This is a tough one to answer on Reddit, because it needs a diagram. Specifically this diagram:
https://astronomy.stackexchange.com/a/21585
The Moon is already locked to the Earth. The Earth’s rotation is slowing down as it is still in the process of locking to the Moon. Let’s focus on the Earth, because it’s easier to understand something ongoing and because I couldn’t find a good diagram for lunar locking. 🙂
1) The Moon raises tides on the Earth: a “tidal bulge” is formed in the ocean and also the rock.
2) The Earth’s rotation carries the bulge around so it’s not aligned with the Moon. (See first diagram)
3) The Moon’s gravity pulls harder on the side of Earth that has a bulge closer to the Moon (see second diagram). This means there’s an unbalanced gravity force that tries to slow down the Earth’s rotation, just like you can stop a spinning tire by pushing on one side of it but not the other.
The exact same process happened to the Moon, but because the Earth is so much bigger its tidal effect on the moon was larger and locking happened faster.
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