Justified True Belief

(●) The mathematician who pointed a homemade telescope at the sky, and whose trial became the West’s favourite story about faith against science. + In the winter of 1609 a middle-aged professor in Padua, badly paid and behind on the dowries he had promised his sisters, ground his own lenses, put them in a lead tube, and looked up. Within four months he had seen mountains on the moon, resolved the Milky Way into individual stars, and watched four small points of light swing back and forth around Jupiter night after night until there was no honest way to call them anything but moons. Nothing in the heavens had been supposed to do that. He published in March, and by summer he was the most famous natural philosopher in Europe. Natural philosopher is what the age called a man who studied how the world works; the word scientist would not be coined for another two hundred years, and there was as yet no such thing as a scientific career to have. He matters to Christian readers for two reasons, and the second one is not the trial. The first is that he wrote the clearest early statement of how a believer should hold Scripture and natural knowledge together when they seem to collide, and that statement is still what most thoughtful Christians say. The second is that his condemnation in 1633 became the single story everyone reaches for when they want to show that the church is the enemy of knowledge. Both things are his, and a reader who knows only the second one has been handed half a man. Here is the fact most people find surprising: Galileo was a believing Catholic to the end, he never questioned the authority of Scripture, both of his daughters entered a convent, and the astronomers who first confirmed his discoveries, and then held a public ceremony in his honour at Rome, were Jesuit priests.

(●) 1564 AD: Born in Pisa to a musician who had already picked a public fight with received authority. + Galileo was born at Pisa on 15 February 1564, the eldest of the children of Vincenzo Galilei and Giulia Ammannati. The family moved to Florence when he was a boy. His father was a lutenist and a music theorist of real standing, and he is worth a moment of the reader’s attention, because the son’s whole method is visible in him first. Vincenzo had a quarrel with the Pythagorean rule, inherited from antiquity and repeated for two thousand years, that musical intervals answer to simple whole-number ratios: an octave two to one, a fifth three to two, and so on down the scale, the whole of music resting on a handful of tidy numbers. It is a beautiful idea, and it had the weight of every authority behind it. Vincenzo did not settle the matter by finding better authorities of his own. He hung weights on strings, measured what he actually heard, and showed that the tidy old numbers did not survive contact with a real instrument, then said so in print, against his own teacher. A boy who grows up watching his father do that does not easily learn to end an argument with a quotation. Galileo enrolled at the University of Pisa in 1580 to read medicine, which was where a promising young man with no money was sent, medicine being the one course that reliably led to a living. He never took the degree. A mathematics tutor at the Tuscan court, Ostilio Ricci, showed him Euclid, and that was the end of medicine. He left Pisa in 1585 without a qualification of any kind and spent the next four years teaching privately and looking for a post.

(●) 1589 AD: He got the mathematics chair at Pisa, lost it, and settled at Padua for eighteen years of experiments on how things fall. + The Pisa chair came in 1589 and paid sixty scudi a year. That sounds like very little because it was very little: roughly a thirtieth of what the university’s best paid professor of medicine took home, which tells you exactly what the universities of the day thought mathematics was for. He kept the chair three years and did not have it renewed. In 1592 he moved to Padua, in the territory of the Venetian Republic, and the eighteen years he spent there he later called the happiest of his life. His work in this period was on motion, which was the problem everybody thought Aristotle had settled and nobody had troubled to reopen. Aristotle taught three things about it: that heavier bodies fall faster in proportion to their weight, that a moving thing needs something to keep pushing it or it stops, and that motion in the heavens is of an entirely different kind from motion down here. Galileo worked at all three, first in an unpublished manuscript now called De motu and then in years of patient experiment, rolling bronze balls down grooved planks set at a slope and timing them by catching the water that ran out of a vessel while they rolled and weighing it afterwards. No clock in the world could measure a second in 1600, so rather than time the ball he weighed the water, and the weight of the water was his stopwatch. The famous story of dropping two weights from the Leaning Tower comes from Vincenzo Viviani, his last pupil, who wrote it down more than sixty years afterwards and long after Galileo’s death; no contemporary source mentions it, and historians treat it as tradition rather than record. His private life was not tidy, and it would be dishonest to leave that out. In Padua he lived with a Venetian woman named Marina Gamba without marrying her, and they had three children: Virginia in 1600, Livia in 1601, and Vincenzo in 1606. Because the girls were illegitimate, no respectable marriage could be arranged for either of them, and the convent was the ordinary solution to that problem in a Tuscan family, so in 1613 he placed both of them at San Matteo at Arcetri, Virginia at thirteen and Livia at twelve. It was a cold thing to do to a thirteen-year-old, and it produced the warmest relationship of his life. Virginia took the name Sister Maria Celeste and became his closest friend, his housekeeper at a distance, and eventually his defender. A hundred and twenty-four of her letters to him survive. His to her do not.

