Science of Discworld

THERE IS A TIDE ...

'HE'S RIGHT ABOUT THE RAIN,' said the Senior Wrangler, who was at the omniscope. 'You've got clouds again. And there's lots of volcanoes.'

'I'mmoving him on further ... Oh. Now he says it's dark and cold and he's got a headache ...' 'Not very graphic, is it?' said the Dean. 'He says it's a splitting headache.' HEX wrote something.

'Oh,' said Ponder 'He's under water. I'm sorry about that, I'm afraid he's a little hard to position accurately. We're still not sure what size he should be. How's this?'

The trumpet rattled, 'He's still under water, but he says he can see the surface. I think that's as good as we're going to get. Just walk forward.'

As one wizard, they turned to watch the suit. It hung in the air, a few inches above the floor. As they watched, the figure inside made hesitant walking motions.

It was not a nice day.

It was still raining, although it had slackened off recently, with sporadic outbreaks during the early part of the millennium and scattered showers during the last couple of decades. Now ten thou­sand rivers were finding their way to the sea. The light was grey and gave the beach a flat, monochrome, and certainly very damp look.

Whole religions have been inspired by the sight of a figure emerging, miraculously, from the sea. It would be hard to guess at what strange cult might be inspired by the thing now trudging out of the waves, although avoidance of strong drink and certainly of seafood would probably be high on its list of 'don'ts'.

Rincewind looked around.

There was no sand underfoot. The water sucked at an expanse of rough lava. There was no seaweed, no seabirds, no little crabs -nothing potentially dangerous at all.

'There's not a lot going on,' he said. 'It's all rather dull.'

'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.