The condemnation of Galileo in 1633 is usually told as a collision between science and religion, and that framing hides the only thing about it that helps explain how measurement eventually won.
The previous lesson left the argument in a specific state. Ptolemy's system was dead, killed by the phases of Venus. Two systems remained, the Copernican and the Tychonic, which are geometrically identical and cannot be separated by any observation of position. Nobody had a physics in which a moving earth was possible. In that situation Galileo Galilei, the most famous natural philosopher in Europe, campaigned publicly for one of the two, and this lesson is about what happened and what it shows about the standing of evidence in 1633.
The first act, 1613 to 1616
Trouble began over dinner. In December 1613 Galileo's pupil Benedetto Castelli was questioned at the Medici court about whether the earth's motion contradicted scripture, in particular the passage in Joshua where the sun is commanded to stand still. Galileo wrote Castelli a long letter setting out how he thought the two should be related, and copies circulated.
In December 1614 a Dominican, Tommaso Caccini, preached against the Galileists in Florence, and in February 1615 another Dominican submitted a distorted copy of the Castelli letter to the Roman Inquisition. Galileo responded by expanding his argument into the Letter to the Grand Duchess Christina of 1615, which is one of the sharpest statements ever written of the claim that natural philosophy should be left to settle its own questions. Its core move is that scripture accommodates itself to ordinary speech and is not a textbook of astronomy, and it quotes Cardinal Baronio's line that the Bible teaches how one goes to heaven, not how the heavens go.
The Inquisition acted in 1616. On 24 February a panel of eleven consultants judged the proposition that the sun is the immobile centre of the world to be foolish and absurd in philosophy and formally heretical, and the proposition that the earth moves to be at least erroneous in faith. On 5 March the Congregation of the Index suspended De revolutionibus until corrected, the corrections appearing in 1620 and amounting to about ten passages altered so that the earth's motion reads as a hypothesis. Foscarini's book defending the system was banned outright. Galileo himself was not condemned. He was called in by Cardinal Robert Bellarmine and told not to hold or defend the opinion, and a disputed minute in the file, unsigned, records a stricter injunction not to teach it in any way whatever. That minute is what he was tried on seventeen years later.
Bellarmine's condition
The most important document of the whole affair is not a verdict but a letter Bellarmine wrote to Foscarini on 12 April 1615, before any of the machinery moved. It says three things, and they need separating.
First, there is no difficulty in treating the earth's motion as a mathematical supposition, a way of computing that saves the appearances. That is Osiander's position from two lessons ago, and it was uncontroversial.
Second, if there were a true demonstration that the sun really stands at the centre and the earth revolves, then one would have to proceed with great care in explaining the scriptures that appear contrary, and would have to say that we do not understand them rather than that what is demonstrated is false.
Third, he does not believe there is such a demonstration and has not been shown one, and until he is, the received reading of the texts stands.
Example. State exactly what Bellarmine's letter makes the question turn on, and whether Galileo could meet the condition in 1615.
It makes it turn on the burden of proof, and it concedes the principle that a demonstration would win. Bellarmine is not saying that evidence about nature is irrelevant to reading scripture; he says the opposite, that a demonstrated truth about nature obliges a rereading. What he denies is that anything on offer amounts to a demonstration, and by the standards of the day he was right. Galileo had the phases of Venus, which as the previous lesson showed refute Ptolemy and leave Tycho untouched; the moons of Jupiter, which remove an objection without establishing anything positive; and mountains on the moon, which are irrelevant to the question. Nothing in that list distinguishes the two live systems. The three observations that would eventually do it, stellar aberration, stellar parallax and the swing of a long pendulum, lay 113, 223 and 236 years in the future.
Now you. A common reading is that Bellarmine's position is simply that authority beats evidence. Is that fair?
Answer
Not as stated, and the unfairness matters because it hides how the argument was actually won. Bellarmine names a condition under which he would change the reading of the text, which is more than many participants in many disputes ever do, and the condition is evidential. What he is really doing is setting the bar for overturning a settled interpretation extremely high, and requiring demonstration in the strict Aristotelian sense: a proof from causes, not a hypothesis that fits the phenomena. He also had a serious logical point that Pope Urban VIII later made his own, namely that a theory saving the appearances is not thereby true, since another theory might save them equally well, and in this instance one demonstrably did. The fair criticism is not that Bellarmine ignored evidence; it is that his standard admitted no way for evidence to accumulate towards a conclusion, so a question could only ever be settled by proof or not at all. Science mostly does not work that way, which is why the practice that displaced this one had to make a place for evidence that is strong without being conclusive.
The wrong proof
Galileo understood the requirement and spent years looking for a physical demonstration. He believed he had found it in the tides, and the fourth and final day of the Dialogue is devoted to it.
The argument is genuinely ingenious. Combine the earth's daily rotation with its annual motion around the sun. At midnight a point on the surface is moving in the same direction as the earth's orbital motion, and at noon it is moving backwards against it, so the actual speed of that point through space varies over the day. The seas, not rigidly attached, cannot follow the changing speed of their basins, and slosh. Tides follow from the earth's double motion, and if the earth stood still there would be none.
Example. Work out the effect quantitatively, and compare the prediction with what the tides actually do.
The earth's orbital speed is km/s and its equatorial surface speed is km/s, so the modulation is per cent of the orbital speed, once per day. That is a real effect and a small one. The fatal problem is not its size but its period: the mechanism gives one acceleration and one deceleration per rotation, so it predicts one high tide a day, at a fixed hour. The observed tide is semidiurnal, two highs in each period of 24 hours 50 minutes, which is hours minutes apart, and the time of high water shifts by about 50 minutes each day. That 50 minutes is the giveaway: it is exactly the daily lag of the moon.