(●) 1610 AD: Four moons around Jupiter made him famous in a season, and took him home to Florence. + In the summer of 1609 word reached Venice that a Dutch spectacle-maker had put two lenses in a tube and made distant things look near. Galileo did not have the instrument and did not need it; hearing that the thing could be done was enough. Within months he had built his own and pushed it from three power to eight, then to twenty and beyond, grinding and polishing the lenses himself because nobody else could make them well enough. He turned it on the moon in the autumn and watched the terminator, the line where the lit half of the moon meets the dark half, which is simply the place where the sun happens to be rising up there. The line was ragged rather than clean. Bright points kept appearing out in the darkness ahead of it, unattached to anything, and then over the following hours they widened and joined the lit side. Anyone who has stood in a valley before dawn and seen the peaks catch the light first knows what that is, and Galileo knew too: sunrise on mountaintops. Then, on the night of 7 January 1610, he saw three small stars in a line beside Jupiter, and the next night they had moved, and they had moved the wrong way. On 13 January there were four of them. Within days he understood what he was looking at, and he wrote the whole thing up in eight weeks. Sidereus Nuncius, the Starry Messenger, was licensed and printed in Venice in March 1610: a slim quarto, the size you get by folding the printer’s sheets twice, of some sixty pages, and it changed European astronomy. He named the four moons the Medicean Stars, after Cosimo II de’ Medici, Grand Duke of Tuscany, who had been his pupil. It was a courtier’s move and it worked exactly as intended: by September he was Chief Mathematician and Philosopher to the Grand Duke, at a thousand scudi a year with no teaching, and back in Florence for good. Late that year, though, came the observation that actually mattered, and it was not the moons. Venus, watched over months, runs through the same cycle of shapes the moon does, from a thin crescent to a full round disc. A planet can only show you a full disc if it is on the far side of the sun with the sunlight coming past it towards you, and in the old earth-centred system worked out by Ptolemy in the second century, Venus never gets there; it is kept between the earth and the sun, and it can never be full. So that system was finished. What the phases did not do, and Galileo knew perfectly well that they did not, was decide between the sun-centred system of Copernicus and the compromise Tycho Brahe had proposed, in which the planets circle the sun while the sun and its whole train circle a motionless earth. Tycho’s arrangement gives you the phases of Venus just as neatly, and it was to be the sticking point for the rest of his life.

(●) 1616 AD: Rome judged the moving earth an error and told him he could not hold it or defend it. + The trouble began at a breakfast table. In December 1613 a Benedictine named Benedetto Castelli, Galileo’s friend and former student, was dining at the Medici court when the Grand Duchess Christina of Lorraine, the Grand Duke’s mother, pressed him on whether the new astronomy could be squared with the Bible. Castelli reported the conversation, and Galileo answered it with a long letter that he expanded two years later into the Letter to the Grand Duchess Christina, the most important thing he ever wrote that was not about physics. He was not the only one talking. Four days before Christmas 1614 a Dominican preacher in Florence, Tommaso Caccini, took the tenth chapter of Joshua as his text, denounced the mathematicians from the pulpit of Santa Maria Novella, and is reported to have closed on a pun made out of the angels’ words to the disciples, "Men of Galilee, why do you stand looking into heaven?" (Acts 1:11). A copy of the letter to Castelli was handed to the Inquisition soon after. Two Roman offices now come into the story, and they are easy to confuse, so it is worth ten seconds to keep them apart. The Holy Office, which everybody called the Inquisition, was a standing committee of cardinals whose job was to guard Catholic doctrine and try the people accused of departing from it. The Congregation of the Index was a different committee with a narrower job: it kept the list, the Index, of books Catholics were forbidden to read. Both answered to the pope, and in 1616 both were about to act. Galileo went to Rome in December 1615 to argue his case in person, which was a mistake, because it forced a ruling that would otherwise have gone on being quietly postponed. On 24 February 1616 the theological advisers of the Holy Office returned their verdict on two propositions: that the sun is the motionless centre of the world, which they called foolish and absurd in philosophy and formally heretical, and that the earth moves, which they called at least erroneous in faith. Cardinal Robert Bellarmine, the ablest theologian in Rome and by no means Galileo’s enemy, summoned him on 26 February and warned him. Copernicus’ book was suspended by the Congregation of the Index on 5 March until it should be corrected. Bellarmine had already set out his own position in a letter of the previous April to the Carmelite Paolo Foscarini, and it was a good deal more open than the verdict that followed it: if a real demonstration were ever produced that the earth moves, then the passages of Scripture that seem to say otherwise would have to be re-read, and one would have to say that we do not understand them rather than that what is demonstrated is false. A demonstration, in the language of the schools, meant a proof, something that shuts the question rather than merely making one side likelier than the other. So everything turned on that one word. Galileo believed he had a demonstration. He did not, and the rest of the story follows from the gap between those two sentences. There is one more document, and seventeen years later it would be the hinge of his trial. Rumours went round Rome that Galileo had been made to abjure, which is to say to stand up and renounce a belief publicly, on oath, as an error, a humiliating thing to have on your name. He had not, and in May 1616 he asked Bellarmine for a written statement saying so, and got one: he had abjured nothing and done no penance, but had simply been notified that the Copernican doctrine could not be held or defended. He kept that certificate for the rest of his life.

