The Science of Discworld Revised Edition

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 flattish 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