HeliocentricA cosmology with the sun at the centre of the universe. Nicolai Copernicus (1473-1543) is the first modern to suggest that the earth moves round a central sun (heliocentrism) rather than the sun moves around a central earth (geocentrism), with the publication of De Revolutionibus in 1543. However, the reception of this theory is surprisingly muted. It was certainly not accepted immediately, as there are too many problems and too few advantages. Not until Kepler and Galileo is it taken as a serious challenge, in cosmological terms, to the Ptolemaic system. It is seen to be useful by astronomers for simpler methods of predicting the motions of the heavens. Johannes Kepler (1571-1630) greatly simplifies the new cosmology by showing that the planetary orbits are in fact simple ellipses around the sun (discovered 1605, published 1609). He is the first to break with Plato’s idea of regular circular motion. Galileo Galilei (1564-1642) is the first to make systematic use of the telescope (1609/10 on), and finds a great deal of evidence against the old system, and some evidence in favour of the new. He also does important work in producing new theories of motion which do away with many important objections to the heliocentric system, based on Aristotle’s views. Isaac Newton (1642-1727) finally cements this revolution with the 1/d2 gravitational law, which gives a proper explanation of the elliptical orbits of the planets, and of motion on the earth. Above all, it is important to recognise that the Copernican system was not significantly more accurate than the Ptolemaic system it replaced. However, it is a more elegant explanation of retrogression, as due to relative motion of earth and planets. There is a proper ordering of planets and estimation of planetary distances, for example it yields as explanation of why Mercury and Venus are always seen near sun. The real improvement is its simplicity - instead of those fearsome Ptolemaic devices for the apparent motions of the planets, and in particular retrogressions, we have simple orbits around the sun. Related Topics:
Contributed by: Richard P Whaite
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