The conflict surrounding Galileo Galilei began around 1610 and culminated with his trial and condemnation by the Roman Catholic Inquisition in 1633. Galileo was prosecuted for supporting heliocentrism, the astronomical model in which the Earth and planets revolve around the Sun at the center of the universe. This view contradicted the prevailing geocentric theory based on the theories of Aristotle and Ptolemy that the Catholic Church commonly accepted. Ultimately, the affair surrounded the tension between accepting truth from an established authority and asserting new truth based on personal and observed experience.
In 1610, Galileo published his Sidereus Nuncius (Starry Messenger), describing the observations that he had made with his new, much stronger telescope by which he supported the heliocentric theory of Nicolaus Copernicus published in De revolutionibus orbium coelestium in 1543. Not long after this publication, Galileo engaged with numerous religious scientists who challenged Galileo's assertions. Although he had a supportive following of scientists—a group that included his student, Benedetto Castelli—by 1614, Tommaso Caccini, a Dominican friar, used the pulpit to publically attack and denounce Galileo's conclusions, calling for official action from the church.
During this period, Galileo hoped to explain the contradiction between the authority of the scriptures and his scientific observations by suggesting that the scriptures offer a reliable insight into spiritual and moral matters rather than physical ones. This, however, only fanned the flames and led to an official report to the Inquisition in 1615. In an effort to plead his case, and against the counsel of friends, Galileo traveled to Rome in the same year, but the trip was unfruitful. In 1616, the Inquisition declared heliocentrism to be "formally heretical."
Throughout this period and for the rest of his life, Galileo remained in contact with several confidants, friends, and opponents through letters. One of his contacts and former pupils, Giovanni Ciampoli, served a particularly important role. Highly educated and a celebrated Latinist, Ciampoli held positions within the church hierarchy that regularly gave him access to a number of the key players in the growing affair. A natural diplomat, he sought to persuade and assuage through reason and dialogue on both sides. To Galileo, he revealed the internal discussions within the Church about his work, counseling him on how to retain majority favor by moderating his position. To Church authorities, like the later Pope Urban VIII, he assured Galileo's adherence to the requests of the Church.
Once Galileo arrived in Rome, the dispute was adjudicated by Cardinal Robert Bellarmine, a respected Catholic theologian who had experience with questions of heliocentrism. Conscious of the optics and the potential blow to the Church's authority, he strongly advised that heliocentrism be considered merely a hypothetical phenomenon rather than an observable reality. In this light, he had no deep concerns with the model generally but saw any deviation as utterly impermissible. The final result of the trial was a reprimand to Galileo that required his complete submission to the official geocentric stance of the Church. In addition, books supporting the Copernican system were officially banned in 1618.
Despite this ban, and partially due to the ascension of Pope Urban VIII in 1623, Galileo published his Dialogue Concerning the Two Chief World Systems in 1632, which defended heliocentrism and was immensely popular. The treatise consisted of a dialogue between a Copernican scientist (Salviati), a sympathetic scientist (Sagredo), and an obstinate Aristotelian who blindly and foolishly supported geocentricity (Simplicio). Throughout the work, Simplicio's stock arguments are repeatedly refuted and ridiculed by the other two. As a result, the work's heliocentric bias was unmistakable and generally depicted geocentrists as ignorant simpletons (an intentional play on the name Simiplicio). In an effort to temper the work's bias, Pope Urban VIII demanded his views be represented; Galileo put them in the voice of Simplicio, an obvious affront to the Pope's authority. As a result, the book was banned and submitted to the Inquisition for examination.
The Roman Inquisition officially tried Galileo in 1633 for openly supporting heliocentrism and heresy. Though some of the inquisitors were sympathetic to Galileo—indeed, the Grand Inquisitor, Cardinal Francesco Barberini, advocated for mere revisions to the treatise and refused to sign the sentencing decree—the panel found him "vehemently suspect of heresy," and sentenced him to house arrest, where he remained until his death in 1642.