SIXTEENTH-CENTURY PHILOSOPHY The discovery of the moons that revolved around Jupiter in its planetary orbit had already disposed of one of the strongest arguments urged against heliocentrism, namely that the moon would only be able to orbit the earth if the earth itself was stationary Galileo was initially cautious in publicly expressing the conclusions he drew from his astronomical discoveries However, after an ecclesiastical commission in Rome had taken oYcial notice of his major observations, he began to propagate heliocentric ideas to a wide circle of friends, and in 1613, in an appendix to a book on sunspots, he declared his adherence to Copernicus A Dominican friar in Florence, in a sermon on Acts 1: 11 (‘Ye Galileans, why stand ye gazing up to heaven?’) denounced heliocentrism as being in conXict with biblical texts, such as the one in which Joshua tells the sun to stand still so that the Israelites may complete their victory over the Philistines Galileo decided to travel to Rome to clarify his theological status In advance he wrote to the powerful Jesuit cardinal, St Robert Bellarmine, urging that the sacred authors who spoke of the sun as moving were merely using popular idiom and were not intending to teach geometry Bellarmine referred the matter to a committee of the Inquisition who determined that the opinion that the sun was the centre of the cosmos was heretical, and the opinion that the earth moved was at the least erroneous On the instructions of Pope Paul V, Bellarmine instructed Galileo that he must not hold or defend either of these opinions If there was a real proof of heliocentrism, he told one of Galileo’s friends, then we would have to re-examine the biblical texts which appeared to contradict it; but as matters stood, Copernicus’ theory was only an unproved hypothesis And indeed, Galileo’s own heliocentric system, though it Wtted the phenomena better, was almost as complicated as the geocentric system of his opponents, demanding constant appeal to epicycles.12 The evidence he had discovered did not justify the degree of certainty with which he maintained his thesis It is often said that in this exchange Bellarmine showed a sounder grasp of the philosophy of science than the age’s greatest scientist and Galileo showed a sounder grasp of biblical exegesis than the age’s most famous theologian The paradox is an agreeable one, but it is not really a fair 12 Galileo did not incorporate Kepler’s discovery of the elliptical orbits of the planets, which was needed to achieve the appropriate simpliWcation of heliocentrism 24