Is it a coincidence that moon keeps one face to Earth?

No, it’s not a coincidence. The constant pull of Earth’s gravity has slowed down the spinning moon so now it’s in what’s called “synchronous rotation.” In other words, the moon spins once for each of its orbits around Earth – and that’s why the moon always keeps a single face turned our way.

Billions of years ago, the moon was spinning faster than it does today – and it was also much closer to Earth. But Earth’s gravity put “the brakes” on the moon – and gradually increased its distance. This happened because Earth pulls one side of the moon more strongly than the other side. And the moon does the same to Earth. Both worlds attempt to “stretch” each other from the shape of a sphere into more of an egg shape. We see this “tidal effect” strongly in ocean tides. But the Earth and moon also raise “tides” of solid ground on each other.

Because their tidal bulges do not line up exactly between the center of Earth and the moon, the rotation rates of both worlds have slowed down over time. Now the moon is locked with one face perpetually turned toward Earth. And the moon is getting farther away from Earth at about 1.5 inches – or 3.8 centimeters – per year.

October 19, 2009

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