(●) 1632 AD: He published the Dialogue with a licence from the censors, and it cost him everything. + In 1623 Maffeo Barberini became Pope Urban VIII. He was a Florentine, a poet, an admirer of Galileo who had written a Latin ode in his honour, and Galileo thought the weather had changed. He dedicated Il Saggiatore (The Assayer) to the new pope, went to Rome in 1624, and was granted six long audiences. Urban gave him leave to write about both world systems on one condition: he must treat them as hypotheses, and he must include the argument Urban considered decisive, that God in his omnipotence could produce the same appearances by means we have not imagined, so no human account of the heavens can claim to be the only possible one. The Dialogue Concerning the Two Chief World Systems came out in Florence in February 1632. Nothing could be printed in Catholic Italy without a licence from a church censor who had read it first, and this book had two, one from Rome and one from Florence, which is worth remembering when the story is told as though he had published in defiance of anybody. It is a conversation over four days between three men: Salviati, who argues for Copernicus and speaks for Galileo; Sagredo, an intelligent layman with no stake in the quarrel, who is gradually persuaded; and Simplicio, who defends Aristotle and Ptolemy. Galileo wrote it in Italian rather than Latin, which is to say he wrote it for anybody who could read rather than only for the universities, and it is genuinely funny, and Simplicio loses every exchange in it. Two things then went wrong, and they went wrong together. The first was the argument Urban had asked for. It does appear in the book, faithfully enough, but Galileo put it in the mouth of Simplicio and gave it to him to deliver on the last page, and Simplicio’s name means roughly "the simpleton". Whatever Galileo intended by that, and he seems genuinely not to have seen how it would look, Urban was told that he had been mocked in print by a man he had spent years protecting. Popes forgive being disagreed with more readily than being made ridiculous. The second was a piece of paper. A file was pulled from the archives of the Holy Office containing an unsigned memorandum of that meeting with Bellarmine in 1616, and the memorandum said something the certificate did not: that Galileo had been ordered not to hold, defend, or teach the doctrine in any way whatsoever. Teaching it as a hypothesis, which is exactly what he had a papal licence to do, would fall under that. Whether the memorandum records what actually happened in the room is still argued by historians, and there are reasons for doubt: it bears no signature of Galileo and none of a notary, and its wording flatly contradicts the certificate Bellarmine wrote out for him three months later. In August the sale of the book was stopped and the unsold copies called in, and on 1 October the inquisitor of Florence arrived at Galileo’s door with a summons to Rome.

(●) 1633 AD: He knelt in a Roman convent, abjured the motion of the earth, and was sentenced to confinement for life. + He was two days short of his sixty-ninth birthday and ill, and he travelled to Rome in the middle of a plague quarantine, arriving on 13 February 1633. He was not thrown in a dungeon and he was never tortured; he lodged in the Tuscan embassy for most of the proceedings and, when detained at the Holy Office in April, was given rooms rather than a cell. Torture is mentioned once in the record, in the formula that he was to be examined about his intention under the threat of it, and there is no evidence it went further than the threat. The trial turned on two questions: what he had actually been forbidden in 1616, and whether the Dialogue defends the doctrine it claims only to describe. On the first he produced Bellarmine’s certificate, which was the strongest card he had. On the second he had nothing, so at his lawyers’ urging he offered the defence that he had not realised how strongly the book came out for Copernicus. It was not true of anybody who had read it, and it cannot have been true of the man who wrote it, and it is painful to watch an old man say it. On 22 June 1633, in the hall of the convent of Santa Maria sopra Minerva, the sentence was read out. He was found vehemently suspect of heresy, which was a formal grade in the Inquisition’s vocabulary: not a conviction for heresy itself, which would have been far worse, but a serious step above the light suspicion that was renounced quietly in front of the inquisitor and a notary, and the grade that required the accused to abjure in public. The Dialogue was prohibited, and he was condemned to formal imprisonment at the pleasure of the Holy Office. Kneeling, with his hands on the Gospels, he read out an abjuration in which he cursed and detested his errors. Of the ten cardinals of the Holy Office, seven signed; three did not, including the pope’s own nephew, Francesco Barberini. He is supposed to have muttered, as he rose from his knees, eppur si muove, and yet it moves. He did not. The line first turns up in print more than a century later, in 1757, and there is no earlier trace of it anywhere. It is a legend that grew because people wanted him to have said it, and it is a good deal easier to admire than what he actually did, which was to go home beaten and write the best book of his life. The prison sentence was commuted the next day. He spent five months as the guest of Archbishop Ascanio Piccolomini in Siena, who treated him as an honoured visitor, and from December 1633 he lived under house arrest at his villa at Arcetri in the hills above Florence.

