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Coincidence

10 The extraordinary Ptolemaic world of epicycles and deferents lasted a surprisingly long time. Despite its complexity it saved appearances and was also said to save souls. Ellipses were in fact studied by early Greek mathematicians such as Appollonius, and as early as 250 BC Aristarchus of Samos was proposing a system of planets orbiting the Sun. However, it was not to be, and for one and a half thousand years the Earth remained in the centre of the universe as the Ptolemaic system was handed down from the Greeks to the Arabs, and then back to the West again. Four early systems are shown opposite after Koestler, and each sphere of each diagram is to be understood as having its own attachment of epicycles and eccentrics. Copernicus, despite in 1543 placing the Sun in the centre top left, remained a devout epicycle man, increasing the number of invisible wheels from the Ptolemaic 39 up to an amazing 48. In the late sixteenth century Tycho de Brahe desperately tried to keep the Earth stationary in the centre of the universe bottom left, whilst an early Greek model, Herakleides, like a later version by Eriugina, attempted a compromise. During the 1600s the tide finally turned and people began to forget that planets sometimes move backwards. A modern model of the Solar System is shown lower opposite with the planets including an asteroid, Ceres, orbiting the Sun, each planet with an orbital shell. This basic model was first conceived by Johannes Kepler in 1596 and it is to his ideas that we now turn. GeocentrIc or helIocentrIc Earth or Sun at the centre 11
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