Does the Moon rotate? Are there other moons that always keep one face toward their planet? (Intermediate)

I noticed that out moon doesn't rotate as it orbits our earth. Is our moon the only moon in our solar system that doesn't rotate?

Be a little careful . . . the Moon does rotate. If you stood on the Moon, the stars would rise and set, just like they do on Earth, except that a lunar day is a month long, the same as the Moon's orbital period. The Moon rotates at just the right speed so that it always keeps one face pointed toward the Earth, which seems like a pretty big coincidence, doesn't it?

Your question is very interesting because the answer is that, no, the Moon is not unique. Almost all moons in the Solar System keep one face pointed toward their planet. (The only exception we know of is Hyperion, a moon of Saturn.) This tells us it's probably not a coincidence, that there is probably a reason for this to happen, a physical process that happens to most moons to slow their rotation.

That process is called tidal friction. You probably know that the Moon's gravity affects the Earth's oceans. Well, the Earth's gravity also affects the Moon. It distorts the Moon's shape slightly, squashing it out so that it is elongated along a line that points toward the Earth. We say that the Earth raises "tidal bulges" on the Moon.

The Earth's gravity pulls on the closest tidal bulge, trying to keep it aligned with Earth. As the Moon turns, feeling the Earth's gravity, this creates friction within the Moon, slowing the Moon's rotation down until its rotation matches its orbital period exactly, a state we call tidal synchronization. In this state, the Moon's tidal bulge is always aligned with Earth, which means that the Moon always keeps one face toward Earth.

Other planets raise tides on their moons, too, so almost all the moons in the Solar System are tidally synchronized. There's even one planet that is sychronized to its moon! Charon, Pluto's moon, is so large and so close to Pluto that the planet and moon are both locked into the same rotational rate. The Moon slows the Earth's rotation, too, but at a very slow rate, increasing the length of the day by a couple of milliseconds each century.

You might be wondering what's up with Hyperion. Gravitational interaction with other moons of Saturn cause Hyperion to tumble chaotically, so Saturn doesn't even get a chance at tidal synchronization before Hyperion's rotational state is changed by another moon. There may be other small moons that behave in this manner, as well, but it is difficult to measure the rotational periods of small moons around distant planets, so we don't know of any yet.

This page was last updated on July 18, 2015.

About the Author

Britt Scharringhausen

Britt studies the rings of Saturn. She got her PhD from Cornell in 2006 and is now a Professor at Beloit College in Wisconson.

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