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Ever wonder why the moon always faces us? Why doesn’t it spin like just about every other thing up there? My physics teacher at school told us that the moon did spin but at exactly the same rate as it went around the earth and that was why it always faced us. I didn’t believe him.

I got it figured eventually. Let’s assume that the moon is perfectly spherical and of uniform matter throughout. Its centre of gravity would be dead centre and it would still be spinning (even if none of that was perfect) because there would be nothing to stop it. That’s why the earth, the sun and all the planets are still spinning along their merry way.

There must be something unusual about the moon. Imagine a bicycle wheel held at the spindle and spinning. It would pretty much go on forever wouldn’t it? But now fit a small weight on one of the spokes ( just off centre) and the wheel will soon spin eratically until the weight no longer makes it to the top and over, but rather causes the wheel to reverse direction and swing back the other way. Then it will start oscillating like a pendulum until eventually it comes to a halt with the weight at the bottom. I figured that if the centre of gravity was not in the centre of the moon but off to one side, then the attraction (the pull) of the earth would be greater when that side was closer to the earth and less when it was away (in the days when the moon did spin). And, just like the bicycle wheel, it would eventually not make it all the way round and would swing back and start oscillating. Now up there, there’s no atmosphere and there’s no spindle with friction, so, unlike the bicycle wheel, there would be nothing to stop the moon oscillating (swinging from side to side) for a very long time.

That meant that if my thinking was correct, the moon should still be oscillating. Well, it is. And now we have the Internet and Google and I could have looked it up, but in those days we didn’t have such luxuries. The non-symmetry is called tidal lock and you can read all about it if you Google “Why does the moon always face us?” There are lots of articles on this – choose one that suits you.

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