Galileo Galilei was an Italian natural philosopher, astronomer, and mathematician who made fundamental contributions to the sciences of motion, astronomy, and strength of materials and to the development of the scientific method. His formulation of (circular) inertia, the law of falling bodies, and parabolic trajectories marked the beginning of a fundamental change in the study of motion. His insistence that the book of nature was written in the language of mathematics changed natural philosophy from a verbal, qualitative account to a mathematical one in which experimentation became a recognized method for discovering the facts of nature. Finally, his discoveries with the telescope revolutionized astronomy and paved the way for the acceptance of the Copernican heliocentric system, but his advocacy of that system eventually resulted in an Inquisition process against him.

Telescopic Discoveries

In the spring of 1609 Galileo heard that in the Netherlands an instrument had been invented that showed distant things as though they were nearby. By trial and error, he quickly figured out the secret of the invention and made his own three-powered spyglass from lenses for sale in spectacle makers shops. Others had done the same; what set Galileo apart was that he quickly figured out how to improve the instrument, taught himself the art of lens grinding, and produced increasingly powerful telescopes. In the fall of 1609 Galileo began observing the heavens with instruments that magnified up to 20 times. In December he drew the Moon's phases as seen through the telescope, showing that the Moon's surface is not smooth, as had been thought, but is rough and uneven. In January 1610 he discovered four moons revolving around Jupiter. He also found that the telescope showed many more stars than are visible with the naked eye. These discoveries were earthshaking, and Galileo quickly produced a little book, Sidereus Nuncius (The Sidereal Messenger), in which he described them. He dedicated the book to Cosimo II de Medici (1590-1621), the grand duke of his native Tuscany, whom he had tutored in mathematics for several summers, and he named the moons of Jupiter after the Medici family: the Sidera Medicea, or "Medicean Stars." Galileo also had discovered the puzzling appearance of Saturn , later to be shown as caused by a ring surrounding it, and he discovered that Venus goes through phases just as the Moon does. Although these discoveries did not prove that the Earth is a planet orbiting the Sun, they undermined Aristotelian cosmology: the absolute difference between the corrupt earthly region and the perfect and unchanging heavens was proved wrong by the mountainous surface of the Moon, the moons of Jupiter showed that there had to be more than one center of motion in the universe, and the phases of Venus showed that it (and, by implication, Mercury) revolves around the Sun. As a result, Galileo was confirmed in his belief, which he had probably held for decades but which had not been central to his studies, that the Sun is the center of the universe and that the Earth is a planet, as Copernicus had argued. Galileo's conversion to Copernicanism would be a key turning point in the scientific revolution. After a brief controversy about floating bodies, Galileo again turned his attention to the heavens and entered a debate with Christoph Scheiner (1573-1650), a German Jesuit and professor of mathematics at Ingolstadt, about the nature of sunspots (of which Galileo was an independent discoverer). This controversy resulted in Galileo's Istoria e dimostrazioni intorno alle macchie solari e loro accidenti ("History and Demonstrations Concerning Sunspots and Their Properties," or "Letters on Sunspots"), which appeared in 1613.

Galileo's Copernicanism

Following the appearance of three comets in 1618, Galileo entered a controversy about the nature of comets, which led to the publication of Il saggiatore ( The Assayer ) in 1623. This work was a brilliant polemic on physical reality and an exposition of the new scientific method. In 1624 Galileo went to Rome and met with Pope Urban VIII. Galileo told the pope about his theory of the tides, which he put forward as proof of the annual and diurnal motions of the Earth. The pope gave Galileo permission to write a book about theories of the universe but warned him to treat the Copernican theory only hypothetically. The Roman censor had a number of serious criticisms of the book and forwarded these to his colleagues in Florence. After writing a preface in which he professed that what followed was written hypothetically, Galileo had little trouble getting the book through the Florentine censors, and it appeared in Florence in 1632. In the Dialogue Galileo gathered together all the arguments (mostly based on his own telescopic discoveries) for the Copernican theory and against the traditional geocentric cosmology. As opposed to Aristotle's, Galileo's approach to cosmology is fundamentally spatial and geometric: the Earth's axis retains its orientation in space as the Earth circles the Sun, and bodies not under a force retain their velocity (although this inertia is ultimately circular). But in the work, Galileo ridiculed the notion that God could have made the universe any way he wanted to and still made it appear to us the way it does. The reaction against the book was swift. The pope convened a special commission to examine the book and make recommendations; the commission found that Galileo had not really treated the Copernican theory hypothetically and recommended that a case be brought against him by the Inquisition. He was pronounced to be vehemently suspect of heresy and was condemned to life imprisonment. However, Galileo was never in a dungeon or tortured; during the Inquisition process he stayed mostly at the house of the Tuscan ambassador to the Vatican and for a short time in a comfortable apartment in the Inquisition building. After the process he spent six months at the palace of Ascanio Piccolomini (c. 1590-1671), the archbishop of Siena and a friend and patron, and then moved into a villa near Arcetri, in the hills above Florence. He spent the rest of his life there. Galileo was then 70 years old. Yet he kept working. In Siena he had begun a new book on the sciences of motion and strength of materials. The book was published in Leiden, Netherlands, in 1638 under the title Discorsi e dimostrazioni matematiche intorno a due nuove scienze attenenti alla meccanica (Dialogues Concerning Two New Sciences). Galileo here treated for the first time the bending and breaking of beams and summarized his mathematical and experimental investigations of motion, including the law of falling bodies and the parabolic path of projectiles as a result of the mixing of two motions, constant speed and uniform acceleration. By then Galileo had become blind, and he spent his time working with a young student, Vincenzo Viviani, who was with him when he died on Jan. 8, 1642.