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Last Updated on May 6, 2015, by eNotes Editorial. Word Count: 2055

Article abstract: Galileo helped establish the modern scientific method through his use of observation and experimentation. His work in mathematics, physics, and astronomy made him a leading figure of the early scientific revolution.

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Early Life

Galileo Galilei was the first of seven children born to Vincenzo Galilei and Giulia Ammanati. His father was a cloth merchant as well as a noted musician who wrote several treatises on musical theory. The Galileis were a noble Florentine family that over the years had lost much of its wealth. It was for financial reasons that Vincenzo left Florence and moved to Pisa to establish his textile trade.

At the age of ten, Galileo and his family returned to Florence. His early education was directed by his father with the help of a private tutor. He also spent some time at the monastery of Santa Maria di Vallombrosa. The content of Galileo’s elementary education is unknown, but it was probably humanistic in character. His father urged him to pursue university studies which would lead to a lucrative profession.

Following his father’s wish, Galileo enrolled as a student of medicine at the University of Pisa in 1581. He showed little interest in his medical studies; it was mathematics that captured his attention. A year after enrolling at the university, Galileo made his legendary discovery of the isochronal movement of pendulums by observing a chandelier in the Pisa cathedral. He confirmed his theory regarding the equal movement of pendulums by conducting a series of experiments. He continued an independent study of science and mathematics, finally convincing his father to allow him to abandon his medical studies. Galileo withdrew from the University of Pisa in 1586 without receiving a degree, and he returned to his family.

Upon his return to Florence, Galileo studied a wide range of literary and scientific texts. In addition, he delivered a series of popular lectures on the Inferno of Dante’s La divina commedia (c. 1320; The Divine Comedy, 1802) at the Florentine Academy. In 1589, he used the influence of friends to obtain an appointment as a lecturer of mathematics at the University of Pisa. His return to Pisa marked a productive and enjoyable time for the young scholar. He conducted a series of experiments relating to falling bodies and wrote a short manuscript which challenged many traditional and generally accepted teachings about physics. In addition to his scholarly activities, Galileo was known for his quick wit, biting sense of humor, and excellent debating ability. Once again his friends intervened on his behalf to arrange an appointment, in 1592, to a more prestigious chair of mathematics at the University of Padua.

Life’s Work

It was at Padua that Galileo began his life’s work, which would bring him both fame and controversy. He quickly established himself as an excellent and popular teacher, both in terms of public lectures and private tutoring. He also wrote a series of short manuscripts on a variety of technical and practical issues. In 1597, he constructed a “military compass” to assist artillery bombardments and army formations.

Although Galileo’s invention of the military compass brought him acclaim and a good source of additional income, it was his work in the study of motion and astronomy that firmly established his reputation as a leading scientist. In 1604, a new star could be seen, and its sudden appearance prompted a fierce debate. According to the dominant theory of the time, Earth was the immovable center of the universe. Based on the work of Aristotle and Ptolemy, most scholars believed that the planets, sun, and stars rotated around a stationary Earth. The universe was thought to reflect a perfect and unchangeable order that had been created by God. The new star raised a problem of how to account for its presence in an already complete and perfectly ordered universe.

The intensity of this debate reflected a larger controversy regarding the work of Nicolaus Copernicus. Copernicus claimed that Earth and the other planets orbited the sun, and the stars were fixed or stationary. The appearance of a new star provided a tangible point of reference to settle a much larger scientific and theological debate on the structure and nature of the universe. It was a debate Galileo wanted to enter. As his correspondence with the astronomer Johannes Kepler indicated, Galileo found Copernicus’ thesis convincing, but he lacked the necessary instrument to test the theory. This problem was remedied in 1609 when on a visit to Venice, Galileo learned about a new “eye-glass by means of which visible objects, though very distant from the eye, were distinctly seen as if nearby.” Based on this limited information, Galileo returned to Padua to design and build his own telescope.

With this new instrument, Galileo turned his gaze toward the sky. He saw that the moon was not a smooth sphere, as previously assumed, but had many craters and mountains. These geological characteristics implied that Earth was not a unique or central planet in the universe. While observing Jupiter, he discovered four moons, disproving the assumption that Earth was the only planet to be orbited by a natural satellite. His observations of Venus forced Galileo to conclude that its phases could not be accounted for within the traditional geocentric model of the universe but could only be explained in terms of the Copernican heliocentric system. His study of the sun revealed its spots, implying that it was spherical and rotated on its axis as did Earth.

In 1610, Galileo published his findings in his Sidereus Nuncius (The Sidereal Messenger, 1880). The book was quite popular and was translated and reprinted in a wide variety of languages. The book implied a strong support of the Copernican solar system. The vast number of stars and movement of planets which Galileo observed could only be explained in the context of a heliocentric model wherein Earth and the other planets orbit the sun, and the stars are fixed points of light which only appear to move because of Earth’s orbital path.

The Sidereal Messenger brought Galileo international fame and set the stage for future controversy. The University of Padua granted him a professorship for life. Instead of accepting the offer, however, he resigned in order to return to Florence and become the grand duke’s chief philosopher and mathematician. The move marked a fateful change for the scientist.

