"I find it difficult to believe," a correspondent wrote some time ago, "that purely as a matter of coincidence, the moon just happens to rotate about its axis at exactly one revolution per orbit around the Earth." He was referring to the same phenomenon which prompted an anonymous Victorian maiden to express her feelings in impromptu verse
O Moon, lovely Moon, with your beautiful face,
Careering at speed through the boundaries of space,
Whenever I see you I think in my mind,
Shall I ever, O ever, behold thy behind?
Viewed from the Earth, the moon appears to complete one revolution of its orbit from new moon to new moon in a synodic month, a period of a little over 29 days which is also referred to as a luxation. But the Moon also rotates on its own axis, and it is a remarkable fact, noted in their individual ways by both the persons mentioned, that the Moon's own rate of rotation exactly matches its period of orbit around the Earth and in the same direction.
Consequently, the Moon always turns the same face, towards the Earth and we never see the other side of it.
As my correspondent suspected, of course, this synchronisation does not arise by pure coincidence it is a result of tidal friction over the millenniums. In the very earliest days of its existence, the Moon rotated much more rapidly than it does now, but the strong gravitational pull of the Earth produced a tendency for a tidal bulge in the direction of the parent planet.
Gradually the Moon's rate of spin decreased, until eventually it stopped completely relative to the Earth. Now the same hemisphere of the Moon, which still has a slight bulge in the direction of the Earth, stays pointed towards us all the time.
As it happens, we can really see a little more than half the "Moon. Variations in the satellite's orbit and in the tilt of its taxis relative to the Earth make the lunar globe appear to oscillate in a period which nearly matches that of its revolution. This apparent oscillation is called libration, and amounts to about six or seven degrees on either side of the mean position.
The result is that some six tenths of the lunar surface comes into our field of view at one time or another, while the remaining four tenths are forever hidden from our eyes.
Indeed, one of the highlights of the early lunar missions was our first glimpse by photograph of this hidden or "dark" side of the moon of, as it were, the moon's behind.