Rotation of the Moon

by Fraser Cain on October 23, 2008

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NASA's image of the Moon

NASA's image of the Moon


Here’s a question, does the Moon rotate?

The answer is yes, but it’s more complex than that. The rotation of the Moon – or a lunar day – lasts around 27.3 days. This is the same amount of time the Moon takes to complete one orbit around the Earth. Because the rotation of the Moon and the length of its orbit are exactly the same, the Moon always presents the same face to the Earth.

Imagine that the Moon and the Earth are connected with an imaginary cable. The cable is bolted down on the Moon, but it’s connected to a ring around the Earth. So, the Earth can rotate at its own speed of turning once every 24 hours, and the Moon can go around it once every 27 days. But the Moon always shows the same face towards the Earth.

Astronomers call this situation tidal locking. And it wasn’t always the case. In the ancient past, the rotation of the Moon was at a different rate than its orbit around the Earth. But the Earth’s gravity kept tugging onto it, pulling different parts of the Moon at different rates. This pull slowed down the rotation of the Moon until its rotation matched its orbital period.

Most of the other large moons in the Solar System are tidally locked to their parent planets. In the case of Pluto, both Pluto and its largest moon Charon are tidally locked to each other. If you stood on the surface of Pluto, Charon would always be in the same spot in the sky, and vice versa. Because the Moon is tidally locked, we can never see the far side of the Moon from here on Earth.

Here’s an article about how the far side of the Moon would be a great place for a telescope. Here’s an article about the rotation of the Sun.

Want more information on the Moon? Here’s NASA’s Lunar and Planetary Science page, and here’s NASA’s Solar System Exploration Guide.

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?

About 

Fraser Cain is the publisher of Universe Today. He's also the co-host of Astronomy Cast with Dr. Pamela Gay.

Donald Porter December 31, 2008 at 11:39 AM

If your cable was “bolted” to the Moon and the Moon actually had any rotation at all then eventually would the Moon wrap the cable around itself? Put another way. If the Moon had any rotation at all then at some time during the 29.5 days wouldn’t the far side of the Moon have to face the Earth?

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