The Sleepwalkers: A History of Man's Changing Vision of the Universe

by Arthur Koestler

In the centre of his roundabout, Philolaus placed the ‘watch-tower of Zeus’, also called ‘the Hearth of the Universe’ or the ‘central fire’. But this ‘central fire’ is not to be confused with the sun. It could never be seen; for the inhabited part of the earth – Greece and its neighbours – was always turned away from it, as the dark side of the moon is always turned away from the earth. Moreover, between the earth and the central fire Philolaus inserted an invisible planet: the antichthon or counter-earth. Its function was, apparently, to protect the antipodes from being scorched by the central fire. The ancient belief that the far-western regions of the earth, beyond the straits of Gibraltar, are shrouded in eternal twilight2 was now explained by the shadow which the counter-earth threw on those parts. But it is also possible – as Aristotle contemptuously remarks – that the counter-earth was invented merely to bring the number of moving things in the universe up to ten, the sacred number of the Pythagoreans.3 Around the central fire, then, revolved in concentric orbits these nine bodies: the antichton innermost, then the earth, the moon, the sun, and the five planets; then came the sphere carrying all the fixed stars. Beyond this outer shell there was a wall of fiery ether, enclosing the world from all sides.