Tidal locking does not mean the Moon has stopped rotating. In fact, the opposite is true: the Moon rotates once on its axis in approximately 27.3 days, almost exactly the same amount of time it takes to complete one orbit around Earth. Because these two periods match, the same hemisphere continually faces us.
How Did the Moon Become Tidally Locked?
The Moon probably did not begin this way.
Early in its history, the Moon likely rotated much faster. Earth's gravity, however, does not pull equally strongly on every part of the Moon. The side facing Earth experiences a slightly stronger gravitational attraction than the opposite side.
This difference produces a tidal force.
When the young Moon rotated more rapidly, Earth's tidal forces slightly distorted its shape. Because the Moon was rotating, this tidal bulge was not always perfectly aligned with Earth. Earth's gravity pulled on the misaligned bulge, creating a torque that gradually slowed the Moon's rotation.
Eventually, the rotation period matched its orbital period. At that point, the tidal bulge remained approximately aligned with Earth, producing the stable configuration we observe today.
This state is called synchronous rotation.
The “Dark Side” Isn't Actually Dark
The hemisphere facing Earth is called the near side, while the opposite hemisphere is the far side.
Calling the far side the "dark side of the Moon" is misleading. It receives approximately the same amount of sunlight as the near side. During a new moon, for example, the far side is largely illuminated while the near side faces away from the Sun.
Humanity did not see the far side until the Soviet Luna 3 spacecraft photographed it in 1959.
Those images revealed something surprising: the two sides look dramatically different. The near side contains large dark regions called maria, created by ancient lava flows. The far side contains far fewer maria and is dominated by heavily cratered highlands.
Scientists continue investigating why the Moon developed this geological asymmetry.
Is Exactly 50% of the Moon Visible?
Interestingly, tidal locking doesn't restrict us to exactly half of the lunar surface.
The Moon's orbit is slightly elliptical, and its orbital speed changes as it travels around Earth. Meanwhile, its rotation remains relatively steady. Combined with the Moon's tilted orbit and our changing viewing position, this produces an apparent rocking motion called libration.
Over time, libration allows observers on Earth to see about 59% of the Moon's surface, although only roughly half is visible at any single moment.
Tidal Locking Happens Across the Solar System
Our Moon is far from unique. Many large moons in the solar system are tidally locked to their planets, including Europa around Jupiter, Titan around Saturn, and Triton around Neptune.
The same physics may even affect exoplanets. Planets orbiting close to small red-dwarf stars can experience strong tidal forces and may eventually become tidally locked, potentially producing permanent daysides and nightsides.
Earth and the Moon are still exchanging angular momentum today. Tidal interactions cause the Moon to move away from Earth by about 3.8 centimeters per year, a rate measured using lasers reflected from mirrors left on the lunar surface.
The familiar face we see in the night sky is therefore not a coincidence. It is the result of billions of years of gravitational interaction, slowly synchronizing the Moon's rotation with its journey around our planet.