This article traces the history of the Galileo Affair - the sequence of events surrounding Galileo Galilei's conflict with the Roman Catholic Church over heliocentrism, from his early astronomical work through his trial, condemnation, and the affair's long aftermath. For contested interpretations of the affair's meaning and causes, see the Controversies section below. For assessments of the affair's broader significance, see Scientific Martyrdom Viewpoint and Revisionist Viewpoint.

In 1543, Nicolaus Copernicus published De revolutionibus orbium coelestium, proposing that the Earth orbited the Sun rather than the reverse. The Church placed no immediate condemnation on the work, and it circulated among scholars for decades. The dominant cosmological model remained geocentrism, grounded in Aristotelian physics and the Ptolemaic system, which had provided the framework for natural philosophy and biblical interpretation since the medieval synthesis of Thomas Aquinas.

Galileo Galilei was born in Pisa on 15 February 1564 (N.S.). He studied medicine at the University of Pisa before turning to mathematics and natural philosophy. By 1589 he held the chair of mathematics at Pisa, and by 1592 he had moved to the University of Padua, where he remained for eighteen years and conducted much of his foundational work in mechanics and kinematics.

In 1609, Galileo constructed an improved telescope - drawing on reports of Dutch instruments - and turned it toward the sky. His observations yielded results difficult to reconcile with geocentrism: mountains and craters on the Moon (implying it was not a perfect celestial sphere), four moons orbiting Jupiter (demonstrating that not all celestial bodies revolved around Earth), and the phases of Venus (consistent with Venus orbiting the Sun). He published these findings in Sidereus Nuncius (Starry Messenger) in March 1610.¹⁾

Galileo was appointed chief mathematician and philosopher to the Medici court in Florence in 1610. His reputation grew rapidly across Europe. In 1611 he visited Rome, demonstrated his telescope to members of the Collegio Romano, and was received by Pope Paul V. The Jesuits of the Collegio confirmed his astronomical observations, though interpretive disagreements persisted.

Between 1612 and 1613 Galileo engaged in a priority dispute with the Jesuit astronomer Christoph Scheiner over sunspots. His Letters on Sunspots (1613) included his first public endorsement of Copernican heliocentrism.

In 1614, Florentine Dominican friar Tommaso Caccini preached a sermon criticizing Galileo and mathematicians generally as enemies of religion. In February 1615, fellow Dominican Niccolo Lorini forwarded a copy of one of Galileo's letters to the Inquisition in Rome, alleging theological impropriety.

Galileo traveled to Rome in late 1615 to defend himself and to argue that heliocentrism was compatible with scripture. Cardinal Robert Bellarmine, the leading theologian of the Curia, was centrally involved in assessing the matter. In February 1616, a committee of consultors to the Inquisition (the qualificatori) declared heliocentrism formally heretical and the idea that the Earth moved “erroneous in faith.”

On 5 March 1616, the Congregation of the Index issued a decree suspending Copernicus's De revolutionibus pending correction and condemning two other heliocentric works outright. Galileo himself was not condemned publicly. According to the official record, Bellarmine privately instructed Galileo to abandon and not hold, teach, or defend the Copernican view. A separate document in the Inquisition file - whose authenticity became a matter of significant later dispute - states that Galileo was additionally forbidden to discuss heliocentrism in any way. Bellarmine gave Galileo a signed certificate stating only that he had been notified of the decree.²⁾

Following the 1616 decree, Galileo refrained from publishing on heliocentrism for several years. He continued work on other projects, including his study of comets, which drew him into controversy with Jesuit astronomer Orazio Grassi. Galileo's response, Il Saggiatore (The Assayer, 1623), was a work of methodological polemic arguing for a mathematical approach to natural philosophy. It was dedicated to the newly elected Pope Urban VIII - Maffeo Barberini - a Florentine nobleman who had previously been an admirer and patron of Galileo.

Urban VIII received Galileo in a series of audiences in 1624 and appeared to encourage him to write about the competing world systems, provided he treated the matter hypothetically and did not claim to demonstrate heliocentrism as physically true.

Galileo spent the following years composing the Dialogo sopra i due massimi sistemi del mondo (Dialogue Concerning the Two Chief World Systems). The work took the form of a conversation among three characters over four days: Salviati (a Copernican), Sagredo (an intelligent layman), and Simplicio (an Aristotelian geocentrist). Though nominally presenting both sides, the Dialogue was widely read as a sustained argument in favor of Copernicus. The Inquisition's imprimatur was obtained in Rome; a second imprimatur was issued in Florence, where the work was published in February 1632.³⁾

Within months, the book attracted hostile attention in Rome. Urban VIII reportedly felt that Simplicio - the least persuasive voice in the dialogue - had been given arguments the Pope himself had suggested, making him the object of ridicule. The Roman imprimatur was suspended in August 1632 and distribution of the book was halted. Galileo was summoned to Rome.

