The struggle between the Roman Catholic Church and 17th-century Italian physicist and astronomer Galileo has become symbolic of the clash between authority and intellectual freedom, but Galileo himself did not foresee any conflict. Using one of the first telescopes, Galileo found evidence to support the Copernican theory that the Earth and the other planets revolved around the Sun. Galileo believed that his scientific findings fell far outside the theological realm. Author Stillman Drake explores Galileo’s shock and disbelief as his disagreement with the church escalated.

From Galileo

Conflicts with astronomers and theologians

By Stillman Drake

While Galileo was writing his book on floating in water, a book about sunspots was published pseudonymously by the German Jesuit Christopher Scheiner. Forbidden by his superior to risk discredit to his order, he wrote in the form of letters to Mark Welser of Augsburg, who had previously sent Galileo the German attack against lunar mountains. Welser, a banker to the Jesuits who was soon made a member of the Lincean Academy, printed Scheiner's letters over the name of 'Apelles' and sent them to Galileo for comment, remarking that he did not suppose sunspots were anything new to the Italian.

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Galileo received this material on a visit to Florence to place his new book in the hands of a printer. His former pupil, a Benedictine abbot named Benedetto Castelli, had arrived to assist him and was asked to see the book through the press and to make daily observations of sunspots as carefully as possible. Castelli recorded those so accurately that the daily movement of a spot could be measured, enabling Galileo to prove that the spots must be on the sun's surface and that the sun rotated about once a month. Scheiner had concluded that what were called sunspots were really tiny planets revolving around the earth or the sun and obstructing our vision. He wrote additional letters which Welser printed, also answered by Galileo, whose three Letters on Sunspots were published at Rome in 1613 under the auspices of the Lincean Academy. The Linceans insisted on adding a preface that Galileo disliked, asserting his priority of discovery.

Galileo had shown sunspots to others while at Rome in 1611, and later the Jesuit mathematician Paul Guldin said that at that time he had sent word of this to Scheiner. Even before Scheiner's book there had been another on sunspots by Johann Fabricius. Yet Scheiner was angered by the priority assertion in the Roman preface, as were many other Jesuits. It started a long and bitter resentment in him that was eventually to have serious consequences for Galileo.

Both Scheiner and Galileo argued many astronomical and other issues than just sunspots. Galileo took the position that all celestial phenomena should be interpreted in terms of terrestrial analogies, against Aristotle's basic postulate of essential differences. He also asserted that the essences of things cannot be known and that science is concerned only with properties of things and observed events. This amounted to a declaration of independence of science from philosophy.

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For the first (and only) time, Galileo came out in print unequivocally in favour of the Copernican astronomy. He had avoided that in the Starry Messenger, and even after discovering the phases of Venus he had no astronomical evidence against the Tychonic system [a compromise between the Copernican and Ptolemaic systems put forward by Danish astronomer Tycho Brahe]. It was only in an appendix to his Sunspot Letters that Galileo mentioned what had been for him the clinching fact. This was the discovery of eclipses of Jupiter's satellites and of a simple means of predicting such events. The importance of this as scientific evidence for Copernicanism justifies a brief explanation, especially because Galileo said little of it.

In order to predict positions of the satellites, it was necessary to introduce a correction for the earth's motion—or the sun's motion, in the old astronomy. This step had a clear meaning in the Copernican system, for omission of it was equivalent to shifting the observer to the sun. That meaning gave Galileo immediately the key to predictions of satellite eclipses when, in 1612 he first realised that such eclipses took place. Now, from a purely mathematical standpoint, the Tychonic system is identical with the Copernican. Yet in 1614, when Simon Mayr claimed priority of discovery of Jupiter's satellites and published tables of their motions a bit more accurate than those used by Galileo in 1612, Mayr admitted that he had never seen a satellite eclipse and offered no way to predict them. The same correction mentioned above was used in his tables, but to this Tychonian astronomer it had no meaning beyond that of an empirical adjustment of the 'sun's motion'. Mayr simply could not imagine himself as moving around the sun, or think in terms of what would be seen from the sun. Astronomers who would not regard the earth's motions as real were under a great handicap in understanding the motions they observed, regardless of 'mathematical equivalence'.

Outside his appendix to the Sunspot Letters Galileo published nothing about satellite eclipses, for two reasons. The first was that he hoped to sell a certain scheme of his for determining longitudes, so he kept his method of calculation secret. The second reason was that Galileo was prevented from ever again treating the earth's motions as real [because of Christian theologians’ objections]…and so highly technical a matter as satellite eclipses could not be explained to lay readers of his Dialogue—let alone their explanation assuming the earth to be fixed.

…It is now often said that incontrovertible evidence for the earth's annual motion was not found until early in the nineteenth century, when the high precision of astronomical instruments first permitted detection of parallax in certain fixed stars. Direct evidence of the earth's daily rotation is similarly said to have awaited the Foucault pendulum in the mid-nineteenth century. Such statements are titillating, but they misrepresent the grounds of scientific conviction. No scientist in the nineteenth century had lingering doubts which he gave up at the time of those events. The issue of the earth's motions had been effectively settled for scientists by Newton's law of universal gravitation, which linked innumerable astronomical measurements and the occurrence of tides to the existence of the earth's two motions.

