The Galileo affair was an early 17th century political, religious, and scientific controversy regarding the astronomer Galileo Galilei's defence of heliocentrism, the idea that the Earth revolves around the Sun. It pitted supporters and opponents of Galileo within both the Catholic Church and academia against each other through two phases: an interrogation and condemnation of Galileo's ideas by a panel of the Roman Inquisition in 1616, and a second trial in 1632 which led to Galileo's house arrest and a ban on his books.
In 1610, Galileo published his Sidereus Nuncius (Starry Messenger) describing the observations that he had made with his new, much stronger telescope, amongst them the Galilean moons of Jupiter. With these observations and additional observations that followed, such as the phases of Venus, he promoted the heliocentric theory of Nicolaus Copernicus published in De revolutionibus orbium coelestium in 1543. Galileo's opinions were met with opposition within the Catholic Church, and in 1616 the Inquisition declared heliocentrism to be both scientifically indefensible and heretical. Galileo went on to propose a theory of tides in 1616, and of comets in 1619; he argued (incorrectly) that the tides were evidence for the motion of the Earth.
In 1632, Galileo published his Dialogue Concerning the Two Chief World Systems, which defended heliocentrism while describing geocentrists as "simpletons". Responding to mounting controversy, the Roman Inquisition tried Galileo in 1633 and found him "vehemently suspect of heresy", sentencing him to house arrest. At this point, heliocentric books were banned and Galileo was ordered to abstain from holding, teaching or defending heliocentric ideas after the trial.
The affair was complex, with Pope Urban VIII originally being a patron and supporter of Galileo before turning against him. Urban initially gave Galileo permission to publish on the Copernican theory so long as he treated it as a hypothesis, but after the publication of the Dialogue in 1632, the patronage was broken off. Historians of science have since corrected numerous false interpretations of the affair.
Initial controversies
Galileo began his telescopic observations in the later part of 1609, and by March 1610 was able to publish a small book, The Starry Messenger (Sidereus Nuncius), describing some of his discoveries: mountains on the Moon, lesser moons in orbit around Jupiter, and the resolution of what had been thought to be very cloudy masses in the sky (nebulae) into collections of stars too faint to see individually without a telescope. Other observations followed, including the phases of Venus and the existence of sunspots.
Galileo's contributions caused difficulties for theologians and natural philosophers of the time, as they contradicted scientific and philosophical ideas based on those of Aristotle and Ptolemy and closely associated with the Catholic Church. In particular, Galileo's observations of the phases of Venus, which showed it to circle the Sun, and the observation of moons orbiting Jupiter, contradicted the geocentric model of Ptolemy, which was backed and accepted by the Roman Catholic Church, and supported the Copernican model advanced by Galileo.
Jesuit astronomers, experts both in Church teachings, science, and in natural philosophy, were at first skeptical and hostile to the new ideas; however, within a year or two the availability of good telescopes enabled them to repeat the observations. In 1611, Galileo visited the Collegium Romanum in Rome, where the Jesuit astronomers by that time had repeated his observations. Christoph Grienberger, one of the Jesuit scholars on the faculty, sympathized with Galileo's theories, but was asked to defend the Aristotelian viewpoint by Claudio Acquaviva, the Father General of the Jesuits. Not all of Galileo's claims were completely accepted: Christopher Clavius, the most distinguished astronomer of his age, never was reconciled to the idea of mountains on the Moon, and outside the collegium many still disputed the reality of the observations. In a letter to Kepler of August 1610, Galileo complained that some of the philosophers who opposed his discoveries had refused even to look through a telescope:
My dear Kepler, I wish that we might laugh at the remarkable stupidity of the common herd. What do you have to say about the principal philosophers of this academy who are filled with the stubbornness of an asp and do not want to look at either the planets, the moon or the telescope, even though I have freely and deliberately offered them the opportunity a thousand times? Truly, just as the asp stops its ears, so do these philosophers shut their eyes to the light of truth.
In 1611, the same year that Galileo visited the Collegium Romanum, his theories first came to the attention of the Roman Inquisition. A commission of cardinals working with the inquisition made inquiries into Galileo's activities, and asked the city of Padua if he had any connections to Cesare Cremonini, a professor at the university of Padua who had been charged with heresy by the Inquisition. These inquiries marked the first time Galileo's name was brought before the inquisition.
Geocentrists who did verify and accept Galileo's findings had an alternative to Ptolemy's model in an alternative geocentric (or "geo-heliocentric") model proposed some decades earlier by Tycho Brahe – a model in which, for example, Venus circled the Sun. Tycho argued that the distance to the stars in the Copernican system would have to be 700 times greater than the distance from the Sun to Saturn. (The nearest star other than the Sun, Proxima Centauri, is over 28,000 times the distance from the Sun to Saturn.) Moreover, the only way the stars could be so distant and still appear the sizes they do in the sky would be if even average stars were gigantic – at least as big as the orbit of the Earth, and of course vastly larger than the Sun. (See the articles on the Tychonic System and Stellar parallax.)
Galileo became involved in a dispute over priority in the discovery of sunspots with Christoph Scheiner, a Jesuit. This became a bitter lifelong feud. Neither of them, however, was the first to recognise sunspots – the Chinese had already been familiar with them for centuries.
