Terry Pratchett - The Science of Discworld I

'It'll be dawn in a moment,' said Bonder's voice in his ear. 'We'll be interested to see what you think of it.'

Strange way of putting it, Rincewind thought, as he watched the sun come up. It was hidden behind the clouds, but a greyish-yellow light picked its way across the landscape.

'It's all right,' he said. 'The sky's a dirty colour. Where is this? Llamedos? Hergen? Where aren't there any seashells? Is this high tide?'

All the wizards were trying to speak at once.

'I can't think of everything, sir!'

'But everyone knows about tides!'

'Perhaps some mechanism for raising and lowering the sea bed would be acceptable?'

'If it comes to that, what causes tides here?'

'Can we all please stop shouting?'

The babble died down.

'Good,' said Ridcully. 'Over to you, Mister Stibbons.'

Stibbons stared at the notes in front of him.

'I'm ... there's ... it's a puzzler, sin On a round world the sea just sits there. There's no edge for it to pour off.'

'It's always been believed that the sea is in some way attracted to the moon,' the Senior Wrangler mused. 'You know ... the attraction of serene beauty and so on.'

Dead silence fell.

Finally, Ponder managed: 'No one said anything to me about a moon.'

'You've got to have a moon,' said Ridcully.

'It should be easy, shouldn't it?' said the Dean. 'Our moon goes around the Disc.'

'But where can we put it?' said Ponder. 'It's got to be light and dark, we've got to move it for phases, and it's got to be almost as big as the sun and we know that if you try to make things sun-sized here they, well, become suns.'

'Our moon is closer than the sun,' said the Dean. 'That's why we get eclipses.'

'Only about ninety miles,' said Ponder That's why it's burned black on one side.'

'Dear me, Mister Stibbons, I'm surprised at you,' said Ridcully. 'The damn great sun looks pretty big even though it's a long way away. Put the moon nearer.'

'We've still got the big lump that the Dean knocked out of the planet,' said the Senior Wrangler. 'I made the students park it around the Target.'

'Target?' said Ponder.

'It's the big fat planet with the coloured lines on,' said the Senior Wrangler 'I made them bring the whole lot out to the new, er, sun because frankly they were a nuisance where they were. At least when they're spinning round you know where they're coming from.'

'Are the students still sneaking in here at night to play games?' said Ridcully.

'I've put a stop to that,' said the Dean. 'There's too many rocks and snowballs around this sun in any case. Masses of the things. Such a waste.'

'Well, can we get the lost lump here soon?'

'HEX can manipulate time from Rincewind's point of view,' said Ponder. 'For us, Project time is very fast ... we should get it here before the coffee arrives.'

'Can you hear me, Rincewind?'

'Yes. Any chance of some lunch?'

'We're getting you some sandwiches. Now, can you see the sun properly?'

'It's all very hazy, but yes.'

'Can you tell me what happens if I do ... this?'

Rincewind squinted into the grey sky. Shadows were racing across the landscape.

'You're not going to tell me you've just caused an eclipse of the sun, are you?'

Rincewind could hear faint cheering in the background.

'And you're quite certain it's an eclipse?' said Ponder.

'What else is it? A black disc is covering the sun and there's no birdsong.'

'Is it about the right size?'

'What kind of question is that?'

'All right, all right. Ah, here are your san, what? How? Excuse me ... now what? ...'

The senior wizards were puzzled again, and demonstrated this by prodding Ponder while he was trying to talk. The wizards were great ones for the prod as a means of getting attention.

'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 [24] Moreover, until the last few decades of human history, most women did not cycle. Nearly all the time, they were either pregnant or lactating. And for the great apes, the cycle is a week or so longer than for humans, and for gibbons it's shorter. So it looks as though the relation with the Moon is coincidental. . 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.