Lockla

Meet Lockla, the Moon, who spins exactly once for every trip around Earth, so she always shows us the same face.

Earth's gravity raises tidal bulges in the Moon, just as the Moon raises them in Earth's oceans. If the Moon spins faster than it orbits, its bulges get carried ahead of the Earth–Moon line. Earth's pull on them then acts as a brake. Over time that tidal torque slowed the Moon's spin until its rotation period matched its orbital period, about 27.3 days. In that tidally locked state the bulges stay aligned and the braking stops.

That is why we always see the same side. The far side is not permanently dark; it gets as much sunlight as the near side, just at different times.

Tidal locking is common. Most large moons in the solar system, including Jupiter's Galilean moons, are locked to their planets. Pluto and its moon Charon are locked to each other.

Because the Moon's orbit is elliptical and tilted, we actually see about 59 percent of its surface over time, through a gentle wobble called libration.