The Moon keeps one face turned towards us permanently. Every photograph taken from Earth for the whole of human history shows the same hemisphere, the same craters, the same dark patches. The far side was not seen by anyone until a Soviet probe photographed it in 1959.
The usual shorthand is that the Moon does not rotate. That is exactly wrong, and the real explanation is more satisfying.
The Moon rotates. That is why we see one face.
The Moon turns on its axis once every 27.3 days, and it orbits the Earth once every 27.3 days. Because those two periods match, it turns through exactly the same angle in the time it takes to go once round us, and the same side stays pointed inward the whole way.
If the Moon did not rotate at all, we would see every part of it over the course of a month, as the orbit carried it around and its fixed orientation exposed each side in turn. The locking requires rotation, not the absence of it.
You can demonstrate this by walking around a chair while keeping your face towards it. You end up facing every wall of the room in turn, so you have rotated once. Someone sitting in the chair only ever sees your face.
Why the two periods match
This is called tidal locking, and it is not a coincidence. It is the state the system was always going to settle into.
The Earth’s gravity pulls slightly harder on the near side of the Moon than on the far side, because the near side is closer. That difference stretches the Moon very slightly into an elongated shape, with a bulge pointing towards us.
Early in its history the Moon rotated faster than it orbited. Each rotation had to drag that bulge around, and the bulge was constantly being pulled back into alignment with the Earth. Flexing solid rock repeatedly turns energy into heat, and that heat came out of the Moon’s rotation. Over hundreds of millions of years the spin bled away until the rotation matched the orbit, at which point the bulge stopped moving relative to the Moon and the braking stopped. It has been stable there ever since.
The same process is common in the solar system. Most large moons of the giant planets are tidally locked to their planets, and Pluto and Charon are locked to each other, so each permanently shows the other one face.
We actually see rather more than half
About 59 per cent of the lunar surface can be seen from Earth at one time or another, thanks to a wobble called libration.
- Libration in longitude. The Moon’s orbit is elliptical, so it speeds up when closer to Earth and slows when further away. Its rotation, however, is steady. The two drift in and out of step across a month, letting us peer a little around each edge in turn.
- Libration in latitude. The Moon’s axis is tilted relative to its orbit, so we see slightly over the north pole at some times and the south pole at others.
- Diurnal libration. The Earth’s own rotation carries an observer several thousand kilometres sideways between moonrise and moonset, giving a marginally different viewing angle.
The far side is not the dark side
The far side receives just as much sunlight as the near side. When we see a full moon, the far side is in complete darkness; when we see a new moon, the far side is fully lit. It has a day and night cycle like anywhere else.
It does look strikingly different. The near side is covered in the large dark plains called maria, ancient basalt flows that fill impact basins. The far side has very few of them and is dominated by heavily cratered highlands. The leading explanation is that the far side’s crust is significantly thicker, so rising magma reached the surface far less often.
If this was interesting
For more everyday physics explained from first principles, see why the sky is blue, or why an octopus has three hearts for the same approach applied to biology. For another consequence of the mismatch between rotation and orbit, why we have leap years.
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