This article traces the life, work, and trial of Galileo Galilei (1564-1642), the Italian mathematician, physicist, and astronomer whose advocacy of heliocentrism brought him into conflict with the Roman Catholic Church. For contested interpretations of his legacy, see the Galileo Galilei Debate page.

Galileo di Vincenzo Bonaiuti de' Galilei was born on 15 February 1564 (N.S.) in Pisa, in the Duchy of Florence, to Vincenzo Galilei, a lutenist and music theorist, and Giulia Ammannati. He was the eldest of six children. His father's work on musical tuning and string tension - empirical investigation of physical relationships - may have shaped Galileo's later methodological commitments.

In 1574 the family relocated to Florence. Galileo was educated at the Camaldolese monastery of Santa Maria di Vallombrosa, where he briefly considered entering religious life. His father redirected him toward medicine, and in 1581 Galileo enrolled at the University of Pisa to study that subject. He left without a degree in 1585, having shifted his interests to mathematics under the informal influence of Ostilio Ricci, a court mathematician of the Medici.

During the years 1585-1589 Galileo taught mathematics privately in Florence and began his investigations into motion, developing early critiques of Aristotelian mechanics. He composed an unpublished treatise, La Bilancetta (c. 1586), describing a hydrostatic balance for determining the specific gravity of objects, consistent with Archimedes' method.

In 1589 Galileo was appointed to the chair of mathematics at the University of Pisa, a comparatively low-prestige and low-paying position relative to chairs in philosophy or medicine. There he composed De Motu (On Motion), an unpublished manuscript criticizing Aristotle's account of falling bodies. The work remained in draft form, suggesting Galileo was unsatisfied with its conclusions.

In 1592 he secured a more prestigious appointment at the University of Padua, in the Republic of Venice, where he remained for eighteen years. Padua proved intellectually fertile. Galileo lectured on Euclidean geometry and the Ptolemaic system (as required by his position), while privately investigating mechanics, hydraulics, and instrumentation. He developed an early thermoscope and improved the design of the sector, a calculating instrument used in artillery and engineering.

During this period Galileo fathered three children with Marina Gamba, a Venetian woman: Virginia (1600), Livia (1601), and Vincenzo (1606). The two daughters, born illegitimate, were later placed in the convent of San Matteo in Arcetri. Virginia, who took the name Maria Celeste, maintained a close correspondence with her father until her death in 1634.

In 1604 a new star appeared in the sky - later identified as a supernova (now called Kepler's Supernova). Galileo gave public lectures arguing that the object lay beyond the sphere of the moon, contradicting Aristotelian cosmology, which held the heavens to be unchanging.

In 1609 Galileo learned of the invention of a magnifying spyglass in the Netherlands. He rapidly improved on the design, producing an instrument of roughly 20x magnification and demonstrating it to the Venetian Senate, which recognized its military and commercial value. The Senate rewarded him with a doubling of his salary and a lifetime appointment at Padua.

Galileo turned the telescope on the night sky and recorded his observations in Sidereus Nuncius (Starry Messenger), published in Venice in March 1610. The work reported four principal findings:

• The moon's surface was mountainous and irregular, not a perfect sphere as Aristotelian cosmology held.
• The Milky Way resolved into vast numbers of individual stars.
• Jupiter was accompanied by four satellites (now called the Galilean moons: Io, Europa, Ganymede, and Callisto), demonstrating that not all celestial bodies orbited the Earth.
• The number of fixed stars visible through the telescope vastly exceeded those visible to the naked eye.

The Sidereus Nuncius was widely read across Europe. Galileo sent a copy with a telescope to the court of the Holy Roman Emperor Rudolf II and corresponded with Johannes Kepler, who responded supportively. The work made Galileo famous.

In 1610 Galileo accepted an appointment as Chief Mathematician and Philosopher to Cosimo II de' Medici, Grand Duke of Tuscany, and as mathematician at the University of Pisa without teaching obligations. The title “Philosopher” - not merely “Mathematician” - was significant; it positioned Galileo to argue about the nature of the physical world, not merely to calculate.

Continuing his telescopic observations, Galileo discovered that Venus exhibited a full range of phases analogous to the moon's, a finding consistent with Venus orbiting the sun rather than the Earth, and inconsistent with the pure Ptolemaic system (though compatible with the Tychonic system, in which the planets orbited the sun while the sun orbited the Earth).