Galileo, then 68 and in poor health, arrived in Rome in February 1633 after lengthy delays. He was questioned by the Inquisition in April. The 1616 document forbidding him to discuss heliocentrism in any form was central to the proceedings; Galileo produced Bellarmine's certificate, which contained no such absolute prohibition, and claimed he had not been given so strict a command. The Inquisitors concluded he had violated the 1616 injunction regardless.

On 22 June 1633, Galileo was required to kneel before the full Congregation of the Holy Office and abjure his errors. He was condemned for being “vehemently suspect of heresy” - a canonical category below formal heresy - for holding and defending the proposition that the Earth moves. He was sentenced to formal imprisonment at the pleasure of the Inquisition and ordered to recite penitential psalms weekly for three years. His Dialogue was banned.

The sentence of imprisonment was commuted almost immediately to house arrest. Galileo was first confined to the residence of Archbishop Ascanio Piccolomini in Siena, who treated him hospitably, then permitted to return to his villa at Arcetri, near Florence, in December 1633.

Under house arrest at Arcetri, Galileo continued to work. He dictated and corresponded with students and foreign scholars, including correspondence with figures in France, the Dutch Republic, and England. He completed Discorsi e dimostrazioni matematiche intorno a due nuove scienze (Discourses and Mathematical Demonstrations Relating to Two New Sciences), a systematic treatment of kinematics and the strength of materials, representing in some respects his most durable scientific contribution. Lacking an Italian publisher willing to print it (the Inquisition's prohibition applied throughout Catholic territories), the work was published in Leiden in 1638 by the Elsevier firm.⁴⁾

Galileo went blind in 1638. He died at Arcetri on 8 January 1642 (N.S.). He was initially buried without ceremony; a monument planned by his admirers was blocked by the Inquisition. His remains were moved to the main body of Santa Croce in Florence in 1737, where a monument was erected.

The condemnation stood formally for 150 years. The Dialogue remained on the Index of Forbidden Books until 1835. In the intervening period, heliocentrism gradually became accepted within Catholic institutions. In 1757, under Benedict XIV, the Congregation of the Index removed the general prohibition on works teaching heliocentrism as a physical fact, though individual condemned titles - including Galileo's Dialogue - remained listed until 1835.⁵⁾

In 1979, Pope John Paul II called for a re-examination of the case. A commission was established in 1981 under Cardinal Gabriel-Marie Garrone and later Cardinal Paul Poupard. In October 1992, John Paul II addressed the Pontifical Academy of Sciences and acknowledged that errors had been made by the theologians who condemned Galileo, attributing the error to a failure to distinguish faith from a particular cosmological interpretation. The statement stopped short of overturning the condemnation formally or declaring Galileo's persecutors to have acted in bad faith.⁶⁾

Whether the 1616 document placing an absolute prohibition on Galileo was authentic, planted, or the product of a procedural irregularity remains disputed among historians; see Galileo Affair Debate.⁷⁾

Whether Urban VIII's personal anger at perceived mockery, rather than theological principle, was the primary driver of the 1632-33 proceedings is a contested causal question; see Galileo Affair Debate.⁸⁾

Whether the affair represents a structural conflict between science and religion as institutions, or an anomalous political episode within a largely cooperative relationship, is disputed; some historians argue the former, others the latter; see Conflict Thesis Viewpoint and Revisionist Viewpoint.⁹⁾

• Maurice A. Finocchiaro, The Galileo Affair: A Documentary History, Berkeley: University of California Press, 1989.
• Giorgio de Santillana, The Crime of Galileo, Chicago: University of Chicago Press, 1955.
• Stillman Drake, Galileo at Work: His Scientific Biography, Chicago: University of Chicago Press, 1978.
• John L. Heilbron, Galileo, Oxford: Oxford University Press, 2010.
• Annibale Fantoli, Galileo: For Copernicanism and for the Church, 3rd ed., Vatican Observatory Publications, 2003.
• Rivka Feldhay, Galileo and the Church: Political Inquisition or Critical Dialogue?, Cambridge: Cambridge University Press, 1995.
• Pietro Redondi, Galileo: Heretic, trans. Raymond Rosenthal, Princeton: Princeton University Press, 1987.