The ancient Greek mathematician Archimedes became his intellectual hero. He supplemented classes in natural philosophy at Pisa with private mathematical study in Florence. He left Pisa without a degree in 1585 and became a mathematics tutor in Florence, where he established the isochronous nature of the pendulum—the fact that the frequency of a pendulum is a constant. In 1589, his Archimedes-inspired work won him the mathematics chair at Pisa.
In 1592, Galileo became professor of mathematics at the University of Padua, Europe’s leading scientific university. Whatever the personal and financial stresses of the Padua years, they were Galileo’s most intellectually fruitful time. He moved from a highly mathematical approach to knowledge to a greater interest in experiment.
He began to elaborate a non-Aristotelian approach to the problems of moving bodies. His most famous result was the discovery that the distance covered by a falling body varies with the square of the time of the fall—the “law of falling bodies.”
Galileo’s work with the telescope in the early 17th century catapulted him to European fame. From what information he could gather, he designed his own, superior to the contemporary Dutch telescopes, in 1609. He observed the previously unknown moons of Jupiter. These were the first satellites of a planet (other than the Moon) ever known.
The fact that the system of the planets could have more than one center helped support the Copernican theory. Galileo’s other discoveries included the mountains of the Moon, the phases of Venus, and the composition of the Milky Way out of innumerable stars.
Galileo wanted to move to Tuscany in Florence. The naming of Jupiter’s moons the “Medicean stars” after the ruling Medici family of Tuscany was a brilliant stroke to win the duke’s favor, securing Galileo’s appointment as court mathematician.
Galileo insisted that he be given the title not merely of mathematician, but philosopher as well. Since the actual physical nature of the universe was the province of natural philosophers, Galileo as a philosopher could make cosmological claims that he could not make as a mere mathematician.
It was from Rome that Galileo faced what would prove to be the greatest challenge of his career, that of the church’s condemnation of Copernicanism. Church authorities were increasingly opposed to Copernicanism and Galileo as its principal Catholic champion.
Copernicus’s On the Revolution of the Celestial Spheres was placed on the church’s Index of Forbidden Books in 1616. Galileo argued that Copernicanism had no relevance to theology, but church authorities did not accept this position. Galileo’s works were still not specifically condemned.
Despite his enormous importance in the development of astronomy, Galileo was not at all what the early modern period considered an astronomer. He was not concerned with the precise observations and elaborate calculations necessary to predict the courses of the stars that absorbed the vast majority of the labor of working astronomers.
Galileo was more interested in making telescopic discoveries and establishing cosmological theory. The most significant work he wrote on astronomy after The Starry Messenger (1610) was Dialogue on the Two Chief Systems of the World (1632). In this work, Galileo used the motion of the Earth to explain the tides.
Galileo’s trial and conviction have been interpreted in many ways by historians. There were two dangers in Dialogue on the Two Chief Systems of the World. One was its bold statement of support for the Copernican system. The other as that the pope, Urban VIII (Maffeo Barberini), became convinced after the dialogue’s publication, which in all probability he himself had licensed, that the dull-witted Simplicio was a satire of him.
Urban reacted to Galileo’s ridicule by suppressing the Dialogue and establishing a commission to investigate the whole matter. After reading the commission’s report, Urban referred the Galileo case to the Roman Inquisition. The Inquisition summoned Galileo to Rome in the winter of 1632–33, a savage requirement to impose on an old man in ill health during a plague epidemic.
On his arrival in Rome in February, he was imprisoned. Negotiations between Galileo and the inquisitors, who threatened torture, produced a public confession. On June 22, 1633, he was condemned to house arrest and the recitation of penitential psalms. He spent his arrest first in Rome, and from the end of 1633 to his death, at his own house outside Florence.