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

by Arthur Koestler

In all these ‘dangerous innovations’ astronomers of the Jesuit Order, of which Bellarmine was the General, had played a prominent part. They had quietly abandoned Ptolemy, and progressed to the Tychonic system: the planets circle the sun, and with the sun the earth (just as the four ‘Medicean stars’ circle Jupiter, and with Jupiter, the sun). This is as far as both metaphysical prudence and scientific caution permitted them to go – even if some Jesuits were Copernicans at heart. The reasons for metaphysical prudence were theological; the reasons for scientific caution empirical: so long as there existed no observable stellar parallax, no apparent displacement in the position of the fixed stars caused by the earth’s motion through space, that motion remained unproven. Under these circumstances, the system of the universe which seemed to agree most closely with observed fact, was the Tychonic system. It also had the advantage of a compromise; by making the sun the centre of planetary motion, it prepared the way for a complete heliocentric system, should a stellar parallax be found, or some other discovery tilt the balance in its favour. But that, as we shall see, was another compromise that Galileo rejected.