The Science of Discworld IV

which is the reason for things happening. Mister Stackpole, all the evidence says the world is on the back of an enormous turtle. Unlikely, yes: an unlikely turtle, but nevertheless the turtle in front of us, or rather below us. And it is therefore the truth, surely?’
    Watching him carefully, Marjorie thought she recognised the Reverend Stackpole now; he was one of the
left-ear people
who you often got in libraries – people who talked
at
you while staring at your left ear and who would never, it seemed, look one directly in the eye. At the same time they willed you to believe, as an example, that the government was poisoning the water supply because of over-population. The worst of them, if you couldn’t shake them off, were the ones who at some point used the word ‘Aryan’ and declared that the master race was already in orbit around Jupiter, just waiting for the Chosen. Library rules forbade physical violence, but sometimes she wanted to go and have a wash afterwards, apologising to her ears for what she had made them hear, and to her fists for clenching them red.
    In this world, she had no idea if she was somehow on the back of a turtle or not; but from her reading she recalled that it was a very long time before humanity knew that it even
lived
on a planet, and even then the concept took some time to permeate. As did concepts like looking after said planet. She remembered her grandmother saying,‘I take all my bottles to the bottle bank to help save the planet,’ and Marjorie had for a moment rejoiced that in some way or other a new message was getting through even to a confused old lady.
    Right now, she wondered if Lord Vetinari, by phrasing his points as questions, was being kind to the man, or was simply trying to ascertain the depths of his delusions.
    But Mister Stackpole was not giving in; in fact, he was fighting back. ‘My Lord, we look at the sky and see round things; for example the Moon is round, and the Sun is round.
Sphericality
, surely, is everywhere. Don’t you think it is trying to tell us something?’
    The Reverend got applause from some quarters of the room for this.
    Lord Vetinari, on the other hand, had not allowed his expression to change by one iota. Once the noise had subsided, he banged on his gavel and said, ‘Thank you, Mister Stackpole. Please be so good to go back to your seat.’ The gavel struck again and the Patrician said, ‘There will now be an adjournment of fifteen minutes; refreshments for all are available in the black gallery.’
    Every wizard’s face brightened immediately. Free food – well,
that
was worth coming along for. The sound of the gavel had barely faded away before Marjorie found herself alone on the bench. The wizards had stampeded – in a genteel way – to the gallery.

SIXTEEN
----
SPHERICALITY SURELY IS EVERYWHERE

    The Reverend Stackpole’s appeal to the ubiquity of round objects strikes a chord. The storytelling ape has a strong preference for neat, simple geometric forms. Circles and spheres featured prominently in early theories of planetary motion, such as those of Ptolemy and his successors: see chapter 22 . To some extent today’s science, with its neat, simple mathematical laws, derives from an ancient tradition in which particular shapes and numbers have mystical significance. Stackpole appeals to the sphericality of several objects that are
not
Discworld to argue that the Disc must also be spherical. He is using a ploy that is all too common among people trying to promote some belief system: introducing ‘evidence’ that undeniably exists, observing that it is consistent with his beliefs but skirting quietly round a big logical gap. Namely: is the belief system the
only
possible explanation for the evidence concerned, or is it consistent with alternatives?
    When it came to the shape of the universe, the cosmologists of the early twentieth century were a bit like Stackpole. They
assumed
that the universe should be spherically symmetric – behave the same way in all directions – to keep the sums easy. When they put that assumption into the equations, and did the sums, the mathematics spat out a spherical universe. This shape quickly became the perceived wisdom. However, there was very little independent evidence to support their original assumption. The logic was – well, circular.
    What shape
is
the universe, then?
    It’s a big question. We somehow have to work out the shape of everything that exists, from one location on