years.

Finally, let's return to our assumption of a small mass imbalance. Is this true? Almost certainly - all we need is the fact that the moon isn't a truly perfect sphere.

Answer 3

Comment to Spencer Nelson's answer: Consider a spherically symmetric satellite. Let us, for simplicity, consider the idealized situation where all atoms of the satellite are organized into one big crystal held together by covalent bonds, so that all the individual atoms are only allowed to make movements that doesn't break any of their covalent bonds. Even such an idealized satellite can get tidally locked with Earth, because it would still get elastically stretched into an oval shape by tidal forces in the direction of Earth.