'You can see there's only one moon,' said the Senior Wrangler, for the third time.

'All right ... how about this?' said Ponder. 'Let us suppose that in some way this world has got both water that likes moons and water that can't stand moons at any price. If it's got about the same amount of both, then that at least explains why there seem to be high tides on both sides at once. I think we can dispose of the Invisible Moon theory, interesting though it was, Dean.'

'I like that explanation,' said Ridcully. 'It is elegant, Mister Stibbons.'

'It's only a guess, sir.'

'Good enough for physics,' said Ridcully.

HUMANITY HAS ALWAYS KNOWN the Moon is impor­tant. It often comes out at night, which is useful; it changes, in a sky where change is rare; some of us believe our ancestors live there. That last one might not be capable of experimental verification, but nevertheless humanity in general got it right. The Moon reaches out ghostly tentacles, gravity and light; it may even be our protector.

The wizards are right to worry that they've forgotten to give Roundworld a Moon, though as usual they're worried for the wrong reasons.

The Moon is a satellite of the Earth: we go round the Sun, but the Moon goes round us. It's been up there for a long time, and in its quiet way it's been exceedingly busy. The Moon affects people as well as baby turtles. The main way it affects us is by causing tides. It may affect us in other, less obvious ways, although many common beliefs about the moon are, to say the least, scientifically controver­sial. The female menstrual cycle repeats roughly every four weeks, much the same time that it takes the moon to go round the Earth -one month, in fact, a word that comes from 'moon'. In popular belief this numerical similarity is no coincidence, as for example in 'the wrong time of the month'. On the other hand, the Moon is the epitome of regularity, as predictable as the date of Christmas day, which cannot be said of the menstrual cycle. Lovers, of course, swoon and spoon beneath the Moon in June ... It is also widely held that people go mad when there is a full Moon, or, a more extreme type of madness, those who are suitably afflicted turn into wolves for a night.

The werewolf legend plays a central role in Men at Arms. Most of the time lance-constable Angua of the Ankh-Morpork city watch is a well-built ash-blonde, but when the Moon is full she turns into a wolf who can smell colours and rip out people's jugular veins. But it does play havoc with her private life. 'It was always a problem, growing fangs and hair every full moon. Just when she thought she'd been lucky before, she'd found that few men are happy in a relationship where their partner grows hair and howls.' Fortunately Corporal Carrot is unperturbed by these occasional changes. He likes a girlfriend who enjoys long walks.

The Moon is unusual, and it is quite likely that without it, none of us would be here at all. Not because of the alleged effect on lovers, who find a way Moon or no, but because the Moon protects the Earth from some nasty influences that might have made it dif­ficult for life to have arisen, or at least to have got beyond the most rudimentary forms. What makes the Moon unusual is not that it is a companion to a planet: all of the planets except Mercury and Venus have moons. It is remarkable because it is so big in compari­son to its parent planet. Only Pluto has a satellite, Charon, discovered in 1978 by Jim Christy, that is comparable in relative size to our Moon. It's not stretching things much to say that we live on one half of a double planet.

We know the Moon is very different from the Earth in all sorts of ways. Its gravity is weaker, so it wouldn't be able to keep an atmosphere for very long, even if it had one, which it doesn't by any sensible use of the term. The Moon's surface is rock and rock dust, with no seas anywhere (water easily escapes too), although in 1997 NASA probes discovered substantial quantities of water ice at the Moon's poles, hidden from the warmth of the Sun by the perma­nent shadows of crater walls. That's good news for future lunar colonies, which could act as bases for the exploration of the solar system. The Moon is a good place to start from, because your spaceship doesn't need much fuel to escape the Moon's pull; the Earth is of course a bad place to start from, because down here gravity is so much stronger. How typical of humans to have evolved in the wrong place ...

How was the Moon formed? Did it condense out of the primal dustclouds along with the Earth? Did it form separately and get captured later? Are the craters extinct volcanoes, or are they marks made by lumps of rock smashing into the Moon? We know rather more about the Moon than we do about most other bodies in the solar system, because we've been there. In April 1969, Neil Armstrong stepped down on to the surface of the Moon, fluffed his lines, and made history. Between 1968 and 1972 the United States sent ten Apollo missions to the Moon and back. Of these, Apollos 8,9, and 10 were never intended to land; Apollo-11 was that historic first landing; and Apollo-13 never made it down to the surface, suf­fering a disastrous explosion early in its flight and turning into an excellent movie.

The rest of Apollos 11-17 landed, and between them they brought back 800 lb (400 kg) of moon rock. Most of it is still stored in the Lunar Curatorial Facility in NASA's Johnson Space Center at Clear Lake, Houston; a lot of it has never been seriously looked at at all, but what has been analysed has taught us a lot about the ori­gins and nature of the Moon.

The Moon is about a quarter of a million miles (400,000 km) from the Earth. It is less dense than the Earth, on average, but the Moon's density is very similar to that of the Earth's mantle, a curi­ous fact that may not be coincidence. The same side of the Moon always faces the Earth, though it wobbles a bit. The dark markings on it are called maria, Latin for 'seas', but they're not. They're flat-tish plains of rock which at one time was molten and flowed across the lunar surface like lava from a volcano. Nearly all of the craters are impact craters, where meteorites have smashed into the Moon. There are lots of them because there's a lot of rocks floating about in space, the Moon has no atmosphere to shield it by burning up the rocks through frictional heating, and the Moon has no weather to grind them back down again until they disappear. The Earth's atmosphere is a pretty good shield, but once geologists started looking they found remains of 160 impact craters down here, which is interesting given that a lot of them will have eroded away in the wind and the rain. But more of that when we get to dinosaurs.

Today the Moon always turns the same face to the Earth, which means that it rotates once round its axis every month, the same time that it takes to revolve around the Earth. (If it didn't rotate at all, it would always be pointing in the same direction, not the same direction relative to the Earth, but the same direction period. Imagine someone walking round you in a circle but always facing north, say. Then they don't always face you. In fact, you see all sides of them.) It wasn't always like this. Over hundreds of millions of years, the effect of tides has been to slow down the rotation rates of both Earth and Moon. Once the moon's rotation became synchro­nized with its revolutions round the Earth, the system stabilized. The moon also used to be quite a bit closer to the Earth, but over long periods of time it has moved further and further out.

Between 1600 and 1900 three theories of the formation of the Moon came into vogue and out again. One was that the Moon had formed at the same time as the Earth when the dustcloud con­densed to form the solar system, Sun, planets, satellites, the whole ball of wax ... or rock, anyway. This theory, like early theories of the solar system's formation, falls foul of angular momentum. The Earth is spinning too fast, and the moon is revolving too fast, to be consistent with the Moon condensing from a dustcloud. (We mis­led you earlier when we said that the dustcloud theory explained the satellites too. Mostly it does, but not our enigmatic Moon. Lies-to-children, you see, now you're ready for the next layer of complication.)