Have you ever noticed that the Moon always looks the same? Sure, the phase changes, but the actual features on the Moon always look the same from month to month.
Does the Moon rotate? What’s going on?
From our perspective here on Earth, the Moon always shows us the same face because it’s tidally locked to our planet. At some point in the distant past, the Moon did rotate from our perspective, but the Earth’s gravity kept pulling unevenly at the Moon, slowing its rotation. Eventually the Moon locked into place, always displaying the same side to us.
But if you looked down on the Earth-Moon system from the north celestial pole, from the perspective of Polaris, the North Star, you’d see that the Moon actually does rotate on its axis. In fact, as the Moon travels around the Earth in a counter-clockwise orbit every 27.5 days, it also completes one full rotation on its axis – also moving in a counter-clockwise direction.
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If you look at a time lapse animation of the Moon moving entirely through its phases over the course of a month, you’ll notice a strange wobble, as if the Moon is rocking back and forth on its axis a bit.
This is known as libration.
On average, the Moon is tidally locked to the Earth’s surface. But its actual orbit is elliptical, it moves closer and then more distant from the Earth.
When the Moon is at its closest point, it’s rotation is slower than its orbital speed, so we see an additional 8 degrees on its eastern side. And then when the Moon is at the most distant point, the rotation is faster than its orbital speed, so we can see 8 degrees on the Western side.
Libration allowed astronomers to map out more of the Moon’s surface than we could if the Moon followed a circular orbit.
Until the space age, half the Moon was hidden from us, always facing away. This hemisphere of the Moon was finally first observed by the Soviet Luna 3 probe in 1959, followed by the first human eyes with Apollo 8 in 1968.
The two hemispheres of the Moon are very different.
While the near side is covered with large basaltic plains called maria, the far side is almost completely covered in craters. The reasons for this difference is still a mystery to planetary scientists, but it’s possible that a second Moon crashed into it, billions of years ago, creating the strange surface we see today.
So yes, the Moon does rotate.
But its rotation exactly matches its orbit around the Earth, which is why it looks like it never does.
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?