Measuring motion
Born in Pisa in 1564, Galileo Galilei abandoned medicine for mathematics and taught at Pisa and then Padua, where he spent his most productive years. Against the Aristotelian physics of his training, he studied how things actually move: balls rolling down inclined planes, pendulums, projectiles. He found that falling bodies accelerate uniformly regardless of weight and that projectiles trace parabolas, results he reached by combining experiment with geometry. The famous demonstration of dropping weights from the Leaning Tower rests only on a late account by his disciple Viviani, and historians doubt it happened as told.
What was genuinely new was the method. Galileo insisted that the book of nature is written in mathematical characters, and he treated carefully contrived experiments, not inherited texts, as the court of appeal. He was also a superb instrument-maker and self-promoter, supplementing his salary with a military compass of his own design and cultivating Medici patrons. The blend of craftsmanship, geometry and ambition that carried him to fame was inseparable from the combativeness that would later help carry him to trial.
The telescope's report
Galileo did not invent the telescope; word of the Dutch device reached him in 1609, and he rapidly improved it. What he did first, and best, was turn it into an argument. In the Starry Messenger of 1610 he reported mountains on the Moon, countless stars invisible to the eye and four satellites circling Jupiter, which he named for the Medici. Later came the phases of Venus, which could not be reconciled with Ptolemy's arrangement, and sunspots, over which he fought a bitter priority dispute with the Jesuit Christoph Scheiner. The heavens, his lenses reported, were neither perfect nor centered on us.
None of this strictly proved that the Earth moves, and rivals such as Simon Marius claimed some discoveries independently. But the observations made Copernicanism, until then a mathematical convenience, feel physically real, and Galileo pressed the case in vivid Italian rather than academic Latin. In 1616 the Church declared the Sun-centered thesis formally erroneous, and Galileo was warned not to hold or defend it. He waited, confident that argument, patronage and a new pope might change the weather.
The trial and the long argument
In 1632, with papal permission he believed he had, Galileo published the Dialogue Concerning the Two Chief World Systems, which left the Earth-centered case in the mouth of a character named Simplicio. Pope Urban VIII, facing wartime pressures and convinced he had been mocked, allowed the Inquisition to proceed. In 1633 Galileo was convicted of vehement suspicion of heresy, forced to abjure, and confined to his villa at Arcetri for the rest of his life. The defiant phrase and yet it moves is a later legend; the abjuration itself is documented.
Confinement did not end the work. In 1638 he published Two New Sciences, the foundation of modern mechanics, smuggled to a Dutch press while its author went blind. The trial has been retold ever since as a simple war of science against religion, but historians describe something messier: a Catholic scientist, a personal falling-out with a former admirer, disputes over scriptural interpretation and courtly honor. In 1992 John Paul II formally acknowledged the errors of the tribunal. The physics, meanwhile, had long since been vindicated by Newton's synthesis.
