Why Does the Moon Always Show Us the Same Face?

Step outside on a clear evening when the Moon is full and you will see the same grey patches your great-grandparents saw: the same dark seas and the same bright craters, arranged in the same face. People have found a rabbit in it, or a man, or a woman carrying firewood, and the picture has never changed. Yet the Moon is a ball, and balls usually turn. So where is the rest of it?

The short answer

The Moon does turn. It simply turns at exactly the pace at which it travels round us. According to NASA, the Moon takes 27.3 days to circle the Earth, and precisely as long to spin once on its axis. Try it with a friend: walk a full circle round them while always facing them. By the time you are back where you started you have also turned once on the spot, and your friend has never seen the back of your head. Astronomers call this arrangement tidal locking.

• 27.3 days one trip round the Earth, and one full turn of the Moon
• 59% the most of the Moon's surface that can be seen from Earth over time
• ≈ 4 cm how far the Moon drifts away from the Earth each year

How a spinning Moon was slowed down

It was not always so. The Earth's gravity pulls on the Moon hard enough to stretch it very slightly, raising a bulge that points towards us. While the Moon was spinning faster, that bulge kept being carried out of line, because rock takes time to shift, and the Earth's gravity kept tugging it back. All that flexing turned the energy of the spin into heat, which was lost. Little by little the Moon slowed, until the bulge stopped moving and one face was left looking at the Earth for good.

The Moon is in good company. NASA notes that all the large moons of the solar system are locked to their planets in the same way, the four big moons of Jupiter among them. Pluto and its moon Charon are locked to each other, so each shows the other a single face. The Earth is slowly heading the same way: the Moon moves about 4 centimetres farther from us every year, and in about 50 billion years, if the Sun lasted that long, only one half of our planet would ever see the Moon.

What people often get wrong

Two mistakes ride along with this fact. The first is that the Moon does not rotate. It does, once per orbit, and that is exactly why we see one face. The second is the phrase the dark side of the Moon. The side we cannot see is the far side, not a dark one. NASA puts it plainly: there is no permanent dark side. The far side gets as much sunlight as the near side, and each side has its day and its night as the Moon turns.

Nor do we see exactly half. The Moon's speed along its path is uneven and its axis is tilted, so from the Earth it seems to rock and nod a little, a motion called libration. Thanks to it, patient observers can see up to 59 percent of the surface over time, although, as a NASA Goddard guide points out, the extra strip lies so near the edge that not much detail can be made out.

How we know

Anyone with a notebook can check the first part: draw the Moon's markings night after night and they stay put, apart from that gentle rocking. The hidden side had to wait for spacecraft. Satellites such as NASA's Lunar Reconnaissance Orbiter have photographed the far side, and mapping it from space settled old puzzles: a feature glimpsed on the very edge, Mare Orientale, turned out to be an impact crater. So the face in the Moon is real, in a way. It is the only side of a very polite neighbour who has never once turned its back on us.