Galileo began his academic career as a professor of mathematics at the University of Pisa in 1589, moving in 1592 to the University of Padua, in the Venetian Republic, where he remained for eighteen years he later described as the happiest and most fruitful of his life. There he conducted early experiments on pendulums and on the motion of falling and rolling bodies, using inclined planes to slow and measure acceleration that would have been too fast to observe in free fall, work that quietly laid the groundwork for the kinematics he would formalize decades later.

It was also at Padua, in 1609, that Galileo heard reports of a magnifying spyglass recently invented in the Netherlands. Working from description alone, he built his own version within months and steadily improved its magnification, turning what had been a novelty item into a serious scientific instrument - and turning it, crucially, toward the night sky rather than distant ships.

Confined to Arcetri and eventually blind, Galileo spent his final years synthesizing decades of earlier research into physics rather than astronomy, since the Inquisition's ban applied specifically to cosmology. The result, Discourses and Mathematical Demonstrations Relating to Two New Sciences, dealt with the strength of materials and with kinematics, the mathematics of motion - including the law that a falling body covers distance proportional to the square of the elapsed time, and the parabolic path of projectiles. Unable to publish in Italy, Galileo had the manuscript smuggled to the Netherlands, where it appeared in 1638, four years before his death.

Two New Sciences is now considered a foundational text of classical mechanics, directly anticipating and enabling the work Isaac Newton would build on a generation later - a fitting final act from a scientist condemned for looking at the sky but remembered equally for how carefully he studied things falling to the ground.

Michael Hart ranked Galileo 12th in The 100, just behind Louis Pasteur, crediting him both with decisive observational evidence for heliocentrism and with helping establish the modern scientific method's emphasis on experiment and mathematical description over inherited authority. His telescope turned an abstract mathematical model into something anyone could look through an eyepiece and verify, while his physics of motion, completed in his final years under house arrest, directly set up the mechanics that Isaac Newton would formalize decades later.

Galileo died in Arcetri on 8 January 1642. He is often said to have died the same year Isaac Newton was born, but the claim rests on a calendar mix-up: Newton's local birth record in England reads 25 December 1642 under the Julian calendar Britain still used, which corresponds to 4 January 1643 on the Gregorian calendar already in use in Galileo's Italy - so the two events, though separated by less than a year, fall in different calendar years once converted to a common system.