The Moon: its origin, its far side, and what it does for Earth

The Moon has a radius of about 1,740 km — less than a third of Earth's width — and sits an average of 384,400 km away. Nobody has ever stood on the ground and seen its far half.

Why we only ever see one face

The Moon takes 27 days to go once around Earth, and exactly 27 days to turn once on its axis. The two periods matching is no coincidence: this is synchronous tidal locking, and NASA describes it as fairly common rather than exceptional.

The mechanism is friction. Earth's gravity stretches the Moon out of shape, and each time it flexes some energy escapes as heat. The spin slowed until it matched the orbit, and from then on the far side has stayed permanently out of view.

To be clear: that is the far side, not the dark side — it still gets sunlight exactly half the time, it simply never turns towards us.

It was born from an impact

The most widely supported hypothesis is that the Moon formed after a Mars-sized body — named Theia — struck the young Earth. NASA keeps its distance here: perhaps a single object, perhaps a series of them.

The Moon came into being roughly 60 million years after the Solar System began to form. It was molten at birth, covered by a magma ocean that lasted tens to hundreds of millions of years.

A world with almost no atmosphere

The Moon's atmosphere is so thin that NASA likens it to the haze thrown up when you beat a dirty rug. With no gas layer to hold heat or block radiation, surface temperatures swing from about 127°C in direct sunlight down to roughly −173°C in darkness. Nothing stops small meteoroids either, so the surface is saturated with craters. Only a few craters near the poles, permanently without sunlight, still hold ancient water ice.

What it does for Earth

The Moon's gravity plays a large part in producing ocean tides, while solar eclipses depend on its orbit.

A third role gets mentioned less often. Earth's axis is currently tilted 23.4°, and over the past million years it has varied only between 22.1° and 24.5°. That tilt is what gives us the four seasons. Work by Laskar and colleagues in Nature in 1993 argues that the Moon is what keeps that tilt stable.

And it is moving away

In 1969 the Apollo 11 crew left a laser retroreflector on the lunar surface. Thanks to it, the average Earth–Moon distance is now measured to better than one part in ten billion, and those measurements show the Moon receding by about 3.8 cm per year.

🌙 The Moon is not an inert ball of rock that happens to accompany Earth. It is most likely a fragment of an ancient collision, held fast by Earth until it turns only one face towards us — and in exchange, it may be what keeps this planet's axis from swinging far.