What is tidal locking?
When a body rotates on its axis and orbits around another body at the same rate, it has the appearance of showing the same “face” to the body it is orbiting at all times. The Moon is a good example: it has a 29.5-day rotation rate and a 29.5-day orbit, so we always see the same side of it. Orbiting bodies do not start in this configuration. Rather, they raise tides on each other, and when they rotate the tidal bulges move to stay on the line between the two bodies. This causes friction, which dissipates energy, and the rotation of the bodies slows down. The moon has slowed all the way down and stopped, but the Earth has much more mass. Earth is slowing down (this has been measured) but will not stop within the expected lifetime of the solar system.
