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STARGAZERS - Does the Moon Rotate ... or Doesn't It?

Week of Aug. 17-23, 2015


Last updated 8/17/2015 at 3:14pm

Well, here we are in August already, and another full moon shines in our sky this week. This one occurs officially on the 29th and, as the full moon always does, will rise in the east at sunset. The August full moon is often known as the Sturgeon moon, since this is the time when this large fish is readily caught.

A few Native-American tribes knew this moon as the Red moon, because it rises with a reddish appearance in the sultry haze low to the horizon, while some called it the Green Corn moon.

All are wonderful stories from history, but there's one fact about that moon that seems to puzzle just about everyone. Sky watchers who have ever watched the moon cycle through the heavens from full moon to full moon have surely 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?"

This is one of the most common questions I hear. And, having discussed this curiosity with sky watchers for more than four decades, I've learned that everyone has their own answer -- either yes the moon rotates, or no it doesn't -- and each person is absolutely 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 or standing 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"?

Hmmm ... seems that in both cases, a terrestrial observer would see lunar features change from week to week. In other words, 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 your viewpoint.

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

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