Why does the earth spin? Is it possible for the earth to not spin? What other planets spin or do not spin?It spins, I guess, because of momentum left over from formation, and also there's probably the issue of gravity being different the closer you get to the sun. But actually the moon and the tides are acting as something of a break. You'll notice that one side of the moon is always facing earth, eventually the earth will always have one side facing the moon, so the day would actually be a month long, except the moon will be farther away and rotating more slowly, so months will be a lot longer.
I don't know, but here's the formula on Wikipedia.Hmmm, plugging into the "simplified formula," one finds a time of four million years for the moon to lock to the earth --- that is, the moon was unlocked for about 0.1% of its history. The same way, I get about 1.1 billion years for the earth to lock to the moon. That assumes that the earth initially rotated once per twelve hours, and so could plausibly be correct if the moon were formed 0.5 billion years ago, during the Cambrian period. Ah, Wikipedia.
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posted by zarq at 9:09 AM on July 7, 2010