Epicycles described in Ptolemy's 'Almagest', 1496.
© Science Museum / Science & Society Picture Library
'The Almagest' by Ptolemy (2nd century AD) was one of the most influential scientific works in history. It described the Ptolemaic system of astronomy and geography. Georg von Peurbach (1423-1461) started editing and translating the work but died after completing the first four books. The work was completed by his pupil Johannes Regiomontanus (1436-1476). This edition reintroduced Greek astronomy and its theories of the universe to the west, in Latin, the language all scholars could read. This page demonstrates how epicycles could be used to explain the apparent motion of Saturn. In Ptolemy's universe the Earth was at the centre, but the planets sometimes appeared to orbit backwards. Ptolemy devised 'epicycles' to explain this retrograde motion.