Galileo Galilei (1564-1642) was an Italian astronomer and physicist. He discovered Jupiter's moons and laws governing falling bodies.

Usually known by his first name, Galileo was born in Pisa; he was the son of a musician and became a medical student, but his interest moved to mathematics and physics. He became the ill-paid professor of mathematics at Pisa when he was 25, moved to Padua in 1591 and later to Florence. Galileo never married but when he was 35 Marina Gamba (a Venetian girl) came to live with him, and they had two daughters and a son. When he moved to Florence in 1610 he left Marina behind and she married soon after.

Galileo's fame rests partly on the discoveries he made with the telescope, an instrument which he did not invent but was certainly the first to exploit successfully. His design used a convex object glass and a concave eyepiece and gave an erect image. In 1610 he observed for the first time mountains on the Moon, four satellites around Jupiter and numerous stars too faint to be seen with the naked eye.

These observations he described in his book Sidereal Messenger (1610), which made him famous. He also discovered the phases of Venus, the composite structure of Saturn (although he was unable to resolve the rings as such: it looked to him like a triple planet) and sunspots. His discovery of heavenly bodies that were so demonstrably not circling Earth, together with his open public support for the Copernican heliocentric cosmology, was to bring him into conflict with the Church.

He wrote his Dialogue on the Two Chief World Systems, Ptolemaic and Copernican in 1632. He tried in the book to make his support for the Copernican view diplomatic, and he seems to have believed that the Church authorities would be sympathetic, but he misjudged their resistance to such novel ideas. The next year he was before the Inquisition, and was shown the torture chamber and forced to recant. He was sentenced to house arrest for life at the age of 69.

Among his notable non-astronomical findings were the isochronism (constant time of swing, if swings are small) of a pendulum, which he timed with his pulse when he was a medical student. (He designed a clock with its escapement controlled by a pendulum and his son constructed it after his death).

He also found that the speed at which bodies fall is independent of their weight. The latter was the result of experiments rolling balls down inclined planes, and not by dropping weights from the leaning tower of Pisa as was once widely believed. His work on mechanics is in his Discourses Concerning Two New Sciences (1638), which completes the claim to regard him as 'the father of mathematical physics'. (The two 'new sciences' which he described are now known as 'strength of materials' and 'dynamics'.) He died in the year in which Newton was born. His work sets the modern style; observation, experiment and the full use of mathematics as the preferred way to handle results.

The gal, named after him, is a unit of acceleration, 10^-2 m s^-2. The milligal is used in geophysics as a measure of change in the regional acceleration due to gravity (g).

Galileo was an able musician, artist and writer - a true man of the Renaissance. His massive contribution to physics makes him one of the small group of the greatest scientists of all time and his startling discoveries, his forceful personality and his conflict with the church help to make him the most romantic figure in science.