Look up at a full moon tonight and you'll see the same dark patches your grandparents saw, and every human before them. The "man in the moon" never turns away. It's tempting to conclude that the Moon doesn't rotate at all. It does. It turns exactly once on its axis for every trip around Earth, about every 27.3 days.
That match is the whole trick. Walk in a circle around a chair while always facing it. By the time you're back where you started you have also turned to face every wall in the room: one full turn. Someone sitting in the chair only ever sees your face. Now try walking around the chair while facing the same wall the whole time. You haven't turned at all, and the person in the chair sees your side, then your back, then your other side.
So the Moon spins, but slowly, and at precisely the right rate. Astronomers call this tidal locking or synchronous rotation. The interesting question is why the rate is so exact.
Earth's gravity pulls harder on the near side of the Moon than on the far side. That difference stretches the Moon slightly along the line to Earth, raising two bulges of rock: one facing us, one facing away. Tides aren't just for oceans. Solid rock flexes too, only less.
Early on, the Moon almost certainly spun faster than it does now. A fast-spinning Moon keeps carrying its bulges round with it, while Earth's gravity keeps trying to pull them back into line. Rock doesn't reshape instantly, so the bulges end up slightly ahead of the Earth–Moon line. Earth tugs on the leading bulge, and that tug acts like a hand on a spinning wheel: it slows the spin. The flexing rock turns the lost spin energy into heat.
This goes on until the spin exactly matches the orbit. At that point the bulges point straight at Earth and stay there, so there's nothing left to push against. The braking stops. Any slight drift gets pulled back, which makes the lock stable. Because the Moon is small and close to a heavy planet, this happened early in its history, and it has stayed locked ever since.
Here is Earth and the Moon seen from above the North Pole. The orange spoke marks one spot on the Moon. The small window shows what someone on Earth sees: the Moon's face, with the orange spot on it when it's on our side.
Things to try:
If the lock were perfect you'd see exactly half the Moon. In practice, over time, you can see about 59% of its surface. The extra slivers come from libration, a slow apparent rocking.
The main cause is the one in the simulation. The Moon's orbit is a slight ellipse, so it moves faster when it's close to Earth and slower when it's far. Its spin, though, stays steady. Sometimes the orbit gets ahead of the spin and sometimes the spin gets ahead of the orbit, so we see a little way round the east and west edges, by up to about 8 degrees. The Moon's axis is also tilted by a few degrees relative to its orbit, so we alternately see a bit over the north and south poles. Watching from different sides of a rotating Earth adds a final degree or so. The remaining 41% has never been seen from the ground. The first pictures of it came from the Soviet probe Luna 3 in 1959.
The same process works in reverse. The Moon raises tides on Earth, and Earth spins much faster than the Moon orbits: once a day, against once every 27 days. So Earth's rotation drags its tidal bulges ahead of the Moon. The Moon's gravity pulls back on them and slowly brakes Earth's spin. Tidal friction alone lengthens the day by about 2 milliseconds per century.
That lost spin has to go somewhere. The leading bulge pulls the Moon forward along its orbit, and a Moon that gains energy climbs into a higher orbit. Laser pulses fired at reflectors left by the Apollo astronauts show the Moon drifting away by about 3.8 centimetres a year, roughly as fast as your fingernails grow.
Over geological time it adds up. Fossil corals from about 400 million years ago laid down daily growth bands, and they record around 400 days in a year. The year was the same length; the days were shorter, closer to 22 hours.
Our Moon is the rule, not the exception. Mars's moons Phobos and Deimos are locked. So are Jupiter's four big moons (Io, Europa, Ganymede and Callisto), Saturn's Titan and Enceladus, the large moons of Uranus, and Neptune's Triton. Big, close, long-lived partners end up in step.
Pluto and its moon Charon go one step further. Charon is so large compared with Pluto that each has locked the other. They orbit every 6.4 days, and Pluto also spins once every 6.4 days. Charon hangs motionless in the sky over one hemisphere of Pluto and is never seen from the other.
In principle Earth could end up the same way, with the Moon parked above one side of the planet. But the braking is so slow that the Sun will swell into a red giant long before that happens. For now the arrangement is lopsided. The Moon shows us one face. We show it all of ours, once a day.