The Moon does turn
It is the most common mistake in the sky: since we always see the same face of the Moon, people assume it doesn’t spin on itself. The opposite is true. If the Moon didn’t spin at all, it would show us each of its faces in turn as it travels around the Earth.
Walk around a table while keeping your face fixed on a chair at its centre: to stay turned toward it, you are forced to pivot on yourself exactly once over a full lap. The Moon does the same. It spins on itself once per orbit, in a little over twenty-seven days. Its rotation and its trip around the Earth are tuned to the same beat.
A matter of tides
This perfect match has a name, synchronous rotation, and it is no coincidence. Long ago, the Moon spun faster. But Earth’s gravity does not pull equally on the near side and the far side of the Moon: it stretches it ever so slightly, gives it a bulge. As long as the Moon spun quickly, that bulge was constantly dragged off to one side, which created friction, a steady brake on its spin.
Over billions of years this braking slowed the Moon’s spin, until it settled exactly onto its orbit and then froze there. It is the very same force that, the other way around, lifts the water of our oceans twice a day: the tidal force. The Earth did to the whole Moon what it does each day to our seas.
The far side isn’t a dark side
One last confusion is worth clearing up. The face we never see is often called the “dark side.” It is not. It receives as much sunlight as the other: when it’s full Moon for us, it is night over there, and the reverse. It is simply hidden from our eyes, not starved of light. Humanity only discovered it in 1959, when the Soviet probe Luna 3 photographed it for the first time.
So the Moon does not turn its back on us out of stubbornness. It has danced with the Earth for so long that it ended up keeping, forever, the same face turned our way.