(●) 1638 AD: Blind and under guard, he got his best book out of the country, and died four years later. + The villa at Arcetri stood a few hundred yards from the convent of San Matteo, where his daughter Maria Celeste had lived since she was thirteen, so the house arrest that was meant as a punishment put him closer to her than he had been in twenty years. She had spent the trial writing to him, keeping his house in order in his absence, and, when the sentence required him to recite the seven penitential psalms weekly for three years, quietly saying them on his behalf. Then, in April 1634, four months after he came home, she died of dysentery at thirty-three. He wrote afterwards that he kept hearing her voice calling him about the house, and that he could not bring himself to look at her letters. What he did next is the reason working physicists still read him. Forbidden to publish, in his seventies, in mourning, and going blind, he went back to the work on motion he had begun at Padua forty years earlier and finally wrote it up properly. The manuscript was carried out of Italy and printed at Leiden in 1638 by the Elzevirs, the great Dutch printing house, safely beyond the reach of the Roman censors, as Discourses and Mathematical Demonstrations Relating to Two New Sciences. In it are the law that a body falling from rest covers distances in proportion to the square of the time, which is the same one, four, nine that the bronze balls on the ramps had been telling him for forty years (T1); the demonstration that a thrown stone or a fired cannonball travels a parabola, the curve you see in an arc of water from a hose; and the beginnings of the study of why beams and columns break, which is where engineering starts. It is the foundation Newton built on. He had gone completely blind by the end of 1637, before the book ever reached print. Writing to his friend Elia Diodati on 2 January 1638, he said that this heaven, this earth, this universe, which his own observations and demonstrations had "enlarged a hundred, nay, a thousand fold beyond the limits universally accepted by the learned men of all previous ages," was now "shrivelled up" for him into the space his body occupied (Jane Sturge’s translation of 1879). The man who had given Europe a hundred times more universe than it started with could no longer see across a room. Students came to Arcetri anyway, condemned man or not. Vincenzo Viviani joined him in 1639 and Evangelista Torricelli, who would go on to invent the barometer, in 1641, and the two of them were with him at the end. He died there on 8 January 1642, aged seventy-seven. The Grand Duke wanted a monument for him in the basilica of Santa Croce, where Florence buries its great men, but Rome objected and Galileo went instead into a small room off the novices’ chapel, out of sight. He stayed there ninety-five years. In 1737 his remains were carried into the church proper and the monument was finally built, facing the tomb of Michelangelo.

What He Taught

(T1) The universe is a book, and it is written in the language of mathematics. + Think about what it takes to read a letter in a language you do not know. You can see the ink perfectly well. You can describe the shapes, count the marks, admire the handwriting, and still have no idea what the letter says, because seeing is not reading and no amount of staring will supply the grammar. That is Galileo’s complaint against the natural philosophy he was taught. The scholars of his day had a rich vocabulary for explaining why things happen: heavy bodies fall because it is their nature to seek the centre, fire rises because its proper place is above. Words like nature and proper place feel like explanations, but you cannot check them against anything. You cannot say what number they predict. Galileo’s proposal was that the world will answer a different sort of question, the sort with a number in it. How far in the first second, and how far in the next? Roll a ball down a plank a thousand times and the ball itself tells you: the distances go as one, four, nine, sixteen, the squares of the times. That is a sentence in the language he means. Two cautions, because this idea is regularly stretched further than he stretched it. Galileo never claimed that mathematics is all there is, and he never suggested that the questions it cannot answer are unreal; he was a believer who thought the most important knowledge of all came by revelation. What he claimed is narrower and has held up for four hundred years: for the behaviour of bodies in motion, the honest description is a quantity, and a description that resists measurement is not yet knowledge. Christians who came after him found the claim easy to hold, because a world written in a stable and readable language is exactly what you would expect if a rational God made it and made minds that could read it.

