Why Does the Moon Keep One Face Towards Us? A Spin That Got Locked

Gravity weakens with distance, so the near side of a body feels a stronger pull than its far side, which stretches it slightly into an elongated shape with bulges pointing towards and away from its partner. If the body is rotating faster than it orbits, that bulge is continuously carried ahead of the line joining the two bodies, and the partner's gravity pulls back on the leading bulge, which applies a braking torque. The rotation therefore slows until it matches the orbital period, at which point the bulge stays aligned and the torque disappears, leaving the body locked. The process also dissipates energy as friction inside the body, heating it. How quickly it happens depends strongly on distance, on the mass ratio and on how readily the body deforms, and for the moon it took on the order of tens to hundreds of millions of years, which is brief compared with its age, which is why the lock is complete and why it happened long before anyone was watching.

The locked face is not exactly half the moon, because several wobbles reveal a little more:

• •Libration in longitude, since the moon's orbit is elliptical so its orbital speed varies while its rotation is constant, letting us see slightly around each eastern and western edge in turn
• •Libration in latitude, since the moon's rotation axis is tilted relative to its orbit, so we see a little over the north and south poles alternately
• •Diurnal libration, a small effect from the observer's position shifting as the earth rotates
• •Together these expose about fifty-nine percent of the surface over time, leaving roughly forty-one percent never visible from earth
• •The far side was first photographed by the Soviet Luna 3 mission in 1959 and first seen directly by the crew of Apollo 8 in 1968
• •It is the far side rather than the dark side, since it receives sunlight exactly as often as the near side does, a point the popular phrase gets wrong

The far side turned out to look startlingly different, and explaining it is an active question. The near side is covered by large dark plains of solidified lava, called maria, which occupy a substantial share of its surface. The far side has almost none, being dominated by heavily cratered highlands, and its crust is measurably thicker. Several explanations have been proposed, including that the thinner near-side crust allowed molten rock to reach the surface more easily, and that the difference in crustal thickness arose because the early moon was tidally locked while still hot and the earth, then much closer and still glowing from its own formation, heated the near side differently so that it cooled and crystallised at a different rate. Another proposal involves a collision with a second smaller moon that accreted onto the far side. The far side also hosts the South Pole-Aitken basin, one of the largest impact structures known in the solar system.

Tidal locking is common rather than exceptional. Almost all large moons in the solar system keep one face towards their planet, including the four large moons of Jupiter and Saturn's Titan. Pluto and Charon are mutually locked, each keeping one face towards the other, which is the eventual end state of any such system given enough time. Mercury sits in a three to two resonance with the sun rather than a one to one lock, rotating three times for every two orbits, which was only discovered by radar in 1965 after a century of assuming it was locked. The earth is being slowed by the moon's tides at a measurable rate, lengthening the day by roughly two milliseconds per century, confirmed by ancient eclipse records and by growth lines in fossil corals showing more days per year in the distant past. The corresponding transfer of angular momentum pushes the moon outward by a few centimetres per year, measured directly by bouncing lasers off reflectors left by Apollo missions.

Tidal forces stretch a body into a bulge, and if it spins faster than it orbits, its partner's gravity pulls back on the leading bulge and brakes the rotation until spin and orbit match. The moon reached that state within a few hundred million years. Librations caused by its elliptical orbit and tilted axis reveal about fifty-nine percent of its surface. The same process locks most large moons, and the moon's tides are lengthening earth's day by about two milliseconds per century.