heliocentric model

A model of the solar system that places the Sun at the centre with the planets in orbit around it. Although such a system had been suggested as early as c. 200 BC by Aristarchus of Samos, it offered no particular advantage at the time for predicting the positions of the planets and the idea of a moving Earth was philosophically unacceptable. The geocentric model, refined by Ptolemy (c. AD 100-170), was in general use until the work of Copernicus (1473-1543). By this time, the idea that the Earth was the centre of the created universe was strongly rooted in religious dogma. In his book De revolutionibus, Copernicus argued the advantages of considering the solar system as Sun-centred. However, the idea did not gain general acceptance until the observational work of Galileo (1564-1642) and Kepler (1571-1630), whose results made better sense in the context of a heliocentric system. In the Copernican system, the planetary orbits were assumed to be circular. This meant that the theory was no more successful on a practical level than the Ptolemaic theory in predicting planetary positions, though it was more elegant and provided a natural explanation for the retrograde motion of the planets. Kepler's discovery that the planetary orbits are elliptical resolved this problem and the first telescopic observations by Galileo revealed phenomena, such as the phases of Venus, that could be explained only on the basis of a heliocentric model.