In the early centuries just after the cataclysmic formation of the moon, the infant Earth probably rotated once every six hours. But the moon's gravitational force has slowed down Earth's rotation to once every 24 hours, and continues to slow us down—about two milliseconds per century.

Earth, in turn, has slowed down the moon to the point where today its rotational period matches its orbital period. With Earth slowing down too, eventually both Earth and the moon will be locked with just one face toward each other.