The Science of Discworld Revised Edition

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?’
    ‘H EX 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.

TWENTY
A GIANT LEAP FOR MOONKIND

    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. 1 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