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Skywatch: Solving the riddle of the moon's rotation

Another month, and another full moon will shine in our sky the week beginning Monday, March 29. Anyone who has ever watched the moon cycle through our sky each month has noticed that we always see the same side of the moon.

This simple observation naturally begs the question "Does the moon rotate or doesn't it?"

Having discussed this curiosity with stargazers for more than three decades, I've concluded that there are two schools of thought about this issue - yes and no - and each person is certain that their reasoning is correct. So let's try a practical demonstration to help clear up the matter.

In this experiment, you will represent the Earth, sitting in the center of a room. Get a friend to represent the moon; this person will need to walk completely around you in a lunar "orbit." The walls, ceiling and floor of the room will represent the distant stars.

Let's first make the "moon" orbit the Earth without rotating on its axis. In other words, have your friend choose a point on a distant wall and face it constantly as he or she circles you. From your position at the center, what do you see of your friend during their entire orbit?

OK, now try the same thing with your friend twirling around as he or she circles you. Now, with the "rotating moon," what can you see from the central "Earth"?

It seems that in both cases, a terrestrial observer would see lunar features change from week to week. Over time, we should see different sides of the moon. But that's not at all what we see. So what's going on?

The secret is that, in order to keep the same features aimed in the Earth's direction, the moon must spin on its axis at the same rate as it orbits our planet.

To demonstrate this, have your friend walk one quarter of the way around the orbit; in order to keep the same face toward you, he or she must "rotate" one quarter of the way around. Another quarter of an orbit, another quarter of a rotation. And so on.

It seems that the question "Does the moon rotate or doesn't it?" has two answers, depending upon whom you ask!

From the central Earth, the answer is "No, the moon doesn't rotate." But I'll bet that your "lunar friend," having spun himself into dizziness, will disagree!


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