Galileo Galilei

Early life and education

Galileo Galilei was born February 15, 1564, in Pisa, in the Duchy of Tuscany. His father Vincenzo Galilei was a musician and music theorist whose own work on the mathematical analysis of musical consonance was influential in the development of early modern empirical science. The Galilei family was minor Florentine nobility but financially modest; Vincenzo struggled to support his children's education on his musician's income.

Galileo was initially educated by monks at the monastery of Vallombrosa, where he showed interest in becoming a Camaldolese monk. Vincenzo Galilei retrieved him and arranged for him to study at the University of Pisa from 1581, initially for medicine. Galileo abandoned medicine for mathematics in 1585 and left the university without a degree. He supported himself through private tutoring in mathematics through his twenties.

In 1589 he received an appointment as chair of mathematics at the University of Pisa, then in 1592 moved to the chair of mathematics at the University of Padua, where he remained for 18 years - generally regarded as the most productive period of his career. The Padua appointment was within the Republic of Venice, a politically independent state whose distance from Roman ecclesiastical authority gave Galileo more freedom than he had had at Pisa.

The telescope and the Sidereus Nuncius (1609-1610)

In 1609 Galileo learned of a Dutch invention - the "spyglass" or refracting telescope - and within months had built improved versions in his workshop in Padua. His first telescope had approximately 3× magnification; he rapidly improved to 8×, then 20×, then 30×. The 30× telescope was the instrument with which he made his most famous observations.

Through the winter of 1609-1610 Galileo turned his telescopes on the night sky. He discovered (a) that the Moon has mountains and craters - contradicting the Aristotelian view that celestial bodies were perfect spheres; (b) that the Milky Way is composed of innumerable separate stars; (c) that the planet Jupiter has four moons orbiting it - which became the central piece of evidence against the geocentric view that all celestial motion was centered on the Earth.

In March 1610 he published Sidereus Nuncius (The Starry Messenger), announcing his discoveries. The book made him an international celebrity within months. He used the new reputation to negotiate a return to Florence as the chief mathematician of the Grand Duke of Tuscany - a politically dangerous move that brought him back under Roman ecclesiastical authority but provided patronage at a higher level than the Padua salary.

The Copernican question and the first Inquisition warning (1611-1616)

Galileo's telescope observations were widely interpreted as supporting the heliocentric system proposed by Nicolaus Copernicus in 1543. The phases of Venus - which Galileo observed in 1610 - were straightforwardly explained by the Copernican system and ruled out the Ptolemaic geocentric system as it had been understood for over a thousand years. The Jovian moon system showed that not all celestial motion was Earth-centered.

In 1615 the Roman Inquisition convened a commission to investigate the theological implications of Copernicanism. The commission ruled in 1616 that heliocentrism was "foolish and absurd in philosophy and formally heretical." Galileo was personally warned by Cardinal Robert Bellarmine not to "hold or defend" Copernicanism. The exact wording of the 1616 warning has been disputed in subsequent scholarship and was a central legal issue at Galileo's later trial.

After the 1616 warning Galileo turned to other projects - work on sunspots, on the cause of the tides (which he believed to be evidence for Earth's motion - he was substantially wrong about the mechanism), and on the Jesuit-controlled astronomical politics of his period. He maintained a low profile on Copernicanism for the next decade.

The Dialogue, the Inquisition trial (1633), and house arrest

In 1632 Galileo published Dialogue Concerning the Two Chief World Systems, a book-length comparative discussion of the Ptolemaic and Copernican systems in dialogue form. The book was approved by the Roman censors before publication; Galileo believed he had complied with the 1616 warning by presenting both systems as hypotheses. The publication was widely received as a thinly-veiled defense of heliocentrism.

In 1633 he was summoned to Rome by the Roman Inquisition, charged with violating the 1616 warning. The trial lasted from April to June 1633. Galileo was found "vehemently suspected of heresy" - a category short of formal heresy but with significant legal weight - and was forced to formally recant heliocentrism. The recantation included specific theological language about the Earth's immobility that Galileo did not in fact believe.

He was sentenced to indefinite imprisonment, immediately commuted to house arrest, where he remained for the last nine years of his life. The Dialogue was placed on the Index of Forbidden Books, where it remained until 1835.

The Two New Sciences and the legacy

During house arrest at his villa in Arcetri near Florence, Galileo wrote his most scientifically substantial book: Discourses and Mathematical Demonstrations Relating to Two New Sciences (1638). The book covered the mathematics of strength of materials, the parabolic trajectory of projectiles, and the law of uniformly accelerated motion. It is the founding text of modern physics as a quantitative-mathematical discipline.

Two New Sciences could not be published in Italy because of Galileo's house arrest. The manuscript was smuggled to Leiden in the Dutch Republic and published there by the Elzevir firm. The book became standard reading for the following generation of natural philosophers in Northern Europe, including the young Newton.

Galileo went blind in his late seventies and died at Arcetri on January 8, 1642 - the year of Newton's birth. He was initially buried in a small unmarked room because of the heresy conviction; in 1737 his remains were moved to a prominent tomb at the Basilica of Santa Croce in Florence, opposite Michelangelo's tomb. The Catholic Church formally accepted the heliocentric model in 1822 and formally vacated the 1633 verdict against Galileo in 1992.

Timeline

• 1564Born in Pisa, Duchy of Tuscany.
• 1581Enters the University of Pisa for medicine.
• 1585Abandons medicine for mathematics; leaves the university without a degree.
• 1589Chair of mathematics at the University of Pisa.
• 1592Chair of mathematics at the University of Padua, in the Republic of Venice.
• 1609Builds his first telescope.
• 1610Discovers four moons of Jupiter and the phases of Venus. Publishes Sidereus Nuncius. Returns to Florence as Grand Duke's chief mathematician.
• 1616Cardinal Bellarmine warns him not to hold or defend Copernicanism.
• 1632Publishes Dialogue Concerning the Two Chief World Systems.
• 1633Inquisition trial. Forced to recant. Sentenced to house arrest.
• 1638Two New Sciences published in Leiden.
• 1642Dies at Arcetri at age 77.
• 1822Catholic Church formally accepts the heliocentric model.
• 1992Catholic Church formally vacates the 1633 verdict against Galileo.

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