(1564-1642) Italian mathematician, astronomer, and physicist. First to use a telescope to observe the skies. Born at Pisa; died at Arcetri (near Florence). Galileo Galilei was born 3 days before Michelangelo died. His first name resulted from a Tuscan tradition which used a variation of the last name for first name of the eldest son. His father had planned a career in medicine for him, until Galileo heard a lecture on geometry, and was exposed to the works of Archimedes. Shortly thereafter he convinced his father to allow the switch to mathematics and science. His approach to science transcended mere observation. Instead, he would attempt to devise a critical experiment which would clearly demonstrate his theories. After quantifying observables during an experiment, he would set upon the task of deriving a mathematical relationship to describe a phenomenon with simplicity and generality. During services at the Cathedral of Pisa, he observed the movements of a chandelier swinging, caused by air currents. Some swings were short, while others were made in wide arcs. But, the time required to complete the swing seemed to be constant. When he later tested his theory with two pendulums of equal length, using arcs of different sizes, he found that the periods of the swing remained constant. Not until after Galileo's death was the principle of the pendulum used (by Huygens) to regulate a clock. During his lifetime, the lack of an accurate clock to measure small intervals of time would hamper many of the experiments of Galileo. He devised a thermoscope to measure temperature; but this invention was quite inaccurate. He also designed a hydrostatic balance, and published a description of same in 1586. His experimentation with falling bodies has become nearly legendary. He is supposed to have dropped two cannon balls of unequal weight (in the ratio 10:1) from the Tower of Pisa, showing that both stuck the ground at the same instant. It is doubtful that this experiment actually took place; but, it was performed earlier by Stevinus. By rolling bodies down an inclined plane, Galileo was able to slow down the actions, and clearly demonstrate that the rate of fall of a body was independent of its weight. This result showed that the physics of Aristotle were in error. The philosophic ramifications of this result were profound: According to Aristotle, it was necessary to apply a continuous force in order to keep a body in motion. Thus, the heavenly bodies were kept in motion by the perpetual efforts of angels. Buridan, however, claimed that no force was required to maintain motion, save the initial push given by God during the act of creation. Galileo further showed that the velocity of a falling body increased linearly, and the total distance increased as the square of the time. His analysis of multiple forces created a science of gunnery. Lacking the advances in mathematical analysis achieved by the application of algebra to geometry, which was later provided by Descartes and Newton, Galileo formulated all his proofs using the geometric methods of the Greeks. His book on mechanics also treated the square-cube law for strength of materials. Then, in 1609, he learned of the telescope (which was invented in Holland). With his own version of the telescope, he began to explore the heavens, and the age of telescopic astronomy was born. He found that there were mountains on the Moon, spots on the Sun, that the Sun rotated upon its axis each 27 days, and that Jupiter posessed four companions which revolved about that planet (the Galilean satelites: Io, Europa, Ganymede, and Callisto). Again, his findings were at odds with the writings of Aristotle. The satellites of Jupiter offered definite proof that not all astronomical bodies circled the earth, giving strong support for the system of Copernicus. In addition, the phases exhibited by Venus were required by the Copernican theory, but could not be accounted for with the Ptolemaic theory. He publicized his findings in a periodical called Sidereus Nuncius (Starry Messenger). Thinking that Pope Urban IV would be friendly to his views, he published his Dialogue on the Two Chief World Systems, in which two people (who advocated the Ptolemaic and Copernican viewpoints, respectively) argue their cases before a third individual. But, the Pope was convinced that the book's character, Simplicio, was an insulting charicature of himself, deliberately invented for that purpose. Thus, Galileo was charged with heresy, brought before the Inquisition, and made to renounce all but the Ptolemaic viewpoint. It was not until 1835, that Galileo's Dialogue was to be removed from the Catholic Index of Prohibited Books. And finally, in 1965, Pope Paul VI praised the work of Galileo, thus admitting the wrong committed by the Church.