At this time, Galileo also engaged in a dispute over the reasons that objects float or sink in water, siding with Archimedes against Aristotle. The debate was unfriendly, and Galileo's blunt and sometimes sarcastic style, though not extraordinary in academic debates of the time, made him enemies such as the Florentine nobleman Lodovico delle Colombe. During this controversy one of Galileo's friends, the painter Lodovico Cardi da Cigoli, informed him that a group of malicious opponents led by delle Colombe, which Cigoli subsequently referred to derisively as "the Pigeon league", was plotting to cause him trouble over the motion of the Earth, or anything else that would serve the purpose. According to Cigoli, one of the plotters asked a priest to denounce Galileo's views from the pulpit, but the latter refused. Nevertheless, three years later another priest, Tommaso Caccini, a member of the "Pigeon League", did in fact do precisely that, as described below.
Bible argument
In the Catholic world prior to Galileo's conflict with the Church, the majority of educated people subscribed to the Aristotelian geocentric view that the Earth was the centre of the universe and that all heavenly bodies revolved around the Earth, though Copernican theories were used to reform the calendar in 1582.
Geostaticism agreed with a literal interpretation of Scripture in several places, such as 1 Chronicles 16:30, Psalm 93:1, Psalm 96:10, Psalm 104:5, Ecclesiastes 1:5 (but see varied interpretations of Job 26:7). Heliocentrism, the theory that the Earth was a planet, which along with all the others revolved around the Sun, contradicted both geocentrism and the prevailing theological support of the theory.
One of the first suggestions of heresy that Galileo had to deal with came in 1613 from a professor of philosophy, poet and specialist in Greek literature, Cosimo Boscaglia. In conversation with Galileo's patron Cosimo II de' Medici and Cosimo's mother Christina of Lorraine, Boscaglia said that the telescopic discoveries were valid, but that the motion of the Earth was obviously contrary to Scripture:
Dr. Boscaglia had talked to Madame [Christina] for a while, and though he conceded all the things you have discovered in the sky, he said that the motion of the Earth was incredible and could not be, particularly since Holy Scripture obviously was contrary to such motion.
Galileo was defended on the spot by his former student Benedetto Castelli, now a professor of mathematics and Benedictine abbot. The exchange having been reported to Galileo by Castelli, Galileo decided to write a letter to Castelli, expounding his views on what he considered the most appropriate way of treating scriptural passages which made assertions about natural phenomena. Later, in 1615, he expanded this into his much longer Letter to the Grand Duchess Christina.
Tommaso Caccini, a Dominican friar, appears to have made the first dangerous attack on Galileo. Preaching a sermon in Florence at the end of 1614, he denounced Galileo, his associates, and mathematicians in general (a category that included astronomers). The biblical text for the sermon on that day was Joshua 10, in which Joshua makes the Sun stand still; this was the story that Castelli had to interpret for the Medici family the year before. It is said, though it is not verifiable, that Caccini also used the passage from Acts 1:11, "Ye men of Galilee, why stand ye gazing up into heaven?".
In late 1614 or early 1615, one of Caccini's fellow Dominicans, Niccolò Lorini, acquired a copy of Galileo's letter to Castelli. Lorini and other Dominicans at the Convent of San Marco considered the letter of doubtful orthodoxy, in part because it may have violated the decrees of the Council of Trent:
...to check unbridled spirits, [the Holy Council] decrees that no one relying on his own judgement shall, in matters of faith and morals pertaining to the edification of Christian doctrine, distorting the Scriptures in accordance with his own conceptions, presume to interpret them contrary to that sense which the holy mother Church... has held or holds...
— Decree of the Council of Trent (1545–1563). Quoted in Langford, 1992.
Lorini and his colleagues decided to bring Galileo's letter to the attention of the Inquisition. In February 1615, Lorini accordingly sent a copy to the Secretary of the Inquisition, Cardinal Paolo Emilio Sfondrati, with a covering letter critical of Galileo's supporters:
All our Fathers of the devout Convent of St. Mark feel that the letter contains many statements which seem presumptuous or suspect, as when it states that the words of Holy Scripture do not mean what they say; that in discussions about natural phenomena the authority of Scripture should rank last... . [The followers of Galileo] were taking it upon themselves to expound the Holy Scripture according to their private lights and in a manner different from the common interpretation of the Fathers of the Church...
— Letter from Lorini to Cardinal Sfrondato, Inquisitor in Rome, 1615. Quoted in Langford, 1992
On March 19, Caccini arrived at the Inquisition's offices in Rome to denounce Galileo for his Copernicanism and various other alleged heresies supposedly being spread by his pupils.
Galileo soon heard reports that Lorini had obtained a copy of his letter to Castelli and was claiming that it contained many heresies. He also heard that Caccini had gone to Rome and suspected him of trying to stir up trouble with Lorini's copy of the letter. As 1615 wore on he became more concerned, and eventually determined to go to Rome as soon as his health permitted, which it did at the end of the year. By presenting his case there, he hoped to clear his name of any suspicion of heresy, and to persuade the Church authorities not to suppress heliocentric ideas.
In going to Rome Galileo was acting against the advice of friends and allies, and of the Tuscan ambassador to Rome, Piero Guicciardini.
Bellarmine
Cardinal Robert Bellarmine, one of the most respected Catholic theologians of the time, was called on to adjudicate the dispute between Galileo and his opponents. The question of heliocentrism had first been raised with Cardinal Bellarmine, in the case of Paolo Antonio Foscarini, a Carmelite father; Foscarini had published a book, Lettera ... sopra l'opinione ... del Copernico, which attempted to reconcile Copernicus with the biblical passages that seemed to be in contradiction. Bellarmine at first expressed the opinion that Copernicus's book would not be banned, but would at most require some editing so as to present the theory purely as a calculating device for "saving the appearances" (i.e. preserving the observable evidence).