(Q) "Philosophy is written in this grand book, the universe, which stands continually open to our gaze. But the book cannot be understood unless one first learns to comprehend the language and read the letters in which it is composed." + Source: Galileo, The Assayer (1623), section 6 (Stillman Drake translation, Discoveries and Opinions of Galileo, 1957). The sentence that follows names the language: it is written in the language of mathematics, and its characters are triangles, circles and other geometrical figures, without which, Galileo says, one wanders about in a dark labyrinth. The book was an attack on a Jesuit astronomer’s account of the comets of 1618, and this passage is a digression in the middle of a quarrel (T1).

(T2) Scripture and nature cannot contradict each other, because the same God is behind both. + Suppose a man leaves two letters, one written in his own hand and one carried by a trusted friend who has it by heart. If the two seem to disagree, you do not conclude that the man contradicted himself. You conclude that somebody has misread one of them, and you go back and look again. That is the shape of Galileo’s central theological argument, and it is not original to him; he took it from Augustine and said so. The argument runs like this. God is the author of Scripture, which the Holy Spirit dictated. God is also the author of the natural world, which carries out his commands without ever disobeying or bending. Two true things cannot conflict. So when a passage of the Bible, read a particular way, appears to be contradicted by something we have genuinely established about the world, the conflict is between our reading and the world, not between God and God, and the reading is the thing to examine. Galileo was careful about the word established, at least in principle: he agreed that where a natural claim is merely probable, Scripture as the church has always read it should hold the field. The reason this cut so deeply in 1615 had less to do with astronomy than with who was allowed to say it. The Council of Trent, the long series of sessions in which the Catholic church had answered the Reformation and tightened its own discipline, closed two months before Galileo was born, and among its rulings was one against private individuals interpreting Scripture contrary to the unanimous consent of the fathers. The fathers are the Christian writers of the first several centuries, Augustine and Jerome and Chrysostom and the rest, whose readings the church treated as a settled inheritance. And here was a layman with no theological training whatever, telling cardinals how to read Joshua. On the substance, though, he was standing on very old ground. Augustine had warned twelve hundred years earlier that a Christian who talks nonsense about the natural world while claiming Scripture’s authority does the faith real damage, because the unbeliever who knows better will assume the rest is nonsense too.

(Q) "the holy Bible and the phenomena of nature proceed alike from the divine Word, the former as the dictate of the Holy Ghost and the latter as the observant executrix of God’s commands." + Source: Galileo, Letter to the Grand Duchess Christina (written 1615, printed 1636) (Stillman Drake translation, Discoveries and Opinions of Galileo, 1957). An executrix is the person charged with carrying out the terms of a will exactly as written. Nature, on this picture, is not a rival authority to Scripture but an obedient servant of the same master, which is why Galileo thought the two could never really be at odds (T2).

(T3) Scripture speaks in the ordinary language of ordinary people, so it was never meant to settle the arrangement of the heavens. + Every weather forecast you have ever heard gives a time for sunrise. Nobody thinks the meteorologist is teaching astronomy, and nobody accuses the forecast of error. It is speaking the way people speak, from where people stand, about what people can see. Theologians call this accommodation: God, in speaking to human beings, stoops to the way they already talk, the way an adult crouches to a child’s eye level rather than lecturing over their head. Scripture says God has hands and God repents, and readers have understood for as long as there have been readers that this is speech fitted to the hearer. Galileo’s move was to extend the same principle to astronomy. When Joshua asks the sun to stand still and the sun stands still (Joshua 10:12–13), the text is telling us that God answered a soldier’s prayer and gave Israel the daylight to finish a battle. It is not filing a report on celestial mechanics, any more than Ecclesiastes 1:5 is, or Psalm 104:5. He drove the point home with a line he borrowed from Cardinal Cesare Baronio, the church historian, and it is the most quoted sentence he ever wrote. The Bible is a book about salvation. It tells you the road to heaven, not the machinery of the sky. What it does say about the sky it says gladly and often, in the register of praise rather than of measurement: "The heavens declare the glory of God, and the sky above proclaims his handiwork." (Psalm 19:1) Galileo would have said that his telescope had merely made the declaring louder.

(Q) "That the intention of the Holy Ghost is to teach us how one goes to heaven, not how heaven goes." + Source: Galileo, Letter to the Grand Duchess Christina (written 1615, printed 1636) (Stillman Drake translation, Discoveries and Opinions of Galileo, 1957). Galileo is quoting here, not coining. He credits the line only to an ecclesiastic of the most eminent degree, and the man he means is Cardinal Cesare Baronio, an Oratorian, which is to say a member of the community of priests Philip Neri had gathered in Rome, and the author of the great Catholic church history of the age. Where the two men met is not recorded, and one account puts it in Padua in 1598, while Baronio was travelling in northern Italy with Clement VIII. The pun works in Latin and Italian as it does in English, and it survived because it fits in one breath (T3).

