Does the Moon Rotate?

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The Moon is familiar; it always looks the same. We know that the Earth rotates, that’s why the Sun, Moon and stars seem to move through the sky every day. But does the Moon rotate? And if the Moon rotates, why do we alway see the same side – it never seems to change.

Well, the Moon does rotate. In fact, the Moon takes 27.3 days to turn once on its axis. But the Moon also takes 27.3 days to complete one orbit around the Earth. Because the Moon’s rotation time is exactly the same amount of time it takes to complete an orbit, it always presents the same face to the Earth, and one face away.

Because it only presents one face to the Earth, astronomers say that the Moon is tidally locked to the Earth. Although the Moon looks like a perfectly smooth ball, it has slight differences in the shape of its gravity field. A long time ago, the Moon did rotate. But each time it turned, the Earth’s gravity tugged at it, slowing down its rotation until it only presented one face to the Earth. At that point, the Moon was tidally locked, and from our perspective, it doesn’t seem to rotate.

Many other moons in the Solar System are also tidally locked to their planet. In fact, most of Jupiter’s large moons are tidally locked.

So, to answer the question: does the Moon rotate? The Moon rotates once every 27.3 days; the same amount of time that it takes to go around the Earth, and so it always presents the same face to the Earth.

Here’s an article about the far side of the Moon; the one we never see from Earth. Here’s an article about the rotation of the Sun.

Here’s the same question answered from Ask an Astronomer, and here’s the same question from Bad Astronomy.

You can listen to a very interesting podcast about the formation of the Moon from Astronomy Cast, Episode 17: Where Did the Moon Come From?