Galileo was born near Pisa, on February 15, 1564. His father, Vincenzo Galilei, played an important role in the musical revolution from medieval polyphony to harmonic modulation. Just as Vincenzo saw that rigid theory stifled new forms in music, so his eldest son came to see Aristotelian physical theology as limiting scientific inquiry. Galileo was taught by monks at Vallombrosa and then entered the University of Pisa in 1581 to study medicine. He soon turned to philosophy and mathematics, leaving the university without a degree in 1585. For a time he tutored privately and wrote on hydrostatics and natural motions, but he did not publish. In 1589 he became professor of mathematics at Pisa, where he is reported to have shown his students the error of Aristotle’s belief that speed of fall is proportional to weight, by dropping two objects of different weight simultaneously from the Leaning Tower. His contract was not renewed in 1592, probably because he contradicted Aristotelian professors. The same year, he was appointed to the chair of mathematics at the University of Padua, where he remained until 1610.

At Padua, Galileo invented a calculating "compass" for the practical solution of mathematical problems. He turned from speculative physics to careful measurements, discovered the law of falling bodies and of the parabolic path of projectiles, studied the motions of pendulums, and investigated mechanics and the strength of materials. He showed little interest in astronomy, although beginning in 1595 he preferred the Copernican theory (see Astronomy: The Copernican Theory)—that the earth revolves around the sun—to the Aristotelian and Ptolemaic assumption that planets circle a fixed earth. Only the Copernican model supported Galileo’s tide theory, which was based on motions of the earth. In 1609 he heard that a spyglass had been invented in Holland. In August of that year he presented a telescope, about as powerful as a modern field glass, to the doge of Venice. Its value for naval and maritime operations resulted in the doubling of his salary and his assurance of lifelong tenure as a professor.

By December 1609, Galileo had built a telescope of 20 times magnification, with which he discovered mountains and craters on the moon. He also saw that the Milky Way was composed of stars, and he discovered the four largest satellites of Jupiter. He published these findings in March 1610 in The Starry Messenger (trans. 1880). His new fame gained him appointment as court mathematician at Florence; he was thereby freed from teaching duties and had time for research and writing. By December 1610 he had observed the phases of Venus, which contradicted Ptolemaic astronomy and confirmed his preference for the Copernican system.

Professors of philosophy scorned Galileo’s discoveries because Aristotle had held that only perfectly spherical bodies could exist in the heavens and that nothing new could ever appear there. Galileo also disputed with professors at Florence and Pisa over hydrostatics, and he published a book on floating bodies in 1612. Four printed attacks on this book followed, rejecting Galileo’s physics. In 1613 he published a work on sunspots and predicted victory for the Copernican theory. A Pisan professor, in Galileo’s absence, told the Medici (the ruling family of Florence as well as Galileo’s employers) that belief in a moving earth was heretical. In 1614 a Florentine priest denounced Galileists from the pulpit. Galileo wrote a long, open letter on the irrelevance of biblical passages in scientific arguments, holding that interpretation of the Bible should be adapted to increasing knowledge and that no scientific position should ever be made an article of Roman Catholic faith.

Early in 1616, Copernican books were subjected to censorship by edict, and the Jesuit cardinal Robert Bellarmine instructed Galileo that he must no longer hold or defend the concept that the earth moves. Cardinal Bellarmine had previously advised him to treat this subject only hypothetically and for scientific purposes, without taking Copernican concepts as literally true or attempting to reconcile them with the Bible. Galileo remained silent on the subject for years, working on a method of determining longitudes at sea by using his predictions of the positions of Jupiter’s satellites, resuming his earlier studies of falling bodies, and setting forth his views on scientific reasoning in a book on comets, The Assayer (1623; trans. 1957).

In 1624 Galileo began a book he wished to call "Dialogue on the Tides," in which he discussed the Ptolemaic and Copernican hypotheses in relation to the physics of tides. In 1630 the book was licensed for printing by Roman Catholic censors at Rome, but they altered the title to Dialogue on the Two Chief World Systems (trans. 1661). It was published at Florence in 1632. Despite two official licenses, Galileo was summoned to Rome by the Inquisition to stand trial for "grave suspicion of heresy." This charge was grounded on a report that Galileo had been personally ordered in 1616 not to discuss Copernicanism either orally or in writing. Cardinal Bellarmine had died, but Galileo produced a certificate signed by the cardinal, stating that Galileo had been subjected to no further restriction than applied to any Roman Catholic under the 1616 edict. No signed document contradicting this was ever found, but Galileo was nevertheless compelled in 1633 to abjure and was sentenced to life imprisonment (swiftly commuted to permanent house arrest). The Dialogue was ordered to be burned, and the sentence against him was to be read publicly in every university.

Galileo’s final book, Discourses Concerning Two New Sciences (trans. 1662-65), which was published at Leiden in 1638, reviews and refines his earlier studies of motion and, in general, the principles of mechanics. The book opened a road that was to lead Newton to the law of universal gravitation that linked Kepler’s planetary laws with Galileo’s mathematical physics. Galileo became blind before it was published, and he died at Arcetri, near Florence, on January 8, 1642.

Galileo’s most valuable scientific contribution was his founding of physics on precise measurements rather than on metaphysical principles and formal logic. More widely influential, however, were The Starry Messenger and the Dialogue, which opened new vistas in astronomy. Galileo’s lifelong struggle to free scientific inquiry from restriction by philosophical and theological interference stands beyond science. Since the full publication of Galileo’s trial documents in the 1870s, entire responsibility for Galileo’s condemnation has customarily been placed on the Roman Catholic church. This conceals the role of the philosophy professors who first persuaded theologians to link Galileo’s science with heresy. An investigation into the astronomer’s condemnation, calling for its reversal, was opened in 1979 by Pope John Paul II. In October 1992 a papal commission acknowledged the Vatican’s error.