(T4) An instrument can show the eye what no one has ever seen, and a single new sight can bring down an old system. + Before anything else, it is worth saying why an intelligent, careful person believed the old astronomy, because the story is usually told as though only a fool could have. Look up on any clear night: the sky plainly turns, and the ground plainly does not. Drop a stone from a tower and it lands neatly at the foot of the tower, which it could hardly do if the tower were being carried eastward at several hundred miles an hour while the stone was in the air. Generation after generation of observers had recorded a sky that never changed, never blemished, never aged, so it was reasonable to conclude that the heavens are made of some different and incorruptible stuff. And then there was parallax. Hold your finger up at arm’s length, shut one eye, then the other, and the finger jumps against the far wall; that jump is parallax, and it is how you know you have moved. An earth swinging from one side of a great orbit to the other over six months ought to make the stars jump in exactly that way, and nobody could measure any jump at all. Every one of those is a real argument from real evidence, and taken together they are why the question had stayed closed for as long as anyone had been keeping records of the sky. Galileo’s telescope did not refute any of that directly. What it did was worse for the old picture, because it kept producing facts the picture had nowhere to put. The moon turned out to have mountains and hollows, throwing shadows the way a range of hills does at dawn, so at least one body up there is made of ordinary rough stuff after all. The Milky Way turned out not to be a vapour but a crowd of stars too faint and too close together for the eye to separate. Jupiter turned out to have four moons of its own, so whatever else is true, the earth is not the one unique centre that everything in the sky circles. Venus turned out to run through a full cycle of phases from crescent to full disc, which the earth-centred system of Ptolemy simply cannot produce. And the face of the sun turned out to carry spots that come, drift across it, and go, which is change in the heavens, in the one place where change was not supposed to happen. Two honest notes, because the story is often told with the difficulties left out. The first is that the phases of Venus killed Ptolemy without establishing Copernicus, since Tycho Brahe’s system, with the planets circling the sun and the sun circling a fixed earth, predicts those phases just as well. Galileo tended to hurry past that, and his opponents were right to press him on it. The second is that the missing stellar parallax was a serious scientific objection and not an evasion at all. It has an answer, which is that the stars are simply much further away than anyone had imagined, so the jump is far too small to see; but nobody could show that in 1613, and nobody measured a stellar parallax until Friedrich Bessel finally managed it in 1838. So the right conclusion in 1613 was that the old system was dead and the new one was not yet proved, which is an honest place to stand and a deeply uncomfortable one.

(Q) "the surface of the Moon is not perfectly smooth, free from inequalities and exactly spherical, as a large school of philosophers considers with regard to the Moon and the other heavenly bodies, but that, on the contrary, it is full of inequalities, uneven, full of hollows and protuberances, just like the surface of the Earth itself, which is varied everywhere by lofty mountains and deep valleys." + Source: Galileo, The Sidereal Messenger (1610) (Edward Stafford Carlos translation, 1880). The first result he reports, and in some ways the most subversive, because Aristotle’s physics required everything above the moon to be perfect, changeless, and unlike the earth. A mountain range on the moon is a piece of ordinary geography in a place where geography was not supposed to exist (T4).

(T5) Motion you share with everything around you cannot be detected from inside it. + Pour a cup of coffee on a smooth flight at cruising altitude. The stream falls straight down into the cup, exactly as it would in your kitchen, though you and the cup and the coffee are all crossing the ground at five hundred miles an hour. Nothing in the cabin gives it away. Close the shades and there is no experiment you can do with a ball, a fly, or a glass of water that will tell you whether the aircraft is moving or parked. This is the answer to the strongest common-sense objection against a moving earth, and Galileo gave it in the second day of the Dialogue, three centuries before anyone flew. He asks the reader to go below decks in a large ship with some flies, a bowl of fish, and a bottle dripping water into a vessel underneath. Watch everything carefully while the ship is still. Then have the ship sail at any speed you like, so long as it sails steadily and does not pitch or turn. The flies do not pile up at the stern. The drops still fall straight into the vessel. You can jump as far towards the bow as towards the stern. Nothing changes at all, because everything in the cabin shares the ship’s motion, and shared motion is as good as no motion. Now put the earth in place of the ship, and the tower objection (T4) answers itself. The stone dropped from the top is not left behind, because the stone is already travelling east with the tower and simply keeps doing so all the way down. Nobody had to abandon what they had seen; they had to see what it meant. This principle, that the laws of motion look the same to any observer moving steadily, is called Galilean relativity, and it went straight into Newton’s first law and stayed at the foundation of physics until Einstein rebuilt the foundation around it rather than removing it.

