The Science of Discworld Revised Edition

longer than the 26,000 year cycle of precession, but even over hundreds of millions of years the angle doesn’t change
much
. Why? Because, as Laskar discovered when he did the calculations, the Moon helps keep the Earth’s axis steady. So it is at least conceivable that life on Earth owes quite a lot to the calming influence of its sister world, however much it may madden us individually.
    A third influence of the Moon was discovered in 1998: a clear association between tides and the rate of growth of trees. Ernst Zürcher and Maria-Giulia Cantiani measured the diameters of young spruce trees grown in containers kept at constant light levels. Over periods of several days the diameters changed in step with the tides. The scientists interpret this as an effect of the Moon’s gravity on the transport of water within the tree. It can’t be variations in moonlight, which would perhaps affect photosynthesis, because the trees were grown in darkness. But the effect may be similar to one that occurs with creatures that live on the seashore. Because they evolved to live there, they have to respond to the tides, and evolution sometimes achieves this by creating an internal dynamic that runs in step with the tides. If you remove the creatures to the laboratory, this internal dynamic makes them continue to ‘follow’ the tides.
    The Moon has been important in another way. The Babylonians and Greeks knew that the Moon is a sphere; the phases are obvious, and there is also a slight wobble which means that, over time, humans see rather more than one half of the Moon’s surface. There it was, hanging in the sky – a big ball,
not
a disc like the sun, and a hint that perhaps ‘big balls in space’ is a much better way of thinking about the Earth and its neighbours than ‘lights in the sky’.
    All this is a long way from lance-constable Angua – even a long way from the female menstrual cycle. But it shows how much we are creatures of the universe. Things Up There really do affect us Down Here, every day of our lives.
    1 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 in coincidental.

TWENTY-ONE
THE LIGHT YOU SEE THE DARK BY

    THERE WAS NO Dark. This came as such a shock to Ponder Stibbons that he made H EX look again. There had to be Dark, surely? Otherwise, what was there for the light to show up against?
    Eventually, he reported this lack to the other wizards.
    ‘There should be lots of Dark and there isn’t,’ he said flatly. ‘There’s just Light and … no light. And it’s a pretty strange light, too.’
    ‘In what way?’ said the Archchancellor.
    ‘Well, sir, as you know, 1 there’s ordinary light, which travels at about the same speed as sound …’
    ‘That’s right. You’ve only got to watch shadows across a landscape to realize that.’
    ‘Quite, sir … and then there’s meta-light, which doesn’t really travel at all because it is already everywhere.’
    ‘Otherwise we wouldn’t even be able to see darkness,’ said the Senior Wrangler.
    ‘Exactly. But the Project universe has just got the one sort of light. H EX thinks it moves at hundreds of thousands of miles a second.’
    ‘What use is that?’
    ‘Er … in this universe, that’s as fast as you can go.’
    ‘That’s nonsense, because –’ Ridcully began, but Ponder held up a hand. He had not been looking forward to this one.
    ‘Please, Archchancellor. It’s doing the best it can. Just trust me on this one. Please? Yes, I can see all the reasons why it’s impossible. But, in there, it seems to work. H EX has written pages of stuff about it, if anyone’s interested. Just don’t ask me about any of it. Please, gentlemen? It’s all supposed to be logical but you’ll find your brain squeaking around until the ends point out of your ears.’
    He placed his hands together and tried to look wise.
    ‘It really
is
almost as if the Project is aping the
real
universe –’
    ‘Ook.’
    ‘I beg your pardon,’ said Ponder. ‘A figure of speech.’
    The Librarian nodded at him and knuckled his way across the floor. The wizards watched him carefully.
    ‘You really believe that
that
thing,’ said the Dean, pointing, ‘with its moon-hating water and worlds that go around suns