The Moon rotates - in sync with its orbit
It may seem like the Moon does not rotate at all: the same dark patches, the same pattern every night. In fact, it completes one rotation on its axis in about 27.3 days - exactly as long as it takes to orbit Earth. The logic is easy to test: walk around a table without taking your eyes off a cup in the center. From the side, it is clear that you have turned around once, even though you kept facing the cup.
This match is not a coincidence but a necessity. Earth pulls more strongly on the Moon's near side than its far side, and the Moon is slightly stretched along that line - it has a permanent "bulge." While the Moon rotated faster, the bulge moved ahead, and Earth's gravity pulled it back. This brake worked for a very long time, until rotation matched orbit. After that, there was nothing left to slow down: the bulge faces Earth directly.
The lock is not perfect, though. The orbit is not circular, so the Moon moves along it faster at times and slower at others, while rotating evenly on its axis. Its axis is also slightly tilted. Together, this makes the Moon wobble: about eight degrees from side to side and slightly up and down. Month after month, different craters peek out from beyond the edge of the disk.
of the Moon's surface is visible from Earth in total
The far side is not dark, just different
"Dark side" is a poor name: it gets exactly as much sunlight, with its own days and nights lasting two weeks. It was dark only to us. The first images came from the Luna 3 spacecraft in 1959, and the view was surprising: no broad dark "seas" - pale crust, covered in craters. Seas cover about a third of the visible disk and only a couple of percent of the far side.
The difference is real, but the explanation is still preliminary: the far side's crust is noticeably thicker, so ancient lava had a harder time breaking through and flooding the lowlands. Why it is thicker is still debated.
But the locking itself is well understood: it is common in the Solar System. Pluto and Charon face each other, and Jupiter's large moons face Jupiter.
- Compare full Moon photos from different months: the crater pattern at the edge shifts slightly
- Choose a crater right at the edge: in some months it is visible, in others it slips beyond the horizon
- Look at photos of the far side: pale, covered in craters, unlike the familiar Moon
- Don't expect to see the wobble in one evening - it builds over weeks
- Don't call the far side dark: it gets just as much sunlight
- Don't say the Moon does not rotate - without rotation, we would see all of it in a month
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Sources
- The Moon's rotation and orbital periods and libration data - NASA
- its recession of ~3.8 cm per year is measured by laser ranging (Lunar Laser Ranging)
- far-side crust thickness - the GRAIL mission