Galileo demonstrates his first telescope

On this day in 1609 Galileo Galilei demonstrated his first telescope to Venetian lawmakers, including Leonardo Donato, the Doge (ruler) of Venice. At the time, Galileo taught geometry, mechanics and astronomy at the University of Padua, which was part of the Venetian republic. A year after the demonstration, he published Starry Messenger, which described his finding - made with his new telescope - that Venus and the moons of Jupiter have phases, like the Moon. Galileo recognized that his observations supported Nicolaus Copernicus’s theory that the planets revolve around the Sun. His advocacy of the theory culminated in 1632 with the publication of his Dialogue Concerning the Two Chief World Systems. The book and the surrounding controversy led to his conviction by the Roman Inquisition in 1633 of heresy. Galileo was held under house arrest until his death in 1642.

Date in History: 25 August 1609