ON THIS DAY IN HISTORY – September 20th, 1633 (Contributed by BarbraAJohnson5)


On this day, September 20th, 1633, Galileo Galilei was tried before the Inquisition for teaching that the earth orbits the sun. Galileo was born in Pisa, Italy in 1564; his father was a musician and through his mother’s family belonged to the nobility but was not rich. In the early 1570’s he and his family moved to Florence.

In 1581 Galileo attended the University of Pisa, where his father wanted him to study medicine; but instead he began his study of the pendulum. According to legend, Galileo watched a suspended lamp swing back and forth while sitting in the Cathedral of Pisa, timing the arc of the swing by using his pulse beat. This lead to his discovery that the time of the pendulum’s swing does not depend on the arc of the swing and eventually to his ideas for the development of the pendulum clock.

In 1592, Galileo was appointed professor of mathematics at the University of Padua. During this period he made significant discoveries in both pure science (kinematics of motion, and astronomy) and applied science (strength of materials, improvement of the telescope).

In 1610 Galileo published an account of his telescopic observations of the moons of Jupiter, using this observation to argue in favor of the sun-centered, Copernican theory (see next paragraph), as opposed to the dominant earth-centered prevailing theories (of Ptolemy and Aristotle). The next year Galileo visited Rome in order to demonstrate his telescope to the influential philosophers and mathematicians of the Jesuit Collegio Romano, and to let them see with their own eyes the reality of the four moons of Jupiter (Io, Europa, Callisto and Ganymede, 4 other moons were later discovered). While in Rome he was also made a member of the Accademia dei Lincei.

Copernicus’ theory that the earth revolves around the sun rather than the common help belief that universe revolves around the earth, On the Revolutions of the Heavenly Spheres , was published in 1543, almost a century before Galileo came to the same conclusion. Copernicus’ theory was considered implausible by the vast majority of his contemporaries, and by most astronomers and natural philosophers until the middle of the seventeenth century until strong theoretical evidence was finally provided by Sir Isaac Newton’s theory of universal gravitation in 1687.

Galileo’s belief in the Copernican System eventually got him into trouble with the Catholic Church. The Inquisition was a permanent institution in the Catholic Church charged with the eradication of heresies. A committee of consultants declared to the Inquisition that the Copernican proposition that the Sun is the center of the universe was a heresy. Because Galileo supported the Copernican system, he was warned by Cardinal Bellarmine, under order of Pope Paul V, that he should not discuss or defend Copernican theories. In 1624, Galileo was assured by Pope Urban VIII that he could write about Copernican theory as long as he treated it as a mathematical proposition. However, with the printing of Galileo’s book, Dialogue Concerning the Two Chief World Systems, Galileo was called to Rome in 1633 to face the Inquisition again. Galileo was found guilty of heresy for his Dialogue, and was sent to his home near Florence where he was to be under house arrest for the remainder of his life. In 1638, the Inquisition allowed Galileo to move to his home in Florence, so that he could be closer to his doctors. By that time he was totally blind. In 1642, Galileo died at his home outside Florence.


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