The Moon
Earth's only natural satellite was born in a cataclysm, steadies our seasons, drives the tides and remains the only other world humans have walked on.
Look up on almost any night and there it is, the only other world whose surface you can see with your naked eye. The Moon has set the rhythm of calendars, stirred the oceans and pulled at human imagination for as long as there have been humans. It is also a time capsule, preserving a record of the solar system's violent youth that Earth has long since erased.
A violent birth
The Moon is unusually large for its planet, about a quarter of Earth's diameter, and rocks brought back by the Apollo astronauts showed it is made of material almost identical to Earth's outer layers, but with far less iron and almost no water. The best explanation is dramatic. About 4.5 billion years ago, when Earth was newly formed, a planet the size of Mars slammed into it. The collision vaporised the impactor and much of Earth's outer rock, flinging a ring of molten debris into orbit. Within perhaps a century that debris clumped together into the Moon. At first it hung only about 20,000 kilometres away, filling the sky, and Earth's day was perhaps five hours long.
Craters, seas and dust
The Moon has no atmosphere and no weather, so nothing erases its scars. Every impact for four billion years is still there. The highlands, which look bright from Earth, are the original crust, saturated with craters. The dark patches, which early astronomers named maria, Latin for seas, are not water but vast plains of frozen lava that welled up through giant impact basins between about 3.1 and 3.8 billion years ago, when the Moon's interior was still hot.
The whole surface is coated in a layer of fine grey dust and broken rock called regolith, pulverised by billions of years of micrometeorite hits. It is sharp, clingy and smells, astronauts reported, like spent gunpowder.
Daytime temperatures reach about 120 degrees Celsius and plunge to minus 130 at night. Deep craters near the poles are in permanent shadow, and spacecraft have confirmed water ice hidden in them, a potential resource for future bases.
Tides and a steady Earth
The Moon's gravity pulls harder on the near side of Earth than the far side, stretching the oceans into two bulges, one facing the Moon and one opposite. As Earth rotates beneath those bulges, each coast experiences two high tides a day. Those tides also act as a brake, slowing Earth's spin by about two thousandths of a second per century and pushing the Moon slowly outward, 3.8 centimetres a year, a drift measured by bouncing lasers off mirrors the Apollo crews left behind.
The Moon does something subtler too: it steadies Earth's tilt. Without a large moon, the gravitational tugs of the other planets would make Earth's axis wobble chaotically over millions of years, swinging the climate between extremes. The Moon's pull keeps the tilt near 23.5 degrees, giving us stable seasons.
Phases and eclipses
The Moon makes no light of its own. Its phases are simply the changing fraction of its sunlit half that we can see as it circles Earth every 29.5 days. It also keeps the same face toward us always, because Earth's gravity long ago slowed its rotation to match its orbit; the far side was unseen by any human until a Soviet probe photographed it in 1959.
By a coincidence that exists nowhere else in the solar system, the Moon is about 400 times smaller than the Sun and about 400 times closer, so the two appear almost exactly the same size in our sky. When the Moon passes precisely in front of the Sun we get a total solar eclipse, with day turning briefly to night. Because the Moon is receding, in about 600 million years it will appear too small to cover the Sun and total eclipses will end.
Humans on the Moon
Between July 1969 and December 1972, six Apollo missions landed twelve astronauts on the Moon. They stayed a total of less than two weeks, drove about 90 kilometres in lunar rovers and brought back 382 kilograms of rock that rewrote our understanding of the solar system's history. No one has been back since, but that is changing. NASA's Artemis programme, along with Chinese, Indian and private missions, aims to return people within the decade and eventually to stay.
Why it matters
The Moon is Earth's archive. Its ancient surface records the bombardment that shaped our planet in an era from which not a single Earth rock survives. It is also the obvious next step outward: a place to learn to live off another world, three days from home, before attempting Mars.
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Test yourself
4 questions · answers explained at the end
1.How did the Moon most likely form?
2.What are the dark patches on the Moon?
3.What causes most ocean tides on Earth?
4.How many people have walked on the Moon?