Tidal Locking & Libration: One Face, and the 59% We Can See

Why the Moon keeps one face toward Earth

It is a common surprise that the Moon rotates at all. Since it always shows us the same features, the Sea of Tranquility and the bright rays of Tycho night after night, it can look as though it simply hangs there, fixed. In fact it is turning. It rotates exactly once for every orbit it makes of Earth, about every 27.3 days. Because the spin and the orbit take the same time, the same hemisphere stays turned toward us the whole way around. This matched-up turning is called synchronous rotation.

Picture the difference. If the Moon did not spin at all, it would keep pointing in one fixed direction in space while it circled Earth, and over a month we would see every side of it in turn, front, side, far, other side, front again. Because it does spin, once per lap, that reveal never happens. The near side is held toward us as if by an invisible tether.

Why should the spin and the orbit match so precisely? The answer is tidal friction, the same family of forces behind the ocean tides. Earth's gravity stretches the Moon very slightly into an egg shape, raising a small bulge on the side facing us. Long ago the Moon spun faster, and that bulge was dragged a little off the Earth line. Earth's pull on the offset bulge acted as a brake, gradually slowing the Moon's spin, until the spin settled into step with the orbit and the bulge pointed straight at Earth. Once locked, the arrangement is stable, so the Moon has kept one face toward us for billions of years. Most large moons in the solar system are locked to their planets the same way.

Libration: the Moon's slow rock

The lock is not perfectly rigid. If you watch the Moon carefully month after month, its face appears to nod and sway by a few degrees, so that features near the edge slip into view and then hide again. This gentle rocking is libration, and it comes in three parts.

Libration in longitude is the east-west rock, and it comes from the shape of the orbit. The Moon's spin is beautifully steady, always the same rate. Its orbit, though, is a slight ellipse, so by Kepler's second law the Moon travels faster when it is near perigee (closest to Earth) and slower near apogee (farthest). The steady spin therefore runs a little ahead of the orbit for half the month and a little behind for the other half, swinging the face east and west by up to about 7.9° and letting us see a sliver around each side edge.

Libration in latitude is the north-south nod, and it comes from a tilt. The Moon's spin axis is tipped about 6.7° from the axis of its orbit, much as Earth's axis is tilted to its orbit around the Sun. So as the Moon rides around Earth, first its north pole leans toward us and then, half an orbit later, its south pole does, letting us peer a little over the top and then under the bottom.

The third and smallest part is diurnal libration, worth about 1°. It comes from us, not the Moon. As Earth turns through a night, it carries you a few thousand kilometers, from one side of the planet toward the other relative to the Moon, so you view the Moon from a slightly different angle at moonrise than at moonset and see a touch farther around the near edge. That 7.9° is the extreme, not the everyday value. In a typical month the east-west rock is closer to 6.3°; it reaches the full 7.9° only when the Sun's pull has stretched the Moon's orbit to its most elongated, because the eccentricity itself waxes and wanes over the months. The interactive above uses a steady, average-shaped orbit, so its longitude libration tops out near that typical 6.3°.

How we come to see 59 percent

At any single glance, we see exactly one hemisphere of the Moon, half of its whole surface. Libration does not change that instantaneous half; what it does is slowly shift which half we are looking at. The east-west rock brings a curved sliver of the eastern and western edges into view; the north-south nod adds slivers at the top and bottom; and the daily wobble trims a little more all the way around.

Add up all the edges that libration ever swings into view, over a month and then over the years as the different librations drift in and out of step, and the total comes to about 59 percent of the surface. The interactive above keeps a running tally: as you let time run, the fraction climbs from the 50 percent of a single view up toward that 59 percent ceiling. The remaining 41 percent, the true far side, never turns toward Earth and was first seen only in 1959, when the Soviet probe Luna 3 flew behind the Moon and sent back the first grainy photographs.

The far side is not the dark side

It is worth clearing up one of the most common misunderstandings about the Moon. The far side and the dark side are not the same thing, and mixing them up gets the physics backward.

The far side is the hemisphere that permanently faces away from Earth, the roughly 41 percent we cannot see from the ground. It is a fixed piece of the Moon, defined by the tidal lock. The dark side, on the other hand, is just whichever half is not being lit by the Sun at a given moment, and it sweeps steadily around the Moon as the Moon runs through its phases. At new moon, when the near side is dark, it is actually the far side that is bathed in full sunlight. Over a month the far side gets exactly as much daylight as the near side. So the far side is not perpetually dark. It is simply hidden from Earth, and it experiences the same slow lunar day of about 29.5 days as everywhere else on the Moon.