Why do we always see the same side of the moon and never the other side

Why do we always see the same side of the Moon?

In 1609, soon after the telescope’s invention, the great Italian astronomer, Galileo Galilei, adapted it as a means of studying the night sky, its first use in astronomy. Galileo’s telescopes were crude by our standards (his first magnified objects about three times), but they showed him the heavens as nobody had seen them before. For all that, his overall view of the Moon, though clearer, was not much different from that seen by the first humans about three million years ago or by Babylonian astronomers in 2000 bc.

Whether we look at the Moon with our naked eyes or with the most powerful telescopes, we see only one face of it. To understand why this is (and perhaps to explain it to a child) try' this simple illustration, in which you will act as the Earth and the child as the Moon. Stand on a spot and get the child to move in a circle round you by stepping to the side, face turned towards you all the time. As the child moves, you turn slowly so that you are constantly facing one another. When both you and the child have turned a full circle, you will be back where you started without either of you having seen the back of the other. But that can happen only while you keep your rotations perfectly synchronised. If the child made only 99-999 per cent of a turn in every complete orbit, you would eventually see his or her back.

The Moon rotates once on its axis in the twenty seven and a third days that it takes to complete a circuit around the Earth. If it rotated slightly faster or slower, the other face would gradually come completely into view. But that doesn't happen. The rotation of the Earth and of the Moon are so perfectly synchronised to keep the Moon’s other face hidden that they might almost be meshed by gears.
Can that precision be accidental? You might believe so, except that many other moons the satellites of Mars and Jupiter, for example show only one face to their parent planets. For so many moons in the solar system to act in this way is no coincidence. They are governed by what astronomers call ‘tidal locking’.

When our Moon was molten rock, a bulge developed on its side facing towards Earth. This bulge, a tide in the molten rock, was caused by the Earth’s gravitational pull, which acts more strongly on the Moon’s closer face than on its far side. As the Moon spun, the tidal bulge of fluid rocks rose and fell, rubbing against material deeper in the Moon’s core and gradually slowing down the Moon’s spin. While there was a difference between the time it took the Moon to rotate on its axis and for it to revolve round the Earth, tidal friction continued to slow down its spin. Only when the Earth and its moon were perfectly synchronised did the braking action of tidal friction cease. But when it did, the Moon’s other face was hidden from direct view for ever.

Fortunately, we no longer rely on a direct view from Earth to give us information about the Moon and other heavenly bodies. In 1959, the Soviet Union’s space probe Lunik III sent us the first pictures of the far side of the Moon, since when dozens of other missions by the Soviets and by the United States have added enormously to our knowledge.