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Does the Moon Rotate?


Despite covering the topic at length in the classroom, many students inevitably get the question wrong come test time. When I ask them how many times the Moon rotates during each orbit of the Earth, I can always count on at least a few of the students to answer: zero.

Does the Moon rotate? Yes, once for every circling of the Earth. People seem to recognize that we always see the same side of the Moon, but they incorrectly deduce the implication of that on its rotation.

The Slow Pirouette

If the Moon didn't rotate then we would, during each orbit of the Moon about the Earth, see all sides of the grey orb.

Think of it this way, if the Moon were between the Earth and the Sun we would see a certain side. Let's call it side A, and it points towards the Earth and away from the Sun.

Imagine then that the Moon starts its trek around the Earth and, after half an orbit, is now on the other side of the Earth. Now the Earth is between the Moon and the Sun.

If the Moon hadn't rotated at all (done zero revolutions) then side A would still be pointing away from the Sun. But the Earth is now between the Moon and the Sun so people on Earth would then be seeing the other side of the Moon, side B if you will.

The only way for the same side of the Moon to always be facing the Earth is if the Moon rotates exactly in step with its rotation around the Earth. But how is this synchronous rotation possible? Happy chance, or is something else going on?

Tidal Locking and Synchronous Rotation

Everyone knows that the Moon affects the tides on Earth, but what most don't realize is that those same tides affect the rotation of the Moon.

Early on the Moon likely rotated faster than it does now, which would have allowed us to see every face of the Moon. But as the Moon's gravity pulls on the Earth's surface water, it creates a tidal "bulge" around the Earth.

As the Earth rotates, this bulge rotates around with it. The resulting gravitational gradient literally pulls the Moon around. Meaning that the Moon's rotation becomes intrinsically tied to its orbital motion.

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