Galileo, whose full name was Galileo Galilei, made major contributions to most branches of physics (especially mechanics), invented and so deployed the telescope to change our view of the nature of the universe completely, and became engaged in a highly dramatic confrontation with the Church.

Born in Pisa, Italy, Galileo was the son of a scholar and musician of some distinction. He entered the University of Pisa in 1581 to study medicine – a subject in which he showed little interest – and failed to complete the course, developing instead a passion for mathematics. He is thought to have made his first important observation in 1583, two years before he left the university. While in Pisa cathedral he noticed that the lamps swinging in the wind took the same time for their swing whatever its amplitude. He timed the swing against his pulse. In 1586 he invented a hydrostatic balance for the determination of relative densities. In 1589 he was appointed to the chair of mathematics at Pisa and later moved to the chair in Padua, in 1592.

It was while in Padua in 1610 that he designed and constructed a simple refracting telescope. He may not have been the first to do so, and there are many other claimants, but he was certainly the first to use the instrument constructively. His initial reaction was sheer amazement at the number of stars in the sky, “So numerous as to be almost beyond belief” he asserted. Merely looking at the Moon immediately revealed that it is not the smooth unchangeable object of Aristotelian theory. He also discovered sunspots. His most exciting moment came in January 1610, when he observed Jupiter for the first time telescopically. To his astonishment he found that the planet has four satellites. Contrary to received opinion as to what was possible, these were new bodies, unmentioned in Aristotle, and certainly not circling the Earth. He published his observations immediately, in 1610, in Sidereus nuncius (Starry Messenger), dedicating them to his patron and former pupil Cosimo II of Tuscany, in whose honor he named the satellites Sidera Medicea (Medici Stars). Within weeks he had received an invitation to a well-paid research chair in Pisa and returned there at the end of 1610.

In a series of works he was also tackling the problem of motion. His mature views were expressed in his Discorsi … a due nuove scienze… (1638; Discourse on Two New Sciences). In a series of brilliant experiments rolling balls down inclined planes (and not, as is commonly thought, by dropping weights from the leaning tower of Pisa) he showed that the speed with which bodies fall is independent of their weight and correctly formulated the law s = ½at ² (where s is speed, a acceleration, and t time). He lacked the concept of inertia and could only accept circular motion as being natural since, for Galileo, a body without any force acting on it would move in perfect circular motion. This was one aspect of his medieval heritage from which he was unable to break away.

After his return to Pisa, Galileo was beginning to meet opposition. Some of this was merely personal; in his criticism of others he wrote with a wit and savagery that many found wounding and impossible to forgive. He also found himself in dispute with many over his numerous discoveries. In these he was never inclined to take a charitable view over the claims of others, and there were many waiting for a chance to humiliate him on his return from the safety of Padua (then part of the independent Venetian republic). Their chance was his open support for the Copernican system. That the Earth moves round the Sun was so contrary to Scripture for many churchmen that those who enthusiastically campaigned for such a view were seen as heretics if not atheists. Galileo made no secret of his views in his writings, talk, and lectures. He openly ridiculed the Aristotelian scholars and supporters of the Ptolemaic (geocentric) system, many of whom were, or were to become, high officers of the Church. To stop the squabbling, Pope Paul V, on the advice of Cardinal Bellarmine, placed Copernicus on the Index (a list of books banned by the Catholic Church) in 1616, summoned Galileo to Rome, and informed him that he could no longer support Copernicus publicly. At this point Galileo had no choice; continued support for Copernicus would be to call the authority of the pope into question and to send him to the stake as it had sent Giordano Bruno in 1600.

In 1623 an old friend of Galileo's, Cardinal Barberini, was elected pope as Urban VIII. Galileo wasted no time in dedicating his new book Il saggiatore (1623; The Assayer) to him. It was a work that was savagely critical of Aristotle's account of comets. Although Urban was not prepared to go back on the decision of 1616 publicly, Galileo seems to have thought that he would be safe unless he supported Copernicus specifically. One thing that should have made him more prudent was the death of his patron, Cosimo, and the succession of a powerless minor to the duchy of Tuscany.

Galileo thought he could avoid the problem by writing a dialogue between a (Ptolemaic-) Aristotelian and a Copernican without ostensibly committing himself to either side, and the Church gave him permission to do so. Thus he wrote the Dialogue Concerning the Two Chief World Systems (1632). However, the form of the dialogue fooled no one. The Aristotelian, Simplicius, is no match for the brilliance of the Copernican, Salviati. It was even suggested to the pope that the bumbling Simplicius was a portrait of Urban himself. Galileo was once more summoned to Rome, threatened with torture, and forced to renounce Copernicus in the most abject terms. There is a tradition that after his renunciation he whispered, “Eppur si muove” (“Yet it moves”), but this is unlikely. Galileo was truly frightened, and it must be remembered that he was nearly 70 years old and facing a powerful body, which might have tortured and burned him at the stake. In the end he was treated reasonably well. He was allowed to return to his villa at Arcetri in isolation. Later, after he became blind and after the death of his daughter, his disciples Vincenzo Viviani and Evangelista Toricelli were allowed to stay with him.