Galileo Galilei (1564-1642), Italian astronomer and physicist. In 1610, Galileo was the first person to use a telescope for astronomical studies. He discovered mountains on the Moon and the four largest of Jupiter's moons. In physics, he found that pendulums of the same length swing with the same time period and that heavy and light objects fall at the same rate. He supported the Copernican theory that the Sun was at the centre of the solar system rather than the Earth. This belief was judged heretical by the Inquisition of the Roman Catholic Church in 1633, and he was held under house arrest for the rest of his life.