He also observed dark spots on the sun and studied their movement across the solar disk, arguing in Historia e Dimostrazioni Intorno alle Macchie Solari (Letters on Sunspots, 1613) that the spots were on or near the solar surface - implying imperfection in the heavens - and that the sun rotated on its axis. This brought him into priority dispute with the Jesuit astronomer Christoph Scheiner.

In 1613 Galileo wrote a letter to his former student Benedetto Castelli addressing the relationship between Scripture and natural philosophy. He argued that the Bible, written for a general audience, was not intended to convey astronomical truths, and that when apparent conflicts arose, Scripture should be interpreted in light of demonstrated natural knowledge. A copy of this letter circulated and came to the attention of Church authorities.

In 1615 the Dominican friar Niccolo Lorini forwarded a copy of the letter to the Roman Inquisition. A second friar, Tommaso Caccini, testified against Galileo in Rome. Galileo traveled to Rome in late 1615 to defend himself and to advocate for the Copernican system.

In February 1616, a panel of eleven consultors to the Inquisition (the qualificatori) assessed the Copernican propositions. They declared the claim that the sun is stationary at the center of the universe to be “foolish and absurd in philosophy, and formally heretical” as contrary to Scripture, and the claim that the Earth moves to be “at least erroneous in faith.” The full Inquisition and Pope Paul V did not formally adopt these qualifications as binding doctrine but acted on them.

Cardinal Robert Bellarmine, a senior theological authority, summoned Galileo and instructed him that the Copernican opinion could not be held or defended. An Inquisition minute of the same meeting records that Galileo was additionally prohibited from “teaching” heliocentrism “in any way whatever” - a stronger injunction than Bellarmine's own summary. The discrepancy between these two accounts became significant at Galileo's later trial.

Copernicus's De Revolutionibus was placed on the Index of Prohibited Books pending correction; the corrections, removing or qualifying heliocentric passages as hypothetical, were implemented in 1620.

Following the 1616 warning, Galileo did not publish on heliocentrism for several years. In 1619 a dispute arose over the nature of comets, prompted by three comets visible in 1618. The Jesuit Horatio Grassi published a treatise treating comets as real celestial objects at a fixed distance from the Earth. Galileo's associate Mario Guiducci published a response, largely reflecting Galileo's views, that Grassi disputed. Galileo replied with Il Saggiatore (The Assayer, 1623), a work widely admired for its polemical style and its programmatic statements about the mathematical character of natural philosophy. The Assayer was dedicated to the newly elected Pope Urban VIII (Maffeo Barberini), an old acquaintance of Galileo's who had previously shown him favor.

Galileo traveled to Rome in 1624, met with Urban VIII six times, and received encouragement to write on the Ptolemaic and Copernican systems - provided he treated Copernicanism as a mathematical hypothesis rather than a physical reality, and included arguments Urban himself had offered for the limits of human knowledge about the heavens.

Galileo spent the following years composing what became the Dialogo sopra i due massimi sistemi del mondo (Dialogue Concerning the Two Chief World Systems, 1632). The work took the form of a conversation over four days among three characters: Salviati (a Copernican voice, broadly representing Galileo's views), Sagredo (an intelligent layman), and Simplicio (an Aristotelian). Galileo secured the imprimatur of Florentine Church authorities and the book was published in Florence in February 1632.

The Dialogue presented the arguments for and against the two systems but was widely read - including by Urban VIII and his advisors - as decisively favoring Copernicanism. Urban was reportedly angered that arguments he had personally provided to Galileo had been placed in the mouth of Simplicio, the character consistently bested in debate. Publication was halted by order of Rome in August 1632.

In September 1632 the Inquisition summoned Galileo to Rome. He was elderly and ill and delayed the journey; he arrived in February 1633. He was not imprisoned but was housed at the Florentine Embassy and later at a residence of the Inquisition, with relative comfort by the standards of the institution.

The trial, which proceeded across four sessions in April and June 1633, focused on two principal questions: whether the Dialogue had violated the 1616 injunction, and whether Galileo had acted in good faith in obtaining the imprimatur. The Inquisition produced the minute recording the stronger 1616 injunction against teaching heliocentrism “in any way whatever.” Galileo maintained he had not violated it, arguing the Dialogue presented arguments on both sides, and that he had received the imprimatur in good faith.

