Galileo, an Italian astronomer and physicist, has been called the founder of modern experimental science. Galileo made the first effective use of the refracting telescope to discover important new facts about astronomy. He also discovered the law of falling bodies as well as the law of the pendulum. Galileo designed a variety of scientific instruments. He also developed and improved the refracting telescope, though he did not invent it.

Galileo Galilei was born in Pisa on Feb. 15, 1564. In the early 1570's, his family moved to Florence, and Galileo began his formal education at a school in a nearby monastery. Galileo's father, determined that his son should be a doctor, sent him to the University of Pisa in 1581. Galileo studied medicine and the philosophy of Aristotle for the next four years.

Galileo's years as a student at Pisa marked a turning point for him. Never really interested in medicine, he discovered he had a talent for mathematics. In 1585, he persuaded his father to let him leave the university. Back in Florence with his family, Galileo spent the next four years as a tutor in mathematics. During this time, he began to question Aristotelian philosophy and scientific thought. At the same time, he gained his first public notice with his new hydrostatic balance, an instrument used to find the specific gravity of objects by weighing them in water.

In 1589, Galileo was appointed professor of mathematics at the University of Pisa. This position required him to teach courses in astronomy on the basis of the Greek astronomer Ptolemy's theory that the sun and all the planets revolve around the earth. Preparing for these courses deepened Galileo's understanding of astronomical theory. In 1592, he took up duties as professor of mathematics at the University of Padua, where he spent the next 18 years. During this time, he became convinced of the truth of the theory, proposed by the Polish astronomer Nicolaus Copernicus, that all planets, including the earth, revolve around the sun.

In 1609, while still at Padua, Galileo built his first telescope. Turning it to the sky, he saw clear evidence that many of Aristotle's and Ptolemy's claims about the heavens were false. Galileo's first discovery was that, far from being perfectly smooth, as Aristotle and Ptolemy had thought, the moon was mountainous and pitted, much like the earth. He made his most sensational discovery in 1610, when he discovered four moons circling Jupiter. He named these moons the "Medicean Planets," in the hope of winning the favor of the Medicis, the ruling family of Florence.

In 1610, Cosimo de Medici, Grand Duke of Tuscany, named Galileo his personal mathematician. This position brought Galileo back to Florence, where he continued his studies of the heavens. He made observations of sunspots and of Venus, noting that the planet progresses through phases similar to those of the moon. This fact confirmed his doubts about Ptolemaic astronomy and deepened his conviction of the truth of Copernicus' theory that the earth and planets revolve around the sun. Publication of these findings, starting in 1610, brought him wide renown.

Galileo also pursued research on motion -- especially the motion of freely falling bodies. The problem, as he saw it, was that the Aristotelian theory of motion, which referred all motion to a stationary earth at the center of the universe, made it impossible to believe the earth actually moves. Galileo went to work to develop a theory of motion consistent with a moving earth.

Among the most important results of this search were the law of the pendulum and the law of freely falling bodies. Galileo observed that pendulums of equal length swing at the same rate whether their arcs are large or small. Modern measuring instruments show that the rate is actually somewhat greater if the arc is large. Galileo's law of falling bodies states that all objects fall at the same speed, regardless of their mass; and that, as they fall, the speed of their descent increases uniformly.

Galileo's quick wit, which he often used to ridicule his opponents, earned him a number of enemies. In 1613, Galileo wrote a letter in which he tried to show that the Copernican theory was consistent with both Catholic doctrine and proper Biblical interpretation. Some of his enemies sent a copy of this letter to the inquisitors in Rome, who sought out and punished heretics -- people who opposed church teachings. In early 1616, Galileo was summoned to Rome for a determination on the orthodoxy of his views. Although he was cleared of charges of heresy, he was ordered not "to hold or defend" the Copernican theory. That is, he could treat the theory hypothetically but not treat it as if it were true.

In 1632, Galileo published his first scientific masterpiece, the Dialogue Concerning the Two Chief World Systems. In this work, he compared the Ptolemaic-Aristotelian theory to the Copernican theory to show that the Copernican system was logically superior. Once again Galileo was summoned to Rome, this time to answer to the charge of willfully disobeying the order not "to hold or defend" Copernicus' theory. In 1633, the Inquisition found Galileo guilty of the charge, forced him to recant (publicly withdraw his statement [abjuration]), and sentenced him to life imprisonment.

Because of Galileo's advanced age and poor health, the church allowed him to serve his imprisonment under house arrest in a villa outside Florence. There, he passed the remainder of his years in relative isolation, eventually becoming blind. But he managed to complete his second scientific masterpiece, the Discourse on Two New Sciences, published in 1638. In this work, Galileo provided both a mathematical proof of his new theory of motion and an original study of the tensile strength of materials. In 1979, Pope John Paul II declared that the Roman Catholic Church may have been mistaken in condemning Galileo. He instructed a church commission to study Galileo's case. In 1983, the commission concluded that Galileo should not have been condemned. In 1984, at the commission's recommendation, the church published all documents related to Galileo's trial. In 1992, Pope John Paul II publicly endorsed the commission's finding that the church had made a mistake in condemning Galileo.

Historians disagree about Galileo's role as the "founder of modern experimental science." In fact, some of them doubt that experiment, in the modern sense, played an important part at all in Galileo's scientific development. These historians maintain that Galileo's real originality lay in the way he approached scientific problems. First, Galileo reduced those problems to very simple terms on the basis of everyday experience and common-sense logic. Then he analyzed and resolved the problems according to simple mathematical descriptions. The success with which Galileo applied this technique to the analysis of physics, especially the physics of motion, opened the way for the development of modern mathematical physics.

Additional resources

Drake, Stillman. Galileo. Univ. of Toronto Pr., 1990. Galileo at Work. Univ. of Chicago Pr., 1978.

Parker, Steve. Galileo and the Universe. HarperCollins, 1992. Younger readers.

Reston, James, Jr. Galileo. HarperCollins, 1994.