Someone might reply that granting this, it is still true that Galileo had no incontrovertible evidence for the Copernican system, since he died before Newton was born. Quite so, and indeed Galileo refrained from asserting that he had incontrovertible evidence. What he did have was a preponderance of evidence that linked together such things as the phases of Venus, satellite eclipses, planetary speeds and distances from the sun, and the existence of tides; that made these compatible with his terrestrial physics, and that showed Aristotelian cosmology and physics to be mistaken on various matters. There were still a great many remaining puzzles, but in that respect his situation was like Newton's, or for that matter like ours today. Nothing in science is immune from further discoveries. Science proceeds on preponderance of evidence, not on finality. The preponderance of evidence known to Galileo indicated that the earth's motions were actual, and Galileo's belief in them was scientific, even though some of the evidence he relied on was later found to be scientifically inadequate.

With this background we can see why Galileo felt compelled to do all he could to prevent a mistake by the Church that would eventually tend to discredit its wisdom. The difficulty in this was that even persons expert in astronomy did not yet understand the weight of the evidence known to Galileo. It was impossible to explain that to theologians expert not in astronomy and physics but only in their erroneous Aristotelian counterparts, so any attempt to do so would be a waste of time. On the other hand, the founding fathers of Christianity had wisely separated faith from science, precisely to avoid crises of the same kind at the elementary levels that had already existed in their day. Galileo accordingly appealed to their authority in his zeal to save his Church from the very mistake that it actually made in 1616.…

It should also be kept in mind that throughout the arguments of 1613-16, Galileo's purpose was not to prove one side of a scientific question, but to separate purely scientific questions from matters of faith in order that rational discussion might remain free. Many writers say that he wanted the Church to adopt the Copernican system, which is not only false but misses the whole point of the actual debate. Galileo did not want the Church to adopt either side of any scientific question and suppress the other as a matter of faith; if the Church were to suppress anything, he wrote, it should forbid any introduction of scriptural authority into debates that could be settled without it, by experience and reason alone.…It was also the separation urged long before by St. Augustine, who pointed out that a heretic might be better informed than a Christian in astronomical matters; that Christians should not spend time studying astronomy that could be better spent in pious devotions; and that to stake Christianity on such matters would be improper.…

…A professor of philosophy whose speciality was Platonism took occasion to tell Christina [de’ Medici, grand duchess of Florence] that Galileo, who was not present, was wrong to say that the earth moved, because that contradicted the Bible.

After the breakfast Christina detained Castelli to speak as a theologian on this point, and in particular to discuss the biblical miracle of Joshua in which the sun was said to have been stopped. Castelli answered all the questions that were raised, and maintained that purely scientific matters should be decided on their own merits, from which the literal or metaphorical status of scriptural passages could then be determined. He sent to Galileo at Florence an account of the incident, upon which Galileo proceeded to write out his long Letter to Castelli in which he approved everything he had said and added more. This was the first letter in which Galileo argued that freedom of inquiry should be allowed by theologians in all matters that could be decided by appeal to 'sensate experiences and necessary demonstrations' alone. That phrase restricted the scope of science to things unrelated to salvation of the soul. No contradiction could exist between Nature, as the executrix [vehicle] of God's will, and the Bible as the repository of God's word. The bible often spoke metaphorically and always for the easy understanding of ordinary people. Its words are subject to interpretation, which should be left to theologians, while Nature speaks inexorably for herself.

One or two previous incidents involving the Bible had occurred, but this was the first serious one.…Now a professor of philosophy had spoken against Galileo's views to his employers, and Galileo took action.

Years of experience showed him that the best strategy was to separate questions of fact from matters of opinion. Thus when the location of the new star was in question, Galileo determined it by ordinary techniques of measurement. Cremonini appealed to its substance, which, being perishable, could not be celestial. Galileo treated that as an opinion contradicted by fact. In the matter of floating bodies, observational facts contradicted opinions about causes. Concerning sunspots, facts of measurement destroyed the opinion that they were far from the sun's surface.

Observations and measurements sufficiently defined the realm of scientific facts as the highest court of appeal, so far as Galileo was concerned. Whether such facts were or were not recognised by Aristotelian philosophers was a matter of no interest to him. A fairly thoroughgoing [complete] revision of their principles would be necessary to accommodate the facts, but that could be managed if anyone wanted to go to the trouble of working it out. If not, then science would proceed independently of philosophical opinions.

The Bible was quite a different matter. No contradiction of Holy Scripture could be permitted in science, any more than in other things. Fortunately the apparent contradictions between astronomy and the Bible were few in number, since the Bible did not attempt to teach astronomy as did the philosophers. Biblical interpretation was a matter of opinion—of expert theological opinion, which should accordingly be governed by astronomical and physical facts. Science could not proceed independently of expert theological opinion, but agreement between them could easily be assured.

