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The Sciences

Why Do We Always See the Same Side of the Moon?

Pure physics can explain this one.

By Adam HadhazyOctober 30, 2014 12:00 AM
Moon, Front and Back - NASA
(Credit: NASA/GSFC/LRO/Arizona State University)

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Q: Why does the moon always present the same face to us? I find it impossible to believe that this could happen by chance. — Michael Connelly, Toronto

A: Nope, not by chance — it’s pure physics.

For starters, the moon is not stuck in place with one side facing us. Our lunar companion rotates while it orbits Earth. It’s just that the amount of time it takes the moon to complete a revolution on its axis is the same it takes to circle our planet — about 27 days. As a result, the same lunar hemisphere always faces Earth.

How’d this come to be? In a word: gravity. The moon’s gravity slightly warps our planet’s shape and gives us tides. Likewise, Earth tugs at the moon, creating a rocky, high-tide “bulge” facing us. That bulge ended up working like a brake, slowing the moon’s spin down to the current rate, so the lunar high tide permanently faces us.

When that happened, about 4 billion years ago, the moon became “tidally locked,” and it has presented us the same visage ever since.

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