Galileo Galilei (15 February 1564 – 8 January 1642) laid, with Kepler, the foundations of the Scientific Revolution. He created fundamental concepts in kinematics, dynamics, and ballistics. His observations using the telescope played a decisive role in establishing the Copernican system. He was the first to clearly annunciate the relativity principle and the (weak) equivalence principle: both played central roles in Newton’s and later Einstein’s theory of gravity. His principle of inertia was incorrect, however, in identifying circular motions as ‘natural’; Descartes was the first to state it in terms of rectilinear motion.
Galileo’s views were investigated by the Inquisition, first in 1615, and again, following publication of his Dialogue Concerning the Two Chief World Systems (which appeared to attack Pope Urban VIII) in 1633. In the latter inquisition he was condemned and forced to recant his views concerning the motion of the Earth: he was confined to house arrest for the rest of his life. During this time, however, he wrote his masterpiece Discourses and Mathematical Demonstrations concerning Two New Sciences, in which he summarised a lifetime of scientific work. It was smuggled out of Italy and published by Lodewijk Elzevir in Leiden, Holland, in 1638.
Numerous books and plays have been written about Galileo. The most famous is Life of Galileo, by Bertold Brecht. A recent and authoritative biography is J.L. Helibron’s Galileo, Oxford University Press, 2010.