DR JEREMY BURGESS / SCIENCE PHOTO LIBRARY DR JEREMY BURGESS / SCIENCE PHOTO LIBRARY
Illustration of the Ptolemaic world system, a model which attempted to explain the motions of the sun, moon and planets and predict their future positions. Earth is at the centre, with its four elements, earth, water, fire and air. Originally proposed by Apollonius of Perga in the third century BC and developed by Hipparchus, it was completed by Claudius Ptolemaeus of Alexandria in the second century AD. Ptolemy assumed a geocentric system in which the moon, Mercury, Venus, the sun, Mars, Jupiter and Saturn moved around the earth in circular orbits of increasing radius.
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