Such was Galileo's position, for which the most solid precedents existed. The early Church Fathers had recommended against any linkage of Christian faith with matters that were irrelevant to salvation, especially matters which required detailed study that would interfere with time better spent in devout meditations. The Council of Trent had fixed upon unanimous agreement of the Church Fathers as a basis of biblical interpretation, and none of them had advised making worldly knowledge depend on faith. Hence Galileo felt secure in his position.…

It is apparent that theologians were not seeking some pretext to censure Galileo, let alone to intervene in scientific issues. The problem was rather with intriguing personal foes of Galileo, and an ambitious priest [who condemned mathematicians in general and Galileo in particular in December 1614], than with responsible Church officials. In mid-1615 Galileo expanded his Letter to Castelli into the much lengthier Letter to Christina, citing St. Augustine and other authorities whom he was certain that the Church would officially follow if, as he now expected, action were to be taken on the question of banning Copernican books.…[Cardinal]Bellarmine in particular had said that there was no thought of banning Copernicus's book, but at most of removing some passages in it and leaving his astronomical hypothesis intact.…

Bellarmine's recommendation could have been accepted by Galileo without greatly impeding the progress of astronomy, and many scholars believe that Galileo should have accepted it, not only for his own safety but as sound science. His refusal to do so is ordinarily taken as evidence of excessive Copernican zeal …[but] what Galileo did next was to set out at great length his arguments that the Catholic faith should not in any way depend on facts of science. There could be no contradiction between the Bible and science, and what should be done was to make that clear. The Bible should not even be construed as favouring, let alone adopting, one astronomy against another, or as requiring scriptural reinterpretation to accommodate anything that science might ever prove. As Galileo wrote in his old age, in his own copy of the later Dialogue:

Of course Galileo's Letter to Christina was not addressed directly to theologians, though it was intended for their eyes. It would have been improper for Galileo, as a layman, to address them in writing offering his advice on a matter of their expert judgement. The way to make sure that everything would be considered was to circulate in manuscript his personal views, and to go to Rome where he could clear them with friendly officials. Another reason for his going to Rome was that suspicion had been cast on him privately and publicly by [the ambitious priest] Caccini, who had since been examined by the Inquisition, and Galileo wanted to clear his name of any charges.…

…A principal area of contention between Catholics and Protestants was freedom to interpret the Bible, which meant that any new Catholic interpretation could be used by Protestants as leverage: if one reinterpretation could be made, why not wholesale reinterpretations? A dispute between the Dominicans and the Jesuits over certain issues of free will was still fresh in the pope's mind, as he had to take action in 1607 to stop members of the two great teaching orders from hurling charges of heresy at each other. These things suggest that Paul V, if not temperamentally anti-intellectual, had formed a habit of nipping in the bud any intellectual disputes that might grow into factionalism within the Church and become a source of strength for the contentions of the Protestants.

At Rome Galileo argued his astronomy against Aristotelian cosmology in various places and before various groups. His old Paduan friend, Antonio Querengo, reported in letters that although Galileo won few converts to his views, he utterly demolished the position of his opponents. Galileo found it hard, however, to meet personally with some officials to discuss theological issues, and was obliged to deal with them through intermediaries. Early in 1616 he wrote out at length his tide theory, based on the earth's motions, for Alessandro Cardinal Orsini. But when Orsini approached the pope, he was told instead to persuade Galileo to desist from further argument lest the Inquisition (which was entirely at the pope's command) be set in motion against him.

Cardinal Bellarmine, consulted by the pope, advised that the propositions in dispute be submitted to the theological qualifiers who normally decided such issues. Galileo would then be notified of action based on their ruling. That was the procedure adopted; the two propositions submitted and the qualifiers' opinions on them were as follows:

It is of interest that in both cases the censures had been made to hinge on the status of the propositions in Philosophy. Of no less interest is the phrase 'foolish and absurd', not false. Nothing was said of astronomy, it being simply assumed that astronomers were under the jurisdiction of philosophers.…

It is a curious fact that historians have not blamed philosophers rather than theologians for the decision taken against freedom of scientific opinion in astronomy. Yet philosophers alone urged the intervention of theologians, confident that they would be on their side. Galileo appears from his letters at this time to have been equally confident that official action would take neither side; that theologians responsible for the future of the Church would decline to make an article of faith out of a disputed astronomical question. The shift of responsibility for interpretation of the Bible from theology to Philosophy took him by surprise. That ordinary people, and even some priests, would take the literal words of the Bible as supporting Aristotle's cosmology was to be expected. For the Church to adopt that view officially seemed to Galileo an unprecedented action. In his own copy of the later Dialogue he made these notes:

Source: Drake, Stillman. Galileo. Past Masters series. © 1980. Reprinted by permission of Oxford University Press.

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Philosophy, Western; Galileo; Astronomy, History of