Saturn from afar

Abstract

From his home on the island of Rhodes in the Aegean, Hipparchus, the greatest of the ancient Greek astronomers, drew up a catalogue of the positions and motions of the objects in the sky. He interpreted the observations as meaning that the Earth was at the centre of everything, and that the planets revolved around the Earth in circles. Claudius Ptolemaeus (more usually called simply Ptolemy), a Greek living in Alexandria in Egypt, observed that the planets did not precisely follow their predicted paths. However, since the circle was regarded as ‘perfect’ he proposed an ‘epicycle’ scheme in which each planet pursued a smaller circle about its mean position as it progressed around its orbit.