DAVID A. HARDY / SCIENCE PHOTO LIBRARY DAVID A. HARDY / SCIENCE PHOTO LIBRARY
Ptolemaic system. Artwork of a planet orbiting the Earth in the complex Ptolemaic system. This system, invented by Ptolemy in the 2nd century AD, placed the Earth (grey/blue) at the centre of the universe, with every other body orbiting it. This alone did not explain the motions of the planets, however, as some occasionally showed "retrograde motion" (moved backwards for a short time). It is now known that this is due to the Earth overtaking them in their orbits of the Sun. Ptolemy held that each body (for example, the yellow planet at upper right) moved in small circles (epicycles, yellow ring) around a point in its orbit (yellow dot). The system remained popular until the 1600s.
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