Glossary · Thiên văn học
Tidal locking
The state in which a body takes exactly as long to spin once on its axis as to complete one orbit, so it always keeps the same face toward the other body. The Moon is in this state with respect to Earth.
The Moon is not failing to rotate. It rotates, but takes precisely as long to spin once on its axis as to complete its orbit around Earth. The two motions match, so from Earth we always see the same face.
The mechanism is gravity. Earth's pull distorts the Moon into a slightly elongated shape. When the Moon was younger and closer, that bulge shifted as the Moon turned; the shifting dissipated energy as heat and gradually slowed the spin until the rotation period matched the orbital period. NASA calls the result synchronous tidal locking.
The locking so far runs one way. The Moon is locked to Earth; Earth is not yet locked to the Moon — our day is still 24 hours long, not a month long. The same mechanism is slowing Earth's spin, and NASA estimates it will be about 50 billion years before Earth also tidally locks to the Moon.