ELI5. Tidal locking.


ELI5. Tidal locking.

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Gravity’s influence falls off by distance, and orbiting bodies are quite large. The near side is being pulled slightly harder, causing it to bulge. As the orbiting body rotates this bulge pulls away, causing the body to flex as the bulge shifts and siphoning away energy from its rotation.

Overall the result is that over time the orbiting body will tend to settle with one face always pointing towards what it is orbiting. This is called tidal locking.

Gravity gets weaker over distance. Earth’s gravitational pull on the near side of the moon is stronger than its pull on the far side of the moon. This has a stretching effect on the moon as a whole. The moon’s gravity has a similar stretching effect on Earth. This stretching effect is the result of “tidal forces” created by the different strength of gravity.

And yes, this stretching is what causes the tides. Water moves much more freely than dirt, so we see it mostly by a change in sea levels.

If you stretch a planet one way, then rotate the way you stretch it, you end up actually applying a rotating force on the planet. Because of this, the moon is slowly sucking away Earth’s rotation, making the day longer. Conversely, Earth’s rotation is pushing the moon into a slightly higher orbit very slowly.

Given enough time, the Earth’s rotation would slow and the moon’s orbit would rise until Earth rotated at exactly the same speed as the moon orbited us. Ignoring the change in the moon’s orbit, that would mean every day lasted about a month, and the same side of the Earth would always be facing the moon. This is *tidal locking*.

The tidal forces on the moon from Earth’s gravity are far stronger than those from the moon’s gravity on Earth, so the moon is long since tidally locked to Earth.