Galileo Galilei (1564-1642)

Galileo Galilei (1564-1642) in his early forties, a few years before the beginning of his groundbreaking telescopic observations in 1609.

Born February 15 1564 in Pisa, in a declining family of Florentine patricians. In 1581 he was sent to study medicine at the University of Pisa, but never showed much interest in the subject and starting in 1583 devoted himself exclusively to mathematics and philosophy. He left Pisa without a degree, yet in July 1589 he was appointed to the chair of mathematics at that same university. In 1592 he took on the prestigious chair of mathematics at the university of Padua.

Prior to 1609, Galileo had only shown passing interest in astronomical matters, despites privately presenting himself as a Copernican. His research while at Pisa and Padua was mostly concerned with the problem of motion, in particular motion on inclined planes, of the pendulum, and of freely falling bodies. First little known outside of Italy, Galileo's telescopic discovery in 1609 and 1610 instantly propelled him into international fame, and won him a position at the Florentine Court, as chief mathematician and philosopher to the Grand Duke of Tucsany, Cosimo de Medici II.

Galileo's telescopic discoveries, published in his landmark 1610 book Sidereus Nuncius shook the very foundations of the Ptolemaic/Aristotelian cosmology. His observations of the Moon's surface revealed valleys and mountains, instead of the smooth perfectly spherical surface postulated by Aristotle. His observations of multitudes of faint stars gave some credence to Copernicus' suggestion that the universe may be a lot larger than hitherto believed. Perhaps his most striking discovery was that of four moons orbiting Jupiter, in direct contradiction with another Aristotelian postulate, that of the Earth being the center of (circular) motion for all heavenly bodies.

In the following two years Galileo made two new sets of observations that would further undermine the prevailing Aristotelian/Ptolemaic cosmology. The first was the observation of the phases of Venus, and the second the observation of sunspots. Galileo published his views on the latter in his Three Letters to Mark Wesler, in response to the three letters written earlier by Christoph Scheiner to the same Wesler. Controversy over the priority of discovery of sunspots would later turn Scheiner and Galileo into bitter enemies.

Following the 1616 decree suspending for revision Copernicus' De Revolutionibus and an injunction by Cardinal Roberto Bellarmino not to hold or defend the Copernican doctrine, Galileo turned to the problem of the tides, hoping in doing to to provide a proof of the motion of the Earth. Galileo's pro-Copernican campaign culminated with the publication of his 1632 Dialogue concerning the two chief world systems. The Roman ecclesiastic authorities considered the book to violate the 1616 decree. In September 1632 Galileo was summoned to Rome by the Inquisition and was put on trial.

On June 22 1633 Galileo was forced to kneel in front of the Roman Inquisition and recant his beliefs in the Copernican doctrine and the motion of the Earth. He was then sentenced to life imprisonment, which was almost immediately commuted to perpetual house arrest without visitors, ostensibly for having disobeyed a 1616 injunction by Cardinal Bellarmine "...not to defend or teach the Copernican doctrine...". Galileo's Dialogue was put on the Index of Prohibited Books, as well as Copernicus' De Revolutionibus and the books of Kepler dealing with planetary theory.

Galileo's sentence was upheld rather rigidly despites numerous appeals to the Inquisition and the Pope by Galileo himself, as well as numerous prominent scientists and statesmen in Italy and Europe. After Galileo became blind in 1637, the enforcement of his sentence was relaxed somewhat, and he was allowed to receive visitors for extended periods of time. In 1638 he completed yet another landmark work, Discourses on Two New Sciences provided the foundations for the modern science of mechanics. The manuscript was smuggled out of Italy and the book published in Holland.

Galileo died on the evening of January 8, 1642. The Roman ecclesiastic authorities vetoed the public funeral and honor planned by the Florentine state. His books, together with those of Copernicus and Kepler, were removed from the Index in 1835, and only in 1992 did the Roman catholic Church formally admitted to having erred in dealing with Galileo.


Drake, S. 1978, Galileo at work: his scientific biography, Chicago: The University of Chicago Press (1995 Dover reprint).

De Santillana, G. 1955, The crime of Galileo, The University of Chicago Press

Fantoli, A. 1996, Galileo, Vatican Observatory Publications [distributed outside of Italy by the University of Notre Dame Press].

Galileo, G. 1610, Sidereus Nuncius, trans. A. van Helden 1989, The University of Chicago Press.

Galileo, G. 1613, Letters on Sunspots [in S. Drake (trans.) 1957, Ideas and Opinions of Galileo], Doubleday.

Galileo, G. 1632, Dialogues concerning the two chief world systems, trans. S. Drake, 2nd edition 1967, University of California Press

Sharratt, M. 1994, Galileo: Decisive Innovator, Cambridge University Press


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