Why Does the Moon Always Show the Same Face? Rotation Slowed to a Stop

A body orbiting another is pulled more strongly on its near side than on its far side, which stretches it slightly along the line between them, producing bulges on both faces. If the body rotates faster than it orbits, those bulges are continuously being carried out of alignment and reformed, which involves flexing the material and dissipating energy as heat, and the gravitational pull on the misaligned bulge acts as a brake. Rotation therefore slows until the bulge stays aligned, which happens exactly when the body rotates once per orbit, and at that point the braking stops because nothing is flexing any more. The state is stable, so once reached it persists.

The outcome is common across the solar system:

• •Our moon, which shows the same hemisphere continuously
• •Nearly all large moons of the giant planets, for the same reason
• •Pluto and Charon, which are locked to each other mutually, each showing one face to the other
• •Mercury, which is in a different resonance, rotating three times for every two orbits
• •Many exoplanets orbiting close to their stars, which is expected on theoretical grounds
• •The earth, which is slowly being locked to the moon and will not finish

The same process operates on the earth and is measurable. Tidal bulges in the oceans are dragged ahead of the line to the moon by the earth's faster rotation, and the moon's pull on those displaced bulges brakes the rotation, lengthening the day by roughly two milliseconds per century. Angular momentum is conserved, so what the earth loses the moon gains, which moves it into a higher orbit at about four centimetres per year, measured directly by bouncing lasers off reflectors left on the surface. Fossil evidence supports the long-term picture, since growth bands in ancient corals and shells record more days per year in the past, consistent with a shorter day hundreds of millions of years ago.

Saying that one hemisphere always faces us is a simplification, and the correction is observable with binoculars. The moon's orbit is elliptical, so it moves faster when nearer and slower when further, while its rotation stays steady, which means the face runs slightly ahead and then slightly behind over each orbit and we see a little around each edge in turn. Its axis is also tilted relative to the orbit, so we see slightly over the north and south poles alternately. And an observer's position on a rotating earth shifts the viewing angle slightly. Together these effects, called libration, reveal about fifty nine per cent of the surface over time rather than fifty, and watching the moon across a month shows it apparently rocking.

The consequences for a locked planet are substantial and are debated in the context of habitability. A planet locked to its star has a permanently lit hemisphere and a permanently dark one, with a ring of perpetual twilight between, and the temperature contrast would be extreme without an atmosphere to redistribute heat. Modelling suggests a sufficiently thick atmosphere or an ocean does redistribute enough to keep a substantial region habitable, which matters because planets in the habitable zones of small cool stars orbit close enough to be locked, and such stars are the most numerous by a wide margin. Atmospheric collapse on the night side, where gases could freeze out permanently, is the main competing concern, and the question is unresolved and consequential for where to look for life.

Gravity stretches an orbiting body into bulges, and rotating faster than it orbits drags those bulges out of line, which flexes the material and brakes the rotation until it matches the orbit exactly. Most large moons ended up there. The same process lengthens our day by about two milliseconds per century and pushes the moon four centimetres further away each year.