Why the Moon Always Shows the Same Face

Look at the full Moon tonight and you'll see the same pattern of dark patches that your grandparents saw, and that people saw ten thousand years ago. Every culture has its own picture in there, a face, a rabbit, a woman carrying sticks, but they are all looking at the same side. We never see the back.

That doesn't mean the Moon doesn't spin. It means it spins at a very particular speed.

Spinning at exactly the right speed#

The Moon takes about 27.3 days to travel once around Earth. It also takes about 27.3 days to turn once on its own axis. Because those two numbers match, the same side keeps facing us the whole way round.

Try it yourself: walk in a circle around a chair while always keeping your face towards it. By the time you're back where you started, you've also turned a full circle on the spot, and the chair has only ever seen your face. That is exactly what the Moon is doing.

This match is called tidal locking, and it is not a coincidence.

How Earth's gravity grabbed the Moon#

Gravity gets weaker with distance. That means Earth pulls slightly harder on the near side of the Moon than on the far side, and this difference stretches the Moon very slightly along the line between the two bodies. The result is a small bulge on the near side and another on the far side. These are tides, but in solid rock rather than water.

Early in its history, the Moon spun faster than it does now. As it rotated, those bulges were dragged slightly ahead of the line pointing at Earth. Earth's gravity kept pulling the bulges back towards that line, acting like a brake. Over millions of years this brake slowed the Moon's spin, until the bulges stopped moving relative to Earth. At that point there was nothing left to slow down: the Moon's rotation matched its orbit, and it has stayed locked ever since.

We actually see a bit more than half#

Although the Moon is locked, we can see about 59% of its surface over time, not just 50%. That's thanks to small wobbles called libration.

• The Moon's orbit is slightly oval, so it speeds up and slows down along the way, while its spin stays steady. This lets us peek a little way round its east and west edges.
• Its orbit is also slightly tilted, so we sometimes see a little over its north and south poles.

It's the far side, not the dark side#

People often call the hidden half "the dark side of the Moon", but it gets just as much sunlight as the side we see. During a new Moon, when the near side looks dark to us, the far side is in full daylight. It's only "dark" in the old sense of unknown.

Nobody saw it until 1959, when the Soviet probe Luna 3 sent back the first blurry photographs. It turned out to look quite different: the far side has far fewer of the large dark plains, called maria, and is covered in craters. In January 2019, China's Chang'e 4 made the first soft landing there.

The Moon is slowly leaving#

The tidal tug-of-war has another effect. As Earth's spin slows down, the Moon gains energy and moves slightly further away, by about 3.8 centimetres a year, roughly as fast as your fingernails grow. We know this precisely because Apollo astronauts left mirrors on the surface, and observatories still bounce lasers off them to measure the distance.

Not just our Moon#

Tidal locking is common across the Solar System. Most of the large moons of Jupiter and Saturn keep one face turned towards their planet. Pluto and its moon Charon go one step further: they are locked to each other, so each one always shows the same face to the other, and from one side of Pluto, Charon would hang in the same spot in the sky forever.

In short#

• The Moon does rotate, once every 27.3 days, the same time it takes to orbit Earth.
• Earth's gravity raised tidal bulges on the Moon and used them to brake its spin until the two periods matched.
• Libration lets us see about 59% of the surface.
• The far side is not dark; it was just unseen until 1959.
• The same process is lengthening Earth's day and pushing the Moon about 3.8 cm further away each year.