The Science of Discworld IV

there’s no danger of falling off a flat world unless you get too close to the edge.
    There is one effective way to counteract this natural tendency to fall downwards: place something underneath as a support. This support may in turn need something underneath to support
it
, but you can iterate the process many times provided ultimately everything rests on something firm. This process, known as building, was effective enough to erect the Great Pyramid of Khufu at Giza, built in 2560 BC and over 145 metres high. It was the tallest building in the world until 1300, when the architect of Lincoln cathedral cheated by using a lot more up and a lot less sideways.
    A common feature of human-centred thinking is that it often works well until you start to ask questions that transcend the human scale. Then it has a habit of falling to pieces. The line of thoughtjust described seems fairly foolproof until you go for the big picture. Applying the kind of logical reasoning that drives so many Discworld stories, it is impossible not to ask:
What keeps the world up?
Human-centred thinking provides an obvious and compelling answer: something supports it. In Greek mythology, it was Atlas, bearing the world on his sturdy shoulders. Discworld sensibly plumps for a more plausible support cast: the giant world-bearing elephant. As belt and braces, there is not just one of them, but four – or possibly five, if the legend recounted in
The Fifth Elephant
is to be believed.
    All well and good, but both universe-centred science and human-centred myth-making can hardly fail to ask a supplementary question:
What keeps the elephants up?
If the idea of an ordinary elephant hovering in mid-air is ludicrous, how much more so is that of a vast, extraordinarily heavy elephant doing the same? Discworld’s answer is A’Tuin, a giant space-faring turtle. The turtle’s shell provides a firm place for the elephants to stand. As a cosmology, it all hangs together pretty well … but of course a further question arises:
What keeps the turtle up?
    It might seem that we could go on like this indefinitely, but at this point observations of nature come into play. The natural world provides a long list of exceptions to the belief that the natural place of any object is on the ground: celestial bodies, clouds, birds, insects and all water-borne creatures – fish, crocodiles, hippos, whales and, crucially, turtles.
    However, we can prune the list. Birds and insects do not remain aloft indefinitely; wait long enough and they do, in fact, descend to their natural place, typically a tree or a bush. The Sun, Moon and stars do not inhabit the terrestrial realm at all, so there is no reason to expect them to behave in a human-centred way – and they don’t. Assigning them to the realm of the supernatural has so many attractions that it becomes virtually unavoidable. The same arguably goes for clouds, which have a habit of producing awe-inspiring phenomena such as thunder and lightning. Scratch clouds. Crocodilesand hippos are out: they spend a lot of time on land. Fish are not renowned for doing that, but no sensible person would try to fit four elephants on top of a fish.
    Which leaves turtles.
    Small turtles spend a lot of time on rocks, but no one in their right mind would expect a small turtle to hold up four giant world-bearing elephants. Big turtles come out onto land to lay their eggs, but that’s a mystical event and it doesn’t cast serious doubt on the theory that a turtle’s natural place is in water. Where, please notice,
it does not require support
. It can swim. So it stands to reason that any self-respecting giant space-faring turtle will swim through space, which implies that it needs no artificial support to avoid falling. Examining the animal more closely, a world-spanning turtle seems ideal as a support for giant elephants. It is hard to imagine what could perform the task better.
    In short, Discworld is, as stated earlier, the sensible way to make a world.
    By comparison, Roundworld makes no sense. It’s the wrong shape, it’s held up by nothing, and it swims through space unaided despite not being the right shape to swim through
anything
. Basically, it’s a giant rock, and you all know what rocks do when you throw them in the lake. It is hardly surprising that it took the wizards a long time to come to terms with the way Roundworld organises itself. Accordingly, we should not be surprised to find that pre-scientific humanity had the