This otherwise bizarre phenomenon can be explained in terms of a subtle effect generated by gravitation and friction — tidal locking. Through their mutual gravitational attraction, the Earth and the moon create tidal bulges on each other. One bulge faces in the direction of the other body, and one faces away. These bulges generate heat through the friction of rock rubbing against itself. Over time, they siphon energy away from the rotational momentum of both bodies, producing a breaking effect.

Because the Earth's mass dominates the Earth-Moon system, the moon experiences the greater braking effect. Over time, the moon's rotation has progressively slowed until the rate of rotation matches the rate at which the tidal bulge moves around the body