How come the moon spins at the same speed it orbits around earth?

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It takes 27 days for the moon to spin completely and the exact same time for it to orbit around earth, thus creating a situation where we can always only see the same side of the moon.

Surely, this synchronization is not a coincidence but how exactly did we reach that point? I read some articles but found them vague.

In: Planetary Science

9 Answers

Anonymous 0 Comments

The Moon became tidally locked with the Earth. The clue to how that happens is the the name Tidally.

At the start the Moon was spinning. And as now was making tides on Earth. But, the Earth was also making tides on the Moon. Not of water, but of the rock itself. So the Earth pulls the near side of the Moon slight more because it is closer. It also pulls slightly less on the far side, and because the far side is further out it is moving at above orbital velocity, so the backside bulges also. (same thing happens on Earth now. It’s just more noticable with water as it moves easier.)

But the Moon is spinning, and it takes time for these bulges to form and fade. So instead of pointing directly at the Earth the bulge pulled slightly further along the rotation. But now the force of gravity is slightly forward of the direct line between centers of mass of Earth and the Moon. So this slight slows the Moon, and over time stops the rotaton.

You may ask were all that angular momentum went? It went into the Earth Moon system and slightly speed up how quickly they orbit each other.

Fun fact: this is actually currently happening to the Earth. The tides are slooooowly slowing the Earths rotation. And at the same time slowly raising the moons orbit. So it is moving slowly away from Earth.

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