(Q) "You will discover not the least change in all the effects named, nor could you tell from any of them whether the ship was moving or standing still." + Source: Galileo, Dialogue Concerning the Two Chief World Systems (1632), Second Day (Stillman Drake translation, 1953). Salviati has just finished the list: the flies, the fish, the dripping bottle, the jumping, the smoke of a burning incense stick. The conclusion is the whole point of the passage (T5).

(Q) "I do not feel obliged to believe that that same God who has endowed us with senses, reason, and intellect has intended to forgo their use..." + Source: Galileo, Letter to the Grand Duchess Christina (written 1615, printed 1636) (Stillman Drake translation, Discoveries and Opinions of Galileo, 1957). The clause he leaves hanging is that God has not intended us to set reason aside in matters we can reach by observation and demonstration. It is a devout sentence and an argumentative one at the same time, and it says as clearly as anything he wrote why he could not see himself as an enemy of the faith.

What the Church Made of Him

(†) The Jesuit astronomers of Rome checked his discoveries, confirmed them, and held a ceremony in his honour. + When the Starry Messenger appeared, the reaction of serious astronomers was not outrage. It was caution, and the caution was sensible: every claim in the book rested on an instrument nobody else possessed, and anyone who had handled a lens knew that lenses can lie, showing you rings and ghosts and doubled images that are not there. The right response to an extraordinary report from a machine only one man owns is to get the machine and look for yourself. That is what happened. The best mathematicians in Catholic Europe at the time were the Jesuits of the Collegio Romano, led by Christoph Clavius, the man who had done the mathematics for the Gregorian calendar and then defended it in print against all comers. They obtained telescopes good enough for the job and spent months checking, and by April 1611 they had confirmed the lot: the moons of Jupiter, the rough moon, the Milky Way resolved into separate stars, the phases of Venus. Cardinal Bellarmine had written to ask them directly whether the reports were true, and they told him plainly that they were. The college then threw a public celebration for Galileo, at which Odo van Maelcote lectured on the new discoveries with Galileo sitting there listening, and that same spring he was elected to the Accademia dei Lincei, Rome’s new private academy for the study of nature. Clavius, in the last edition of his own textbook on the heavens, printed the observations and remarked that astronomers would now have to find some way of arranging the celestial spheres that could accommodate them. That is the part of the story most often left out. The observations were never the problem, and the men who verified them first were priests.

(†) His rule for reading Scripture was Augustine’s before it was his, and the church came round to it. + The Letter to the Grand Duchess Christina quotes Augustine more than any other authority, and the debt is real. In his commentary on the literal sense of Genesis, Augustine had laid down two rules Galileo simply took over. Where Scripture can be read in more than one way and the natural evidence is decisive, do not lash yourself to one reading so tightly that you fall with it. And do not talk foolishly about the natural world in Scripture’s name, because an unbeliever who knows the subject will laugh, and will then assume that everything else Christians say about heaven is equally uninformed. The church did come round, and it is worth knowing how, because nobody ever stood up and announced a reversal. It happened in quiet administrative steps spread over three hundred years, the way institutions usually change their minds. First the works were allowed to circulate: in 1741 Benedict XIV granted an imprimatur, the church’s formal permission to print, to an edition of Galileo’s complete writings. Then the general rule went: in 1758 the Index dropped its blanket prohibition of books teaching that the earth moves. The particular titles took longest, and Copernicus, Kepler and Galileo were not struck off the list of forbidden books until the edition of 1835, by which time no educated person in Europe believed the sun went round the earth. Finally the judgement itself was faced. In 1979, at the centenary of Einstein’s birth, John Paul II called publicly for the case to be examined again, and the commission he appointed reported in 1992 that the theologians of the day had failed to distinguish their own reading of Scripture from Scripture itself, and had been wrong. Protestant reception ran on a separate track altogether, since Rome’s rulings meant nothing there, and it came out mixed in much the same way. Luther is reported in the Table Talk, the notes his students took of his dinner conversation, to have brushed the moving earth aside by pointing at Joshua, and Calvin’s commentaries simply assume what everyone assumed, that the earth stands still and the sun crosses over it. The famous line in which Calvin is supposed to have asked who will venture to place Copernicus above the Holy Spirit gets quoted in book after book, and it should be dropped, because it has never been found in anything Calvin wrote. What Calvin did teach, at length and on purpose, is accommodation, the same principle Galileo would later need (T3). God in Scripture lisps to us, Calvin said, the way a nurse lisps to an infant, and Moses in Genesis 1 describes the heavens as they look to an unlearned eye standing in a field rather than as an astronomer would describe them. That is Galileo’s rule in Reformed dress, and it was in print before Galileo was born, which is a good thing to remember whenever the accommodation principle is presented as a modern retreat invented to get out of a difficulty.

