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

by Arthur Koestler

Wackher, though a Catholic convert, believed in the plurality of worlds; accordingly, he thought that Galileo’s discoveries were planets to other stars, outside our solar system. Kepler rejected this idea; but he equally refused to admit that the new heavenly bodies could be revolving round the sun, on the grounds that since there were only five perfect solids, there could only be six planets – as he had proved to his own satisfaction in the Cosmic Mystery. He accordingly deduced a priori, that what Galileo had seen in the sky could only be secondary satellites, which circled round Venus, Mars, Jupiter, and Saturn, as the moon circled round the earth. Once again he had guessed nearly right for the wrong reasons: Galileo’s discoveries were indeed moons, but all the four of them were moons of Jupiter.