Galileo Galilei was born in Pisa in 1564 and studied at the city's university. He became a mathematician and natural philosopher, later teaching in Padua. Through experiments and careful observation he investigated motion, mechanics and the heavens. In 1609 he began using an improved telescope to examine the night sky, opening a remarkable new phase of his career.

In 1610 he published Sidereus Nuncius (The Starry Messenger), describing, among other discoveries, the Moon's uneven surface and four moons orbiting Jupiter. His observation of the phases of Venus provided further evidence against traditional cosmology. Galileo publicly supported Copernicus's Sun-centred system, bringing him into conflict with church authorities as well as scientific opponents.

In 1616 Cardinal Bellarmine warned him not to defend the Earth's motion as established fact. After the publication of his Dialogue Concerning the Two Chief World Systems in 1632, the dispute returned to Rome. In 1633 the Roman Inquisition found Galileo vehemently suspected of heresy, and he was placed under house arrest. He died in Arcetri in 1642. His discoveries helped transform experimental science, while his trial became an enduring symbol of the difficult relationship between intellectual inquiry and authority.