Libration: how we see 59 per cent of the lunar surface
Synchronous rotation, Cassini's laws and three apparent wobbles: why from Earth we see more than half the Moon, and how its far side was first photographed.
The Moon always shows us the same face. This observation is as old as humanity and has given rise to countless tales about the face or the figures drawn by the lunar seas. Yet it is not entirely accurate: thanks to a series of slight apparent wobbles, called librations, about 59 per cent of the surface of our satellite can be observed from the Earth over time.
Synchronous rotation
The Moon turns on its axis in the same time it takes to complete one orbit around the Earth, about 27.3 days. This synchronous rotation is the result of tidal forces which, over very long ages, slowed the Moon's spin until it turned the same hemisphere permanently towards our planet. Many satellites in the solar system, among them the four largest moons of Jupiter, are in the same condition.
The rules describing this behaviour were formulated by Giovanni Domenico Cassini in 1693. They state that the Moon rotates uniformly, that its axis forms a constant angle with the plane of the ecliptic, and that the spin axis, the perpendicular to the orbit and the perpendicular to the ecliptic always lie in one and the same plane.
Three wobbles
Libration in longitude arises from the elliptical orbit. The Moon's rotation is constant, while its orbital speed varies: it is greater near perigee and smaller near apogee. For this reason, during the month the Moon seems to turn slightly back and forth, showing us now a strip of its eastern edge, now one of its western edge, by an amount approaching eight degrees.
Libration in latitude depends on the tilt of the lunar axis with respect to the plane of the orbit, about six and a half degrees: on some days of the month we see a little beyond the Moon's north pole, on others beyond its south pole. Finally there is diurnal libration, due to the fact that the observer, carried by the Earth's rotation, views the Moon from slightly different points at moonrise and moonset.
From the first observations to the far side
Librations were noticed with the first telescopes. Galileo described some of them in the 1630s, and Johannes Hevelius, in his Selenographia of 1647, discussed libration in longitude. The regions of the limb, such as the great basin of Mare Orientale, can be seen only foreshortened and at moments of favourable libration, and for this reason their true shape was fully understood only in the twentieth century.
The remaining 41 per cent stayed unknown until October 1959, when the Soviet probe Luna 3 transmitted the first photographs of the far side. It revealed a very different landscape, almost without the great dark seas and densely cratered. In January 2019 the Chinese probe Chang'e 4 made the first soft landing on that side, inside the crater von Kármán.
The face of the Moon
The constancy of the visible face has fed the imagination for millennia. Plutarch devoted a dialogue, On the Face of the Moon, to ancient theories on the nature of the lunar spots. In European traditions people saw in the spots a man with a bundle of thorns, in East Asia and Mesoamerica a hare or a rabbit. Dante, in the second canto of Paradise, has Beatrice explain the origin of the spots, recalling the popular belief that recognised Cain in them.
The far side has often been wrongly called the dark side, although it receives as much sunlight as the visible one: at new moon, indeed, it is fully lit. The true mystery was not darkness but inaccessibility, and libration reminds us that even what seems motionless in the sky in fact performs a slow and regular movement.