That’s an interesting question, closely related to the first physics question I ever wondered about. Most planetary objects spin, because it would take a special accident for the ingredients which coalesce to make the Earth etc. not to have some little net angular momentum (spin) initially. Even accidental collsions with other objects would get something spinning.
The Moon actually spins too. It spins exactly once per month, so that the same half keeps facing the Earth. Now that sounds like a special coincidence, just like non-spinning would be, so it deserves an explanation.
My first scientific idea, in grade school, was that maybe the Moon is a magnet (like the Earth), and that one part of it ended up pointing toward the earth in the same way that one side of a compass needle will end up pointing toward another nearby magnet. That turns out to be wrong, since the moon isn’t magnetic.
The actual explanation turns out to be that when the Moon used to spin faster the Earth's gravity induced big tides in the dust on the Moon. Friction between the Moon and those tides slowed down the spin until a fixed side faced the Earth. Then there were no more tides so the spinning settled down at the current rate.
Mike W.
(published on 10/22/2007)
