Italian mathematician, astronomer, and physicist. He developed the astronomical telescope and was the first to see sunspots, the four main satellites of Jupiter, and the appearance of Venus going through phases, thus proving it was orbiting the Sun. In mechanics, Galileo discovered that freely falling bodies, heavy or light, have the same, constant acceleration and that a body moving on a perfectly smooth horizontal surface would neither speed up nor slow down.
Galileo's work founded the modern scientific method of deducing laws to explain the results of observation and experiment (although the story of his dropping cannonballs from the Leaning Tower of Pisa is questionable). His observations were an unwelcome refutation of the ideas of Aristotle taught at the (church-run) universities, largely because they made plausible for the first time the heliocentric (Sun-centred) theory of Polish astronomer Nicolaus Copernicus. Galileo's persuasive Dialogo sopra i due massimi sistemi del mondo/Dialogues on the Two Chief Systems of the World 1632 was banned by the church authorities in Rome and he was made to recant by the Inquisition.
Galileo was born and educated in Pisa, and in 1589 became professor of mathematics at the university there; in 1592 he became a professor at Padua, and in 1610 was appointed chief mathematician to the Grand Duke of Tuscany. When tried for heresy in 1633, and forced to abjure his belief that the Earth moves around the Sun, Galileo is reputed to have muttered: 'Eppur si muove' ('Yet it does move'). He was put under house arrest for his last years.
Galileo discovered in 1583 that each oscillation of a pendulum takes the same amount of time despite the difference in amplitude. He invented the thermometer and a hydrostatic balance, and discovered that the path of a projectile is a parabola.
Galileo published De motu/On Motion 1590. Having made his own telescopes, he published his first findings in Sidereus nuncius/The Starry Messenger 1610; the book was a sensation throughout Europe. He summed up his life's work in Discorsi e dimostrazioni matematiche intorno a due nove scienze/Discourses and Mathematical Discoveries Concerning Two New Sciences. The manuscript of this book was smuggled out of Italy and published in Holland 1638.