Galileo

I

Introduction

Galileo (1564-1642), Italian physicist and astronomer who, with German astronomer Johannes Kepler, initiated the scientific revolution that flowered in the work of English physicist Sir Isaac Newton. Galileo’s main contributions were, in astronomy, the use of the telescope in observation and the discovery of sunspots, mountains and valleys on the Moon, the four largest satellites of Jupiter, and the phases of Venus. In physics, he discovered the laws of falling bodies and the motions of projectiles. In the history of culture, Galileo stands as a symbol of the battle against authority for freedom of inquiry.

II

Early Years

Galileo, whose full name was Galileo Galilei, was born near Pisa, Italy, on February 15, 1564. His father, Vincenzo Galilei, played an important role in the musical revolution from medieval polyphony to harmonic modulation. Just as Vincenzo saw that rigid theory stifled new forms in music, so his eldest son came to see both the then-dominant physics of Greek philosopher Aristotle and the Roman Catholic theology influenced by it as limiting scientific inquiry. Galileo was taught by monks at Vallombrosa and then entered the University of Pisa in 1581 to study medicine. He soon turned to philosophy and mathematics, and although he left the university in 1585 without a degree, he did receive a useful introduction to the versions of Aristotelian physics current at the time.

III

Aristotelian Physics of Galileo’s Time

Aristotelians made a sharp division between Earth and the heavens. In the heavens there could be no change except the recurring patterns produced by the circular motions of the perfectly spherical heavenly bodies. The sublunar world (the universe below the Moon) was the region of the four elements—earth, water, air, and fire—and was subject to its own distinct laws of natural motion. Fire, for instance, had lightness, which made it rise vertically, away from the center of Earth. Earthy objects fell naturally downward toward the center of Earth: the heavier the object, the faster its fall. “Natural” motions of the elements took them to their natural place, where they rested. Rest was the natural state of an element; it was motion that needed explaining, since every motion must have a cause. This common-sense physics held sway until Galileo began to undermine it.

IV

Galileo’s Work in Physics

The key to Galileo’s new physics lay in mathematics. While registered as a medical student at the University of Pisa, he increasingly devoted his time to the study of mathematics, with the encouragement of court mathematician Ostilio Ricci. After leaving the university he tutored privately for a time and wrote on hydrostatics, but he did not publish anything. In 1589 he became professor of mathematics at Pisa.

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