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, in which he tried to make his support for the Copernican view diplomatic. He seems to have believed that the Church authorities would be sympathetic, but he misjudged their resistance to such novel ideas. The following year Galileo was brought before the Holy Office of the Inquisition to stand trial for heresy (for believing the Sun to be the centre of the ‘known world’). He was found guilty, forced to recant his views, and sentenced to house arrest for life at the age of 69. The Dialogue was prohibited along with the reprinting of his earlier books and this could have prevented him from publishing further work. The immediate effect, however, was that the value of existing copies of Dialogue rose as scholars bought up the banned book.

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, which he completed while under house arrest, is in his Discourses Concerning Two New Sciences (1638). Galileo’s supporters smuggled this work out of Italy and had it printed at Leiden, in Protestant Holland. This work completes the claim to regard him as ‘the father of mathematical physics’. (The two ‘new sciences’ which he described are now known as ‘theory of structures’ in engineering 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.

See also the Chronology of major events in science.

Galileo aged about 60: engraving by Ottavio Leoni.