Galilei, Galileo
   Italian mathematician, astronomer, and physicist, one of the principal figures in the 17th-century reconstruction of physical science. His writings and his work are also important in the development of ideas about scientific method. He was born at Pisa, the son of a prominent Florentine musician, Vincenzo Galilei. He studied classical languages at the monastery of Vallombrosa. In 1581 he matriculated as a medical student at the University of Pisa, but he left without a degree in 1585 and worked as a private teacher of mathematics. He was already interested in mathematical and physical science and in 1589 became professor of mathematics at Pisa. In 1592 he accepted a professorship at the University of Padua, the Venetian state university, where he spent 18 productive years and wrote on astronomical, physical, and mathematical subjects.
   Learning of the invention of the telescope in the Netherlands, in 1609 Galileo designed and built one of his own. The discoveries resulting from his observations were startling, including the existence of mountains on the moon and the satellites of the planet Jupiter. Both of these discoveries contradicted prevailing astronomical theory. His experiments in physics and astronomy at Padua led to his observation of sunspots and discovery of the phases of Venus and his demonstration that Venus rotates around the sun, not the earth, thus providing strong evidence in favor of the heliocentric astronomy of Copernicus. He set forth these findings and his discovery of sunspots (another blow to the credibility of traditional astronomy) in his Sidereus nuncius / The Starry Messenger (1610).
   Galileo's astronomical discoveries attracted the patronage of the grand duke Cosimo II of Tuscany, and in 1610 he left Padua for Florence and became mathematician and philosopher to Cosimo. At this point Galileo openly became a supporter of Copernicus. This position aroused opposition from the Dominican friars, who charged that teaching that the sun stood in the center of the universe and the earth moved around it was contrary to the Bible. Under attack from them, he wrote a famous Letter to the Grand Duchess (1615) demonstrating that Copernican astronomy was not contrary to Scripture. This letter set forth what came to be an influential conception of the relation between natural science and religious authority.
   By 1616, however, the Inquisition at Rome had declared the idea that the earth moved to be heretical, and Galileo received a formal warning from one of his critics, Cardinal Robert Bellarmine, while officials of the Inquisition stood by to serve an injunction against him if he refused to submit. He did agree never again to teach or defend the forbidden opinion. At this time also, the Congregation of the Index forbade publication of Copernicus' De revolutionibus until it was purged of errors (basically, of its author's opinion).
   The accession of a new and much friendlier pope, Urban VIII, in 1623 encouraged Galileo to dedicate to the pope a work that upheld Galileo's opinion on the orbit of comets, Il saggiatore /The Assayer. The pope's gracious acceptance of this dedication encouraged Galileo to continue refining his Copernican ideas. During the rest of the 1620s, he worked on what became his most famous book, Dialogo sopra i due massimi sistemi del mondo / Dialogues on the Two Chief Systems of the World. Because of the ban on defenses of Copernican astronomy, he had difficulty getting permission to publish it and eventually had to add a preface and concluding note declaring that the Copernican system was a purely mathematical hypothesis and not a provable scientific conclusion. The book was then licensed and published in 1632. Pope Urban regarded this publication as a violation of Galileo's promise to write impartially on the Copernican question. The Inquisition prohibited sale and any further publication of the Dialogues and summoned Galileo to Rome to answer charges that he had defended Copernicanism even though it had already been condemned. He eventually was compelled to plead guilty to a lesser charge that he had rashly produced a defense of Copernicus without intending to do so. The pope himself insisted that Galileo must be compelled to take an oath declaring that he did not believe in the motion of the earth; he was forced to abjure his former opinion, and he was then sentenced to house arrest for the rest of his life and forbidden ever to write again on Copernicanism.
   In the last decade of his life, being confined to his country villa, Galileo shifted his scientific work from astronomy to dynamics. The result was a book, Discorsi e dimostrazioni matematiche intorno a due nuove scienze / Two New Sciences, which he smuggled out of Italy to Holland and published at Leiden in 1638. Not until the 19th century did church officials lift the ban on Copernican astronomy, and when a new edition of the Index of Forbidden Books appeared in 1835, Galileo's Dialogues on the Two Chief Systems of the World had at last been removed.

Historical Dictionary of Renaissance. . 2004.

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