Justified True Belief

(●) The imperial astronomer who found the true shape of the planets’ paths, and who called the finding an act of worship. + If you have ever seen a diagram of the solar system with the planets running on slightly squashed circles rather than perfect ones, you are looking at Johannes Kepler’s work. For two thousand years everybody who studied the sky, pagan and Christian alike, had assumed that heavenly bodies must move in circles, because the circle is the perfect figure and the heavens are the perfect place. Kepler spent the better part of a decade fighting that assumption, lost, and told the world so in print. The three rules he left behind still describe how a spacecraft swings around the sun. He is on a Christian apologetics site for a plainer reason than the astronomy. Kepler wanted to be a Lutheran pastor. He was steered into a schoolmaster’s job instead, and he spent the rest of his life insisting that the two callings were one calling: that reading the design of the heavens was a way of praising the designer, and that a man who traced a planet’s path carefully was doing something like what a man does who sings a psalm. He wrote prayers into his astronomy books. They are still there, printed between the geometry. One fact worth carrying away. In the years when Kepler was working out the law that ties a planet’s year to its distance from the sun, his mother was on trial for witchcraft in a Württemberg village, and he was writing her legal defense himself. The harmony he found in the heavens was not found in a quiet life.

(●) 1571 AD: Born sickly in a small imperial city, into a family that was coming apart. + Johannes Kepler was born on 27 December 1571 at Weil der Stadt, a free imperial city in Swabia, in what is now southwestern Germany. A free imperial city answered to the emperor directly and to no duke or count in between, which gave a small place a good deal of pride and not much protection. The family had come down in the world. His grandfather Sebald had served as mayor of the town; his father Heinrich hired himself out as a soldier, went off to the wars in the Netherlands, and at some point in Kepler’s youth simply did not come back. His mother Katharina kept herbs and knew remedies, a common enough trade for a village woman and, as things turned out, a dangerous one. Smallpox nearly killed him as a small boy and left him for life with weak and doubled eyesight and hands that never worked well. It is a strange fact about the man who redrew the heavens that he could not see them clearly. What he could do was calculate, for hours and days and years, with a patience that wore out everyone who worked beside him. Two childhood evenings stayed with him, and he wrote them down decades later. In 1577 his mother took him out of the house to look at the great comet hanging over the town, and in 1580 she woke him to watch the moon go dark and red in an eclipse.

(●) 1589 AD: He went up to Tübingen to train as a Lutheran pastor, and was sent out to teach arithmetic instead. + The dukes of Württemberg ran one of the best scholarship systems in Europe: a bright boy of no money could be carried through Latin school, then the monastery schools at Adelberg and Maulbronn, and on to the university at Tübingen, provided he meant to serve the Lutheran church. Kepler did mean to. He took his degree in 1591 and went on to the theological faculty expecting to end as a country pastor. At Tübingen he fell in with Michael Maestlin, the professor of mathematics, and Maestlin let him in on something that was not taught to undergraduates as true. Fifty years earlier Nicolaus Copernicus had published a system in which the earth is a planet like the others and the sun stands still at the centre, while everyone around him held the older view: a motionless earth at the middle of everything, with sun, moon and planets carried round it. You might reasonably ask how a system could be taught in the lecture hall and not believed. It happened constantly. Most astronomers treated the sun-centred scheme the way a navigator treats the lines of longitude drawn on a chart, as marks that make the sums come out and that nobody expects to find painted on the ocean: a convenient way of running the calculations rather than a description of the world. Maestlin thought it was a description of the world. So, at once and for the rest of his life, did Kepler. In 1594, halfway through his third and last year of theology, the school at Graz in Austria asked Tübingen for a mathematics teacher and Tübingen sent Kepler. He went unwillingly. He was twenty-two, he had wanted to preach, and he later wrote to Maestlin about the wrench of it: "I wanted to become a theologian; for a long time I was restless. Now, however, observe how through my effort God is being celebrated in astronomy." (Letter of 3 October 1595, as translated in Max Caspar’s Kepler.)

