Tidal locking (or synchronous rotation, captive rotation) occurs when the gravitational gradient makes one celestial body face another celestial body with the same surface forever; For example, the moon always faces the same direction of the earth. It takes the same time for a celestial body in the tidal locking to rotate once on its own axis and once on its companion star. This synchronous rotation causes the hemisphere to be fixed towards its partner.
In fact, there are many such tidal locking in the celestial bodies of the solar system. For example, between the sun and mercury, between planets and satellites, and between other stars and planets outside the solar system, there will be such tidal locking phenomena.
The rotation of the moon and the rotation of the earth is a day, because the rotation produces alternation of day and night. Compared with the earth, the moon rotates slowly, with a rotation period of 27.32 days. This is because the rotation and revolution of the moon are basically the same. When the moon rotates once, its revolution is also a revolution. It takes as long as a turn.
Extended data
The capture of the moon by tidal locking didn't happen overnight. Even if the distance between the two places is close enough and the conditions can form tidal locking, it will take millions of years to finally form the lock of sunseeker.
Because earth-moon system is locked to rotate towards the sun watcher, we can see that when the moon rotates around the earth, the white point as a reference also rotates once. Therefore, when we understand this movement, we must take into account the movement characteristics of the other side, rather than just thinking that the moon is motionless against the earth.
There is another strange situation in the solar system, that is, Pluto and its satellite Ka Rong are in mutual tidal locking motion. The simple understanding is that they have each other's side and always face each other.
Baidu Encyclopedia-tidal locking