The Answer
A process called tidal locking slowed down the rotation of the moon to the point where its speed or rotation almost exactly matches it speed of orbit.
When we look at the moon, we always see the same side. What’s up with that?
The earth orbits around the sun and spins on its axis so that the side of the globe that is facing the sun is always changing. It’s a good thing too because if it weren’t this way half the earth would be too hot to be habitable and one half would be too cold.
In contrast, the moon orbits around the earth, but the same side is always facing the earth. This means that it’s not spinning, right? Not really. It actually spins on its axis as well, but the amount of time it takes to spin once on its axis is almost exactly the same as the amount of time the moon takes to complete one orbit of the earth (approx. 27 days).
Is this a coincidence?
No, it is not. It can be presumed that when the moon formed it had a period of rotation that was completely independent of anything that the earth was doing.
The fact that the rotation and revolution have synced up is due to a process called tidal locking.
Just like the gravity of the moon affects ocean tides on the Earth, gravity from Earth affects the moon. But because the moon lacks an ocean, Earth pulls on its crust, creating a tidal bulge at the line that points toward Earth.
The amount of tidal bulge on the moon is about 30cm. This doesn’t seem like much, but this is an otherwise rigid object that is being transformed which produces a lot of stress and friction within the moon. This transformation also takes time, so the bulge ends up being offset from the direction of the earth by a little bit. These factors resist the rotation of the moon.
The rate at which these forces slowed the rotation of the moon has been very slow, but the moon is believed to have been formed about 4.5 billion years ago so it’s had a lot of time for these forces to take effect.
In fact, the process is still happening and the earth itself is slowly becoming tidally locked the moon as well.
In the distant distant future, the Moon will stop moving in the sky, and hang motionless, visible from only half the Earth. … In about 50 billion years, long after the Sun has died, the Earth and the Moon will finally be tidally locked to each other…
Is this situation unique to the earth and its moon?
No. Most of the moons around the planets in our solar system are tidally locked to their partners.
One of the most interesting instances of this is the pair of dwarf planets Pluto and Ceres. According to NASA Science “Charon neither rises nor sets, but hovers over the same spot on Pluto’s surface, and the same side of Charon always faces Pluto”.