The Ptolemaic system is a defunct theory of astronomy that was widely thought to be true until the time of Copernicus and Galileo. It was an attempt to explain why some planets appeared to stop, or even reverse their course, whilst still maintaining that the earth was the centre of the universe. The idea originated in Ancient Greece, although it was based on earlier observations by the Babylonians and others in the world's earliest cities. The basics of the geocentric system were established by Aristotle's time, but refined and 'explained' by Ptolemy in the second century AD. Belief in this system faded after Copernicus published De revolutionibus orbium coelestium in 1543; it was further undermined by Galileo's observations in the early seventeenth century.