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Q & A: spinning planets

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Most recent answer: 08/03/2011
Q:
Why does the earth spin, and not the moon?

- gvjoshi (age 53)
noida,up,india
A:
Thatís 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 isnít 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)

Follow-Up #1: Why do we see only one side of the moon?

Q:
I have a hard time visualizing why, if the Earth is rotating on it's axis, and the Moon is rotating on it's axis, do we always see the same side of the Moon. It seems as though the same side of the Moon is always facing the Earth, but how can that be if it spins on it's axis?
- Karol (age 10)
Dearborn MI. USA
A:
great question. The key is that it spins on its axis just once each time it goes around the earth. That's just enough to keep the same side always facing the earth.  You can try to model this yourself. Have a friend go in a circle around you while they keep facing the same way. Sometimes you'll see their front, sometimes their back. To keep facing you they have to keep turning which way they face as they go from one side of you to the other.

I've marked this as a follow-up, because we had a little explanation in an earlier question.

Mike W.

(published on 08/03/2011)

Follow-up on this answer.