The moon's period of rotation around the Earth is identical to its rotation about its axis. What are the odds of this happening by chance, or are there physics behind this occurrence? originally appeared on Quora, the place to gain and share knowledge, empowering people to learn from others and better understand the world. You can follow Quora on Twitter, Facebook, and Google Plus.

It’s no coincidence. The synchronization between the Moon’s orbital period and its rotation period is due to a process tidal locking.

The details get hairy, but the essence of the process is quite simple. Earth raises strong tides on the Moon, which create twin bulges on the Moon’s near and far sides. When the Moon was young it probably had a much faster rotation period. As it rotated those bulges rotated as well, so that the bulge would race slightly ahead. At the same time, Earth’s gravity acted to pull it back. Here’s a cartoon version of what happened:

The constant pulling back on the Moon acted as a brake, slowing its spin until the rotation period and the orbital period matched. Once that happened, the tidal bulge stayed aligned with the Earth at all times, and there was no more force affecting the Moon’s rotation. That stable arrangement probably happened within the first 100 million years after the Moon formed, so until humans came along no living thing had ever seen the far side of the Moon.

(In case you are wondering: The Moon has the exact same tidal effect on Earth, but our planet is so much bigger that the Moon has managed only to slow our rotation down to 24 hours.)

Note that this situation is far from unique in the solar system. Nearly all of the major satellites are tidally locked to their planets for the same reason. In the case of Pluto and Charon, they are doubly locked: They both rotate with a 6.39-day period, the same as their orbital period. That means that one hemisphere of Pluto always faced toward Charon, and vice versa. That is why the New Horizons probe was able to get a clear look at only one hemisphere of each body when it flew by in 2015.

There are a number of exceptions to the tidal-locking rule, however. Most of them are tiny “irregular” outer moons of Jupiter, Saturn, Uranus, and Neptune, which are generally assumed to be objects that were captured after the planets had formed. Three misbehaving moons in this group stand out, however.

Saturn’s 210-kilometer-wide moon Phoebe has a brisk, 9.3-hour rotation period, much shorter than its orbit. It is believed to be an escapee from the Kuiper Belt that was then captured by Saturn. (Phoebe is also a major plot point in The Expanse, for you SF fans.)

Neptune’s Nereid is nearly as large, 170 kilometers across. It, too, might be a captured object, or it might be a once-orderly satellite whose orbit was disrupted by its much larger, backwards-orbiting neighbor, Triton.

Saturn’s Hyperion (above) is an especially interesting case. This weird, spongy-looking moon has a highly oval shape, 165 kilometers across in the short dimension and 360 kilometers across the long way. It rotates in a chaotic manner, a result of its strongly elliptical orbit and its intense gravitational interaction with its big neighbor, Titan. (Yes, one big moon is named Triton, the other Titan. Too late to do anything about that.)

Planets orbiting very close to other stars are probably tidally locked as well. In our solar system, the only one that comes close is Mercury, which is in a 3:2 spin-orbit resonance. For Earth-size planets in tight orbits around a red dwarf star, like the planets of the Trappist-1 system, tidal locking raises interesting questions about habitability.

It appears that many red dwarf planets have Earth-size planets circling at a distance where their surface temperatures could allow liquid water. Could those planets actually support life? That depends in part on what happens to a nominally Earth-like world that has one hemisphere bathed in constant light, the other in eternal darkness. Studies of the Trappist-1 planets by the upcoming James Webb Space Telescope should start to provide some answers.

This question originally appeared on Quora. More questions on Quora:

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