The Copernican System
 

Nicholas Copernicus (1473–1543 AD) formulated a theory of the Universe that was heliocentric, i.e. the Sun became the centre of the Universe, the Earth being just another planet. Nearly all the known motions of the planets were accounted for by supposing them to revolve about the Sun, in circular orbits but with their centres slightly displaced from that of the Sun. He also had to keep a few small epicycles.
The Moon revolved about the Earth as before but the Earth rotated on its axis, thus accounting for the diurnal rotation of the heavens.
Copernicus perfected his system over thirty years, his great book De Revolutionibus Orbium Coelestium being published only in the year of his death. A preface, not written by Copernicus, suggested that his heliocentric system need not be supposed to be closer to the truth than the Ptolemaic

System but could be looked upon as merely a simpler mathematical device. It is probable, however, that Copernicus himself believed it to be a physical reality. Certainly, it aroused deep psychological and religious opposition, displacing as it did the Earth, the home of mankind, from the centre of the Universe to the status of a satellite.
There were indeed two scientific objections that could be made, illustrating that a little learning is a dangerous thing.
Opponents of the Copernican theory pointed out that if the Earth moved round the Sun (a) the Moon would be left behind, (b) the brighter, and, therefore nearer stars, would show a parallactic yearly shift against the background of the fainter and presumably more distant stars. Neither of these events was observed.
The vindication of the Copernican theory had to await the work of three men, Tycho Brah´e (1546– 1601 AD), Johannes Kepler (1571–1630 AD) and Galileo Galilei (1564–1642 AD). We will consider Galileo’s work first of all.