Chip Off the Old Block

Each day the Moon rises and sets later than it did the previous day -- between half an hour and an hour. This happens because it rotates around the Earth the same way the Earth is turning, so the Earth has to rotate more than a full turn -- which takes more than a day -- until we can see the Moon in the same spot. But is it only a coincidence that the Moon and the Earth rotate the same way?

The leading contender for an explanation of this fact, in pre-Apollo days (before men landed on the Moon, in 1969,) was the "spin-off" theory: the Moon spun off the still-molten ball that was to become the Earth, soon after the formation of the solar system. That would explain why the Moon and the Earth rotate in the same direction.

Obviously nobody was there to witness this event, but we can say now that this explanation is false. Because if it were true, the Moon and the Earth would be made of the same material. Before Apollo, nobody really knew if the rocks on the Moon were like the rocks on the Earth. (They did know, however, that the Moon was not made of cheese.)

One of the goals of Apollo was to bring back rocks from the Moon and compare them with Earth rocks. Well, the Moon rocks had absolutely no water in them, but every rock on the Earth does (not too much; that's what makes drinking rocks so hard.) That demolishes the "spin-off" theory.

Well, now we have two facts to explain: first, that the Moon and the Earth rotate in the same direction; second, that the rocks on the Moon are almost entirely dry. Both these facts can be explained by the "chip off the old block" theory: the young earth was hit by a large asteroid, chipping a piece off. This piece, our Moon, would rotate the same way as the Earth because it was knocked off a rotating Earth, and would be almost entirely dry because of the heat of the asteroid's impact.

This is the best explanation we have now. Maybe it will eventually be shown to be false...possibly, even, as soon as tomorrow.

(01/08/03)