Towards the end of 1609, Galileo Galilei got hold of a newfangled optical device and started pointing it at the night sky above Padua. The following year he published a book describing some of the discoveries he had made with the help of his wonderful telescope: that there are mountains on the moon, that Jupiter has four satellites, and that the Milky Way is not a formless cloud but a vast collection of separate stars. He also expressed sympathy for the idea, recently advocated by Copernicus, that the sun is the centre of the world and that the planets move in regular circles round it. The Catholic hierarchy was at first prepared to tolerate Galileo’s claims, even though they were hard to reconcile with classical astronomy, scholastic philosophy and various passages in the Bible. But Galileo had a taste for controversy and, over the years, proclaimed his support for Copernicus with increasing fervour, taking pleasure in baiting the Jesuit teachers who had appointed themselves guardians of intellectual propriety in the Roman Church. He met his match in 1633, when he was put on trial in Rome, found guilty of heresy, and placed under house arrest for the remainder of his life.