(†) The trial became the founding legend of a war between science and religion, and historians have spent a century taking the legend apart. + The idea that science and Christianity have been at war throughout history is not ancient. It was manufactured in the nineteenth century by two American writers: John William Draper, in his History of the Conflict between Religion and Science of 1874, and Andrew Dickson White, the first president of Cornell, in A History of the Warfare of Science with Theology in Christendom of 1896. Galileo is the star witness in both, and much of what the general reader still believes about him comes from those two books rather than from the trial documents: the dungeon, the torture, the flat earth of the medieval church, the muttered eppur si muove. None of it is in the record. Historians of science have been taking the warfare thesis apart for decades now, and among specialists it is a minority position. Ronald Numbers assembled a whole volume on the myths it produced; David Lindberg and John Hedley Brooke have shown how much of the actual relationship looks like patronage and collaboration rather than combat, which is not the shape you would expect from a war. Some of the evidence is very concrete. The Jesuit order alone supplied a long line of working astronomers, and thirty-five craters on the moon carry Jesuit names. And the men who founded modern science were not defectors from the faith: Copernicus was a canon on the staff of a cathedral, Kepler had trained for the Lutheran ministry and never stopped wanting it, and Boyle and Newton between them wrote more theology than physics. None of them thought they were changing sides. None of that, however, makes the trial anything other than a scandal, and Christians gain nothing at all by softening it. A church court took up a question that was never its business, ruled on it against the weight of the evidence, and silenced a man for the rest of his life. Two things are worth holding together about it. The first is that the ruling was a disciplinary act of two Roman congregations and not a definition of doctrine binding on the whole church, which is precisely why it could be quietly reversed later without anything unravelling. The second is the uncomfortable one: it happened because churchmen tied the authority of Scripture to one particular reading of Scripture that Scripture itself did not require, and then had to defend the reading as though the authority depended on it. Galileo told them so at the time, in writing, and he was right.

(†) Where his own record will not bear defending: he claimed a proof he did not have, and he humiliated the men who asked him for it. + The trial was an injustice. Galileo was also, on the evidence of his own writings, a difficult man and at times a dishonest advocate, and the two facts sit side by side without cancelling each other. Four things in the primary sources will not be defended. - He claimed a demonstration he did not have. Bellarmine had said plainly that a true demonstration of the earth’s motion would oblige the church to re-read the relevant passages of Scripture. That was an open door, and a patient man would have walked through it. Galileo walked at it instead, insisting the demonstration was already in hand, when the phases of Venus fitted Tycho’s arrangement of the heavens exactly as well as Copernicus’ and the missing stellar parallax was still unexplained. His own standard in the Letter to Christina, that a merely probable claim about nature should not dislodge the received reading of Scripture, told squarely against him here, and he did not apply it to himself. - His one alleged proof was wrong. The fourth day of the Dialogue argues that the tides are caused by the earth’s two motions together sloshing the oceans back and forth, the way water slops in a basin you are carrying across a room, and Galileo regarded this as the clincher that settled everything; he had wanted to call the whole book a treatise on the tides. It is simply false. It predicts one high tide a day where there are plainly two, and it can say nothing at all about why the tides keep time with the moon. - He brushed aside better work than his own. Kepler’s Astronomia nova of 1609 supplied the elliptical orbits that actually made the Copernican system work, and Galileo, who had been corresponding with Kepler since 1597, went on drawing perfect circles to the end and never engaged with it. Kepler had also suggested that the moon draws the seas, which is very nearly the right answer. In the fourth day of the Dialogue Galileo lumped that suggestion in with occult properties and puerilities; occult in the period’s sense meant hidden, a power asserted without any mechanism anyone could exhibit, and it was a fair thing to be suspicious of and the wrong place to aim the suspicion. - He was cruel in controversy. The Assayer is a brilliant and merciless attack on the Jesuit Orazio Grassi over the comets of 1618, and on the actual astronomy Grassi was nearer the truth than Galileo, who held that comets were an optical effect in our own atmosphere. Then he took Urban VIII’s own argument and handed it to a character called the simpleton, in a book he owed to Urban’s protection. To be sixty-nine and facing an Inquisition is a terrible place to be, and a good deal of what put him there was a lifelong habit of making enemies of people who had been perfectly prepared to be his friends. None of this earns the sentence he received. It does mean the man in the story is not a martyr of pure reason set upon by pure obscurantism, and the accounts that make him one are not doing history.