Galileo Galilei (February 15, 1564 - January 8, 1642), Italian physicist, mathematician, astronomer and philosopher. Galileo was the first to successfully use a telescope to observe the heavens, discovering new stars, mountains on the Moon, and the phases of Venus. His discovery of moons that circled Jupiter, not the Earth, offered support for the controversial Copernican system that removed the Earth from the center of the Solar system. This brought Galileo into conflict with the Catholic Church. After writing his Dialogue on the Two Chief World Systems in 1632, he was tried and found guilty of heresy and forced to recant his views.