Galileo, properly Galileo Galilei 1564-1642
Italian astronomer, mathematician and natural philosopher

Galileo was born in Pisa, the son of a musician. He matriculated at Pisa University (1581) where he accepted the chair of mathematics in 1589. From watching the movement of a lamp in the cathedral of Pisa, he discovered (1582) the principle of the isochronism of the pendulum (equality in time whatever the range of its swing), which indicated the value of the pendulum as a timekeeper. In his study of falling bodies, Galileo showed that, contrary to the Aristotelian belief that the rate at which a body falls is proportional to its weight, all bodies would fall at the same rate if air resistance were not present. He also showed that a body moving along an inclined plane has a constant acceleration, and demonstrated the parabolic trajectories of projectiles.

In 1592 he moved to the University of Padua, where his lectures attracted pupils from all over Europe. He made his first contribution to astronomy in 1604 when he demonstrated that a bright new star which had appeared in the constellation Ophiuchus was more distant than the planets, confirming Tycho Brahe's conclusion that changes take place in the celestial regions beyond the planets. In 1610 he perfected a refracting telescope, which he used in the course of many astounding astronomical revelations published in his Sidereus Nuncius (1610, 'Sidereal Messenger'). These included the mountains of the Moon, the multitude of stars in the Milky Way, and the existence of Jupiter's four satellites. Galileo was appointed Chief Mathematician and Philosopher by the Grand Duke of Tuscany.

On a visit to Rome in 1611 he was elected a member of the Accademia dei Lincei and f�ted by the Jesuit mathematicians of the Roman College. Further discoveries included the phases of Venus, spots on the Sun's disc, the Sun's rotation, and Saturn's appendages (though not then recognized as a ring system). These brilliant researches led Galileo to affirm the truth of the Copernican system with the sun at its centre, which he defended in his Dialogue on the Two Principal Systems of the World (1632). Its sale was prohibited by the ecclesiastical authorities; Galileo was brought before the Inquisition and under threat of torture recanted. He was finally allowed to live under house arrest in his own home at Arcetri, near Florence; there he continued his researches and completed his Discourses on the Two New Sciences (1638), in many respects his most valuable work, in which he discussed at length the principles of mechanics.

In 1637 he became blind, but he continued working until his death on 8 January 1642. The sentence passed on him by the Inquisition was formally retracted by Pope John Paul II on 31 October 1992.

Bibliography: Pietro Redondi, Galileo: Heretic (1988); Stillman Drake, Galileo at Work - His Scientific Biography (1978) and Discoveries and Opinions of Galileo (1957); M Allan-Olney, The Private Life of Galileo (1970); Georgio de Santillana, The Crime of Galileo (1955).