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

Imagine in front of you is a lever. The lever is all the way down, parallel to the ground. You are only allowed to pull on the lever with a force that is parallel to the ground. When the lever is all the way down, you will be pulling along the length of the lever, and nothing will happen. But if you raise the lever up a bit, now when you pull on it the lever will go back down, because your pulling force is no longer along the length of the lever.

Now onto the Moon. There are two concepts at play here: center of mass and center of gravity. The Moon’s center of mass is the average center of all the Moon’s mass. It will pretty much be at the center of the Moon. The Moon’s center of gravity is the location within the Moon where, if you were able to somehow fall through the Moon’s mass and into the Moon, you would end up floating, not going one way or the other. It’s the gravitational center of the Moon.

Gravity is stronger on things that are closer. The side of the Moon facing the Earth is closer to Earth than the far side of the Moon. So the Earth’s gravity “pulls” the Moon’s center of gravity a little closer to the Earth. The Moon’s center of mass and center of gravity are now not the same spot. Its center of mass is basically at the center of the Moon, but its center of gravity is a little off-center, because it’s pulled closer to the Earth by Earth’s gravity.

This basically creates a lever. The Moon’s center of mass is like the fulcrum of the lever (the base of the lever), and the Moon’s center of gravity is like the end of the lever’s handle. Anytime the Moon rotates such that its center of gravity is no longer in line with its center of mass and the Earth, Earth’s gravity “pulls on the lever”, snapping the Moon’s center of gravity back in line with its center of mass and the Earth.

When the Moon’s center of mass and center of gravity are in line with the Earth, Earth’s gravity “pulls along the length of the lever”, so nothing happens (just like in the first paragraph I wrote). But anytime they are not in line, Earth’s gravity no longer pulls along the length of the lever, so it “snaps the lever back into place”, thus keeping the same side of the Moon facing the Earth.

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