A year after his return to Florence, Galileo made a triumphant visit to Rome. He lectured widely, demonstrated his telescope, and debated a variety of scientific issues. Although he was well received by both the pope and papal court, there were signs of growing opposition to his work. Some theologians claimed that Copernicus, and therefore Galileo, was in conflict with the Bible and the doctrines of the Church regarding the central role and location of Earth in God’s created order. Galileo was warned that he should teach and discuss the Copernican system only as a speculative theory and not as a truthful representation of the universe. In 1615, Copernicus’ book, De revolutionibus orbium coelestium (1543; On the Revolutions of the Celestial Spheres, 1939), was placed on the Catholic church’s index of banned publications. In response to this censorship, Galileo refrained from any public comment on astronomy for a number of years, turning his attention to navigational problems. In 1618, however, three comets appeared, and a Jesuit astronomer maintained that their appearance disproved Copernicus. Galileo broke his silence with the publication of Saggiatore (1623; the assayer). The brief tract not only refuted the attack against Copernicus but also presented an elegant argument in behalf of free scientific inquiry.

As the controversy surrounding this episode subsided, Galileo began writing his most important and controversial book, Dialogo sopra i due massimi sistemi del mondo, tolemaico e copernicano (Dialogue Concerning the Two Chief World Systems, Ptolemaic and Copernican, 1661), published in 1632. Its purpose was to present an “inconclusive” comparison between the Ptolemaic and Copernican models of the universe. Although Galileo carefully presented the various claims in terms of competing theories, it was apparent that he believed that the Ptolemaic geocentric theory was false and that the Copernican heliocentric theory was true. Although the book had received the Catholic church’s imprimatur, it was placed on the index of banned books shortly after its publication. A few months later, the Office of the Inquisition summoned Galileo to Rome to stand trial for heresy. What was at stake was whether he had defied a papal ban “to hold, defend, and teach the Copernican doctrine.” More important, the Church’s authority and ability to enforce compliance with its teachings were also at issue.

After a five month trial, Galileo was convicted because he “held and believed false doctrine, contrary to the Holy and Divine Scriptures.” The punishment would be a public prohibition of Dialogue Concerning the Two Chief World Systems and a prison sentence. Galileo, however, was given an opportunity to recant, which he accepted, swearing, “I will never again say or assert . . . anything that might furnish occasion for a similar suspicion.” Theological authority, for the time being, had silenced the claims of scientific observation.


For the rest of his life Galileo remained under house arrest, first in the village of Siena and later in Arcetri. He was not allowed to take any extensive trips or to entertain many guests. Following the death of his favorite daughter in 1634, he lived a lonely life and became blind in 1637. Despite the attempt to isolate him from the world, his fame grew—such noted figures as Thomas Hobbes and John Milton went out of their way to visit him shortly before his death.

His legacy was to establish science, based on observation and experimentation, as an important intellectual and social force in the world. Unlike Copernicus or Kepler, he was not a systematic or speculative thinker, preferring to base his work on a careful inquiry into the causes of natural phenomena. As indicated by his various inventions, he was also interested in applying his knowledge to practical problems.

Galileo marked an important break between theology and science that was not easily or quickly bridged—Copernicus and he were not removed from the Church’s index of banned books until 1835. Yet despite his conviction for heresy, history has judged his right to seek after truth quite differently. According to legend, as Galileo signed his recantation following his trial, he mumbled, “Eppur si muove” (“And yet it [Earth] moves”). Although Galileo never spoke these words, the legend’s existence and endurance is a fitting indication of the eventual support he received not only for his work but also for the right of the scientist to engage in free and open inquiry.


Boas, Marie. The Scientific Renaissance, 1450-1630. New York: Harper & Row, 1962. A detailed historical account of the major scientific discoveries and conflicts during the Renaissance. Provides good background material regarding the work and accomplishments of Galileo.

Fermi, Laura, and Gilberto Bernardini. Galileo and the Scientific Revolution. New York: Basic Books, 1961. A concise and highly sympathetic biography. The book includes a limited number of illustrations and an appendix which presents a translation of Galileo’s first tract, “The Little Balance.”

Geymonat, Ludovico. Galileo Galilei. New York: McGraw-Hill, 1965. A highly detailed biography which concentrates on examining the development of Galileo’s thinking. Particular attention is placed on reconstructing his emerging philosophy of science.

Koyré, Alexandre. Galileo Studies. Translated by J. Mepham. Atlantic Highlands, N.J.: Humanities Press, 1978. A highly technical and critical examination of the scientific and mathematical principles used by Galileo in his various observations and experimentations.

Kuhn, Thomas S. The Copernican Revolution: Planetary Astronomy in the Development of Western Thought. Cambridge, Mass.: Harvard University Press, 1957. A detailed historical review of the debate inspired by Copernicus. The dramatic intellectual and social implications of the change from a geocentric to a heliocentric worldview are also examined.

Redondi, Pietro. Galileo: Heretic. Translated by Raymond Rosenthal. Princeton, N.J.: Princeton University Press, 1987. A comprehensive and critical examination of Galileo’s trial before the Inquisition. Particular attention is directed toward examining the motivations and issues at stake for the Catholic church in the trial.

Ronan, Colin A. Galileo. New York: G. P. Putnam’s Sons, 1974. A standard biography which not only provides numerous details about Galileo’s life but also places them within the larger context of the intellectual changes taking place. The book also contains numerous illustrations and photographs.

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