(●) 1595 AD: A figure he drew on a classroom board set him the question he worked on for the rest of his life. + By his own account the date was 19 July 1595. Kepler was at the blackboard in Graz, drawing for his pupils the slow circling dance of Jupiter and Saturn around the zodiac, the narrow band of sky along which the planets are always found, when he noticed that the pattern of lines he had chalked up left a smaller circle nested inside a larger one, and that the ratio of the two looked familiar. What if the spacing of the planets was not an accident? What if the six known planets stood at the distances they do because something geometrical held them there? The answer he reached is wrong, and it is worth understanding anyway, because it shows what kind of man he was. Pick up a die from a board game. Six faces, every one of them the same square, every corner exactly like every other corner: that is what geometers mean by a regular solid. The Greeks had proved something startling about such shapes, which is that there are exactly five of them in the whole of space and there can never be a sixth. The die is one of the five. The other four are built from triangles and pentagons, and they have four, eight, twelve and twenty faces. Now count the planets anybody knew about in 1595. Six of them, which leaves five gaps in between. Kepler set the five solids one inside another, slipping a sphere between each solid and the next so that every planet had a sphere of its own to ride on, and found that the spacing came out roughly right. He published it in 1596 as the Mysterium Cosmographicum, the secret of the universe, and it was the first wholeheartedly Copernican book to appear since Copernicus’ own, fifty years before. He sent copies to the men who mattered, among them Galileo in Padua and Tycho Brahe, the Danish nobleman who had spent twenty years measuring the sky more accurately than anyone in history. Tycho wrote back. The young man’s reasoning was far-fetched, he thought, but the mind behind it was worth having.

(●) 1600 AD: Refusing to change his religion cost him his post and sent him to Prague, where Tycho Brahe’s life work fell into his hands. + Graz sat in Lutheran territory ruled by a Catholic archduke, an odd arrangement that had been allowed to stand for a generation, and in 1598 Ferdinand of Inner Austria set about ending it. The Protestant school was shut and its teachers ordered out. Kepler was useful enough to be granted a reprieve, and he was told plainly what would settle the matter for good: convert, and the post is yours. He would not. In 1600 a commission gave the remaining Protestants of Graz a few weeks to conform or leave, and he left, with his wife Barbara, whom he had married in 1597, and a wagon of books. He went to Prague, where Tycho Brahe was now imperial mathematician to the Holy Roman Emperor Rudolf II, and the two men began an alliance that neither of them enjoyed. Tycho was rich, loud, and guarded about his data; Kepler was poor, proud, and wanted all of it. He was handed Mars, the planet whose path had defeated every astronomer who tried it. Then, on 24 October 1601, after a banquet and a short illness, Tycho died. Within days Kepler had his post, his salary on paper, and the thing he actually wanted: thirty years of observations, accurate to a minute or two of arc. A minute of arc is a sixtieth of a degree, a hair’s breadth of sky, and Tycho had spent his working life building instruments large enough and steady enough to hold that standard night after night. No one else in Europe owned anything like it.

(●) 1609 AD: After years on Mars he published the Astronomia Nova, with the ellipse and the sweep of equal areas in it. + He had told a colleague he would settle Mars in eight days. It took him the better part of eight years. He filled hundreds of sheets with arithmetic done by hand, tried one circular scheme after another, and got agonizingly close: a model that matched Tycho’s positions everywhere except at two points, where it was off by eight minutes of arc. Hold that error up against something you can see. The full moon measures about thirty minutes of arc across, so eight minutes is roughly a quarter of a moon’s breadth, a discrepancy no one would have noticed by looking. Every astronomer before him would have shrugged and moved on, and every astronomer before him would have been entitled to, because no earlier set of measurements was good enough for eight minutes to mean anything. Kepler knew whose observations he was holding, and he would not shrug. So he threw the circle away, and after a long detour through egg shapes and ovals he found that the curve Mars actually follows is an ellipse: the figure you get by looping a string around two pins and drawing it taut, a flattened circle with two centres instead of one. The sun sits at one of them. He found also that a planet does not travel at a steady speed. It hurries when it is near the sun and dawdles when it is far away, and it does so by an exact rule. Picture a line drawn from the sun to the planet, sweeping along as the planet goes, the way the hand of a clock sweeps round the face. Over any thirty days of a planet’s year that line covers the same amount of ground as over any other thirty days: a short fat wedge when the planet is close in and hurrying, a long thin one when it is far out and slow. Equal areas in equal times, as it is usually put. The two findings work as a pair. The ellipse says what shape the path is, and the equal areas say where along that path the planet will be found on a given night. The book came out in 1609 as the Astronomia Nova, the new astronomy, and its full title makes a claim no astronomy book had made before: a new astronomy based on causes, or celestial physics. Astronomy had always been the art of predicting where a light would appear. Kepler turned it into the study of what is pushing the light around.

