Tidal Locking

The Moon always presents the same face to Earth, a fact so consistent that no human being had ever seen its far side until a spacecraft photographed it in 1959. This isn't a coincidence, and it isn't because the Moon doesn't rotate -- it does. The Moon's rotation period and its orbital period around Earth have simply become identical, a state called tidal locking, and the mechanism behind it comes down to gravity acting unevenly across an object of significant size. Earth's gravitational pull is slightly stronger on the side of the Moon that's closer to Earth than on the far side, and that difference in pull, however small, is enough to stretch the Moon very slightly into an elongated shape, with a bulge facing toward Earth and a corresponding bulge facing away. Early in the Moon's history, before it settled into its current locked state, it likely rotated faster relative to its orbit, meaning this tidal bulge was constantly being dragged out of alignment as the Moon spun beneath it. Earth's gravity, in turn, exerted a torque on that misaligned bulge, gradually slowing the Moon's rotation down over a very long timescale. This slowing continued until the Moon's rotation period matched its orbital period exactly, at which point the bulge stayed permanently aligned with Earth and the torque dropped to zero, locking the rotation in place. The same process, operating in reverse over a much longer timescale, is also very gradually slowing Earth's own rotation and pushing the Moon into a slightly wider orbit.