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Spinning the Moon

Bad Astronomy: The Moon only shows one face to us because it is not rotating.

Better astronomy: The Moon only shows one face because it is rotating, once every time it revolves around the Earth.

Best astronomy: The Moon does not appear to rotate in the reference frame where the Earth-Moon line is fixed in direction, but it does rotate as seen by an outside observer.

Image of the Earth and Moon taken in 1994 by NASA's Galileo spacecraft, on its way to Jupiter.

How it works: If you go out on several different nights and look at the Moon, you will always see the same features, at about the same position. It looks as if the Moon doesn't rotate! Ah, but it does.

This can be seen using a model. Grab two oranges (or apples, or baseballs, or whatever roughly spherical objects you have handy). Mark one with an "X"; this represents a feature on the Moon. Now put the other one down on a table; this is the Earth. Place the Moon model on the table about 30 centimeters (one foot) away with the X facing the Earth model. Now move the Moon model as if it were orbiting the Earth, taking care to make sure that the X faces the Earth model at all times.

Surprise! You'll see that to keep the X facing the Earth model, you have to rotate the Moon model as it goes around the Earth model. Furthermore, you can see you have to spin it exactly once every orbit to keep the X facing the Earth model. If you don't rotate it, the Moon model will show all of its "sides" to the Earth model as it goes around.

Now, I have been a bit tricky here. We are talking about two different frames of reference; one on the surface of the Earth looking out at the Moon, and one outside the Earth-Moon system looking in. You performed the experiment from the latter frame, and saw the Moon rotating. From the former, however, you can see for yourself the Moon does not rotate. What is being argued here is that in one frame the Moon rotates, in another it does not.

We've actually learned three things:

  • 1) the Moon rotates as it orbits the Earth (as seen by an outside observer);
  • 2) it rotates one time for every orbit (to that observer); and
  • 3) if it didn't rotate, we would eventually see all of the Moon as it orbited the Earth.
  • There is a bit more to this story. We actually can see a bit more than just the one face of the Moon. Because the Moon's orbit is not a perfect circle but actually an ellipse, its spin and rotation don't exactly match up. This means that sometimes the spin lags behind the orbital speed, and sometimes it moves ahead. This in turn means that sometimes we can "peek" around a bit onto the far side of the Moon. This is called "libration". You can see it yourself! If you happen to observe the Moon a week after perigee (closest point to the Earth) and then two weeks later, a week after apogee (farthest point from the Earth) you can see that the face looks like it has rotated a bit. This is easiest to see with binoculars or a small telescope. It's very hard to see with the naked eye, but remember, ancient astronomers knew about this effect long before the invention of the telescope!

    [Note added December 6, 2001: Bad Reader Charlie Kluepfel pointed out to me that I had a mistake in the original version of the above paragraph; I had said libration was maximized at apogee and perigee. He showed me that actually this happens between apogee and perigee, so I corrected the text. You can check this out for yourself: a table of data for the Moon can be found at my colleague Fred Espenak's website. The libration is measured in degrees in the column labeled"Libration", subheading "l". The distance is also marked in the column "Dist"; note that libration is largest when the Moon is midway between its nearest and farthest distance form the Earth.]

    The next question is why does the Moon only show one face to the Earth? But that's a different story... ;-)

    ©2008 Phil Plait. All Rights Reserved.

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