In June 1633 Galileo was found “vehemently suspect of heresy” - a formal Inquisition category below formal heresy - for holding and defending the proposition that the Earth moves and the sun is fixed. He was required to abjure, which he did on 22 June 1633 in the church of Santa Maria sopra Minerva in Rome. The Dialogue was prohibited. He was sentenced to formal imprisonment, immediately commuted to house arrest.

Galileo spent the remaining years of his life under house arrest, initially at Siena in the custody of Archbishop Ascanio Piccolomini, and from December 1633 at his villa in Arcetri, near Florence.

Under house arrest at Arcetri, Galileo continued scientific work. His daughter Maria Celeste, in the nearby convent, died in April 1634. Galileo's eyesight deteriorated progressively; he was effectively blind by 1638.

He completed Discorsi e Dimostrazioni Matematiche Intorno a Due Nuove Scienze (Discourses and Mathematical Demonstrations Relating to Two New Sciences, 1638), a work treating the strength of materials and the kinematics of motion. The book was published by the Elsevier press in Leiden, in the Dutch Republic, beyond the reach of the Roman Index. The Two New Sciences established the mathematical analysis of uniformly accelerated motion and projectile trajectories that later formed the mechanical foundation on which Newton built.

In his final years Galileo dictated observations and received visitors, including the young John Milton, who described the visit in Areopagitica (1644). The Inquisition refused Galileo's repeated requests to be allowed to leave Arcetri to seek medical treatment.

Galileo died at Arcetri on 8 January 1642 (N.S.). Grand Duke Ferdinando II of Tuscany wished to erect a funeral monument, but Pope Urban VIII indicated that to honor a man condemned by the Inquisition would give scandal. Galileo was buried quietly in a small chapel at Santa Croce in Florence. In 1737 his remains were moved to a monumental tomb in the main basilica, where they remain.

The prohibition on Copernican works was gradually relaxed. De Revolutionibus was removed from the Index in 1758. The Dialogue was not removed until 1835.

In 1992 Pope John Paul II, addressing the Pontifical Academy of Sciences, stated that the theologians who condemned Galileo had failed to distinguish faith from a particular cosmological interpretation, and acknowledged that Galileo had suffered. The statement is commonly characterized as a Church rehabilitation of Galileo, though its precise theological scope has been debated.

Several aspects of the Galileo affair remain contested among historians. See Galileo Galilei Debate for full treatment of the following disputes:

• Whether the stronger 1616 injunction recorded in the Inquisition minute - forbidding Galileo from treating heliocentrism “in any way whatever” - was valid, properly served, or fabricated.
• Whether Urban VIII's anger at the Dialogue was primarily theological, personal, or political.
• Whether Galileo's methodology constituted a recognizable form of modern scientific empiricism or reflected a more complex early modern hybrid of mathematical, philosophical, and rhetorical traditions.

1. Galilei, Galileo. Sidereus Nuncius. Venice: Thomas Baglioni, 1610. Translated by Albert Van Helden. University of Chicago Press, 1989.
2. Galilei, Galileo. Dialogo sopra i due massimi sistemi del mondo. Florence: Landini, 1632. Translated by Stillman Drake as Dialogue Concerning the Two Chief World Systems. University of California Press, 1953.
3. Galilei, Galileo. Discorsi e Dimostrazioni Matematiche Intorno a Due Nuove Scienze. Leiden: Elsevier, 1638. Translated by Stillman Drake as Two New Sciences. University of Wisconsin Press, 1974.
4. Finocchiaro, Maurice A. The Galileo Affair: A Documentary History. University of California Press, 1989. Primary documents including the 1616 Inquisition minutes, the 1633 trial proceedings, and Galileo's abjuration.
5. Drake, Stillman. Galileo at Work: His Scientific Biography. University of Chicago Press, 1978.
6. Fantoli, Annibale. Galileo: For Copernicanism and for the Church. 3rd ed. Vatican Observatory Publications, 2003.
7. Heilbron, J. L. Galileo. Oxford University Press, 2010.
8. Blackwell, Richard J. Galileo, Bellarmine, and the Bible. University of Notre Dame Press, 1991.
9. Westfall, Richard S. “The Trial of Galileo: Revisionism Revisited.” Journal for the History of Astronomy 20 (1989): 1-12.
10. John Paul II. Address to the Pontifical Academy of Sciences, 31 October 1992. Vatican City. Published in L'Osservatore Romano, 4 November 1992.