The Science of Discworld IV

occurred many times over: in the motion of the Sun and of all the planets. He could cut the number of epicycles down from Ptolemy’s eighty to a mere thirty-four by transferring this one to the Earth. The Sun then became stationary, and everything else (bar the Moon) revolved around it – Earth included. By adopting an Earth-centred frame of reference, Ptolemy had been obliged to transfer the Earth’s motion round the Sun to
every other body
, by adding a single extra epicycle to all of them. Remove thiscommon epicycle, and the description would be much simpler. But then you are faced with a radical change to the theory: among the many celestial bodies, only the Moon revolves around the Earth. Everything else revolves around the Sun.
    That statement is open to challenge, on grounds discussed for flat Earths in chapter 8 . You can represent the universe in any frame of reference you wish. There is nothing to stop you choosing a coordinate system in which the Earth is stationary, and you can even decide – depending on your assessment of your own importance in the scheme of things – that
you
are at the origin. It is entirely straightforward, for those who play this kind of game, to rewrite all of the laws of nature within that you-centred frame of reference. So there is a sense in which what’s in the middle and what goes round what is entirely arbitrary.
    However, another philosophical principle, Occam’s razor, suggests that this freedom to choose is not terribly meaningful. William of Occam (or Ockham) is credited with the philosophical principle ‘entities should not be multiplied beyond necessity’. fn1 This tends to be interpreted as ‘simple explanations are better than complex ones’, but that goes beyond what William actually said. His point was that it is silly to include features that can be removed without making any significant difference. Complex explanations are often better than simple ones, but only when simpler ones won’t do the job. Interpreting Occam’s razor either way, lots of copies of an epicycle are trumped by just one copy, even if it has to be attached to a different body.
    In a frame attached to the Earth, the laws of motion become extraordinarily complicated. The nearest major galaxy, M31 in Andromeda, about 2.6 million light years away, has to whiz all the way round the Earth once every 24 hours. More distant objects – the current record is about 13.2 billion light years – must undergoeven more outlandish gyrations. In contrast, if we choose a frame of reference centred on the Sun, making it stationary relative to the average positions of the stars, the mathematics becomes far simpler and the physics and metaphysics far more reasonable. Ignoring the gravitational influences of any other bodies, the Sun and Earth
both
orbit their mutual centre of gravity in ellipses. But because the Sun is so much more massive than the Earth, that centre lies well inside the Sun. So … the Earth goes round the Sun. We foolishly
think
the Earth is stationary because it is, relative to us. (Sorry, still too human-centred: make that ‘we are stationary, relative to it’.)
    Lesson learned – after several centuries, a few burnings and a lot of fuss and bother. But that was just the warm-up act. When astronomers realised that distant blobs of light were galaxies – swirling masses composed of billions of stars – it eventually dawned on them that the familiar Milky Way’s river of light is no accident: it is our own galaxy seen edge-on, from inside. Naturally our Sun will be at the galactic centre … Well, no, it is actually in a very nondescript region about two thirds of the way towards the rim: 27,000 light years from the galactic core, close to one of the galaxy’s spiral arms, the Orion Arm. The glorious Sun is merely one star (and a pretty feeble one at that) among thousands in the Local Fluff, which itself lies inside the Local Bubble. The Sun is not even in the galactic plane, though it’s fairly close – about sixty light years.
    After several centuries in which every successive attempt to portray humanity as special was debunked, the Copernican principle became embedded in fundamental physics as a generalisation of Einstein’s basic principle of relativity: there is no such thing as a privileged observer.
    We said earlier that a major motivation behind the scientific method is a conscious awareness that people tend to believe things because they want to, or have been