Why we always see the same side of the Moon

The Moon turns once on its axis in the same 27.3 days it takes to go once round Earth.

It does spin

People often say the Moon does not rotate, because the same pattern of dark plains faces us night after night. In fact it does rotate, just slowly. NASA's fact sheet gives its sidereal rotation period as 655.72 hours, which is 27.32 days, the same as the time it takes to orbit Earth once against the stars. Because the spin and the orbit keep pace, the side facing Earth stays facing Earth. NASA compares it to a dancer circling a partner while always facing them.

You can test this with a chair. Walk once round it while always facing it. When you finish, you have also turned once to face every wall of the room. Now walk round again while always facing the same wall: the chair sees your front, one side, your back and your other side. The second walk is what a non-spinning Moon would do. In the model, switch off "Moon spins" and watch the red dot drift out of view, and the dark plains on the little Moon in the panel slide round to the far side.

How the Moon got locked

This matching is called synchronous rotation or tidal locking, and NASA notes that it is common: all the large moons in the solar system are tidally locked to their planets. For our Moon it began early. The young Moon was hot, partly molten and much closer to Earth, and it was spinning fast. Earth's gravity stretched it into a slight football shape, with a bulge pointing roughly at Earth. As the Moon turned, the bulge had to move through its rock, always lagging a little behind. Gravity kept pulling the bulge back towards Earth, and the constant flexing turned spin energy into heat.

Spin was lost until one turn took exactly as long as one orbit. At that point the bulge no longer had to move through the rock, the heating stopped, and the spin rate stopped changing. The same tug of war, this time between the Moon and Earth's oceans, is still going on. It slows Earth's rotation a tiny amount and pushes the Moon outward by about 3.8 cm a year. NASA estimates that in about 50 billion years, if the Earth and Moon somehow survived the Sun's death, Earth would become locked too, and only one half of the world would ever see the Moon. Pluto and its moon Charon have already reached that state.

Slightly more than half

Over a month we actually see about 59% of the Moon's surface, according to NASA's visualisation studio. The extra comes from libration, a slow rocking that has three main causes. The orbit is an ellipse, so the Moon moves faster near perigee and slower near apogee while its spin stays steady; the spin falls a little behind or runs a little ahead, and we peek round the eastern or western edge. The Moon's spin axis is tilted by about 6.7° to its orbit, so sometimes we look slightly over the north pole and sometimes under the south pole. And Earth's own tilt makes the Moon seem to lean one way and then the other, like a metronome.