The Sleepwalkers: A History of Man's Changing Vision of the Universe

by Arthur Koestler

The third day is concerned with the astronomical arguments for and against Copernicus, and here Galileo is downright dishonest. He first shows that the Copernican system is superior to the Ptolemaic by the familiar arguments from the Jupiter moons and the phases of Venus. He then explains that to ‘save’ the planets’ apparent stations and retrogressions, Ptolemy had to introduce ‘very great epicycles’ with which Copernicus was able to dispense ‘with one single motion of the earth’. But he breathes not a word about the fact that Copernicus, too, needs a whole workshop full of epicycles; he keeps silent about the eccentricity of the orbits, the various oscillations and librations, the fact that the sun is neither in the centre of the motions, nor lies in their plane; in a word, he deliberately evades the real problems of astronomy which had started Tycho and Kepler on their quest. The planets all move in perfect circles at uniform linear velocities around the sun (which, for instance, would make Saturn’s period twenty-four years instead of thirty).15 All problems appear solved ‘with admirable facility’; for ‘in the Ptolemaic hypothesis there are the diseases, and in the Copernican their cure’.16