Now you. Galileo knew the tides track the moon and rejected it. Why, and what did the rejection cost him?
Answer
He rejected it because the only available way of stating the connection was that the moon influences the water across empty space, with nothing passing between them, and that was precisely the sort of occult sympathy the new philosophy existed to abolish. Kepler had proposed a lunar attraction and Galileo said in print that he was astonished Kepler had lent his ear to it. The commitment doing the work here is a good one: explanations should proceed by contact and by mechanism, not by hidden affinities. The cost was that he threw away a correct empirical correlation, in favour of a mechanism whose central prediction was contradicted by any harbour in Europe, and staked his claim to a demonstration on it. It is worth being blunt about the shape of this. The man who was right about the earth had, on his own account, one proof, and it was wrong; and the reason he was wrong is that he applied a methodological principle that was broadly correct and would not be repaired until Newton made action at a distance respectable, at the cost of the same principle.
The second act, 1632 to 1633
In 1623 Maffeo Barberini, a Florentine who had admired Galileo for years, became Pope Urban VIII. Galileo went to Rome, had six long audiences, and came away believing he had permission to write a book treating both systems provided it reached no conclusion. Urban asked that a particular argument be included: since God is omnipotent, any set of appearances could have been produced by means beyond our conception, so no physical account of them can be held to be necessarily true.
The Dialogue Concerning the Two Chief World Systems was licensed and printed in Florence in February 1632. It is a masterpiece of scientific writing and a political catastrophe. It is not balanced: the Copernican speaks with all the arguments and the Aristotelian, named Simplicio, is given the losing side and often made to look foolish. Urban's argument appears, but in Simplicio's mouth, in the closing pages. And the two chief world systems of the title are Ptolemy's and Copernicus's, which means the book stages a contest with a corpse and omits the Tychonic system entirely, the one arrangement that fitted every telescopic observation as well as Copernicus did.
Sales were stopped in August. The trial ran from April to June 1633, and its legal question was narrow: not whether the earth moves, but whether Galileo had violated the 1616 injunction and obtained his licence by concealing it. On 22 June 1633 he was sentenced as vehemently suspect of heresy, made to abjure kneeling, and placed under house arrest for the remaining nine years of his life. Seven of the ten cardinals of the Holy Office signed the sentence.
What the episode actually shows
Three corrections to the usual story, each of which matters for the argument of this subject.
The first is that the opposition was not uniformly clerical or anti-astronomical. The Jesuit mathematicians of the Collegio Romano, the best-equipped astronomers in Italy, confirmed Galileo's telescopic discoveries in 1611 and held a ceremony in his honour. Christoph Clavius had reformed the calendar; Christoph Grienberger and others did serious observational work. Most of them adopted the Tychonic system, and they did so with a defensible reason: it fitted every observation, and it did not require a physics nobody had. The people who knew the most were not persuaded, and their not being persuaded was rational.
The second is that the real question was jurisdictional. The Council of Trent had reserved the interpretation of scripture in matters of faith and morals to the Church, and Galileo, a layman, was publishing rules for reading scripture. Beneath the astronomy sits a fight about who is entitled to settle a dispute when a text and an observation appear to conflict, which is a question about authority rather than about the sky.
The third is that the ruling did not stop the work. Descartes, in the Netherlands, suppressed his own cosmological book Le Monde on hearing the news, and it was not published until after his death, which is a real cost. Italian astronomy did suffer. But Galileo, under house arrest and forbidden to publish, wrote the best book of his life, had it smuggled out to the Elzevirs in Leiden, and published it in 1638.
Example. The condemnation is often used to show that authority can suppress a scientific truth. What does the sequence of events actually establish about the standing of measurement in 1633?
That measurement had real but insufficient standing, and that both sides agreed on the criterion. Bellarmine conceded that a demonstration would compel a reinterpretation of scripture; Urban conceded that hypotheses which save the appearances are legitimate to compute with; Galileo accepted that he needed a demonstration and went looking for one. Nobody in the dispute held that evidence about nature was irrelevant. What the episode establishes is that in 1633 an observation could not yet outrank a text, that the standard for a demonstration was set at a height no astronomical evidence of the period could clear, and that the Copernicans could not clear it because they genuinely did not have the goods. The change that this subject is about is not that people started caring about evidence. It is that within two generations a body of evidence accumulated that met even a demanding standard, and that a method emerged for building such bodies deliberately.
Now you. Why does omitting the Tychonic system from the Dialogue count as a scientific failing and not just a tactical one?
Answer
Because a book claiming to settle a question has to engage the strongest surviving rival, and by 1632 that was Tycho's. Refuting Ptolemy in 1632 established nothing that the phases of Venus had not established twenty years earlier, and every professional reader knew it. Setting the strongest opponent aside makes the victory look decisive when it is not, and it leaves the reader unable to see what would be needed to go further. That is the same defect, viewed from the other side, as the one the previous lesson identified in the vicarious hypothesis: a theory is tested by what it can survive, not by what it can beat. There is a tactical reading too, that engaging Tycho would have obliged Galileo to admit he could not distinguish the systems, but the cost is scientific whatever the motive.
What was missing
Strip out the personalities and the jurisdiction and one gap remains. The objection of the very first lesson was never answered: on a moving earth, why does a dropped stone land at the foot of the tower, and why is nothing flung off?
Galileo had the answer, or most of it, and it had nothing to do with astronomy. It came from rolling balls down inclined planes and timing them, from watching pendulums, and from thinking hard about what happens to a ball on a ship. Under house arrest at Arcetri, forbidden the subject of the earth's motion, he wrote it down. That book, and the science of motion in it, is where the next lesson goes.