(●) 1611 AD: He lost his son, his wife and his emperor in a year, and then spent six years defending his mother against a charge of witchcraft. + Smallpox came through the house in Prague, the same disease that had nearly killed Kepler himself as a small boy, and this time it took his son Friedrich. Soldiers came through the city in the same year, and the fever they carried with them took his wife Barbara. Then Rudolf II, the melancholy emperor who had protected him and rarely paid him, was forced off the throne by his own brother and died soon after, so that in the space of a few months Kepler lost his child, his wife, and the court that was the only reason to be in Prague at all. He asked permission to leave, and in 1612 moved south to Linz as district mathematician of Upper Austria, a salaried post whose duties ran to the yearly calendars and forecasts a provincial government expected from its mathematician. He married again in 1613, a young orphan named Susanna Reuttinger, and he was honest in a letter about the plain shopping list of qualities he had gone through before choosing her. At Linz the chief Lutheran pastor, Daniel Hitzler, refused him the Lord’s Supper, which is to say he was barred from taking communion at the Table with the rest of the congregation. The sticking point was the Formula of Concord, the document that set out what Lutherans held in common and that Lutheran clergy and teachers were expected to sign, and within it one particular teaching, that the body of Christ is present everywhere at once. That teaching was how the Lutheran party accounted for Christ being truly present in the bread. Kepler could not say he believed it, because he thought a body is by its nature somewhere, and he would not put his name to a sentence he did not hold. The signature would have settled the matter and opened the Table to him for the rest of his life, and he would not give it. He was equally unwilling to go over to the Reformed side, the Calvinist churches, whose account kept the body of Christ in heaven and made the bread a sign through which the Spirit feeds a believer on him; that, he thought, was too thin. So he stood between the two, welcome at neither. He appealed at last to Matthias Hafenreffer, his own old theology professor at Tübingen, who wrote back in 1619 that the door was closed. For a man who had wanted the pulpit, being turned away from the Table by his own teacher was the deepest wound of his life. While all that was going on, a second trouble had started in his mother’s village and was working its way toward him. In 1615 a woman in Leonberg who had quarrelled with the family accused his mother Katharina of giving her a bitter drink, and in a Württemberg village in that decade there was only one direction such a complaint could travel. The charge grew into witchcraft, the standing accusation against any old woman who knew herbs, and once it was made it could not simply be dropped. The case ground on for six years. In 1620 Katharina, then in her seventies, was arrested and kept chained in a cell for fourteen months; her son, the imperial mathematician, put his work down, moved to Württemberg, took over her legal defense himself, and argued the case on the evidence, one accusation at a time, until the court let her go in 1621. She died the following spring.

(●) 1618 AD: He found the harmony he had been hunting since Graz, and died twelve years later on the road, chasing wages he was owed. + The question he had asked at the blackboard in 1595 got its real answer twenty-three years later. On 8 March 1618 the relation between a planet’s year and its distance from the sun came into his head; he checked it, botched the arithmetic, and threw it out as false. On 15 May he came back to it, and this time it held. He published it the next year in the Harmonice Mundi, the harmony of the world, a strange and magnificent book that runs from pure geometry through musical theory to the heavens, because Kepler believed all three were the same subject. The rest was war and debt. The Thirty Years’ War broke out in the year he finished the book. Linz was besieged in 1626 and the printing house burned; he carried his papers to Ulm and brought out the Rudolphine Tables in 1627, the planetary tables built on Tycho’s observations and his own laws, which held a planet’s predicted place to a few minutes of arc where the older tables could be wrong by degrees, and remained the working tables of European astronomy for a century. He paid much of the printing himself, because the imperial treasury had been years behind on his salary for most of his career. In the autumn of 1630, at fifty-eight, he rode to Regensburg, where the imperial diet was sitting. The diet was the assembly at which the emperor and the princes of the empire met to settle the business of the empire between them, and it was the one place where a man owed money by the crown might get in front of the people who owed it. He went to press his claim for the arrears, the years of salary that had never been paid. He fell ill on arrival and died there on 15 November. The Protestant churchyard where he was buried was destroyed in the war a few years later and his grave has never been found. The epitaph survived because he had written it himself: Mensus eram coelos, nunc terrae metior umbras. Mens coelestis erat, corporis umbra iacet. I measured the skies, now I measure the shadows of the earth. The mind was of heaven; here lies the shadow of the body.

What He Taught

(T1) A planet travels on an ellipse, with the sun at one focus, and not on a circle at all. + Push two pins into a board, loop a string around them, and pull a pencil against the loop as you go round. The shape you draw is an ellipse: a circle that has been pulled out of true, with two special points inside it instead of one centre. Those points are the foci. Kepler’s first law says that every planet runs on such a curve, and that the sun sits at one focus, off centre, with nothing at all at the other. To see what this cost him, you have to see what he gave up. Circular motion in the heavens was not just a convenient assumption. It was a settled conviction, held from Aristotle through Ptolemy and right through Copernicus, that the sky is the region of perfection and the circle is the perfect figure, so the two belong together. Copernicus had moved the earth and kept the circles. Kepler kept the earth moving and threw the circles out, and he did it for one reason: the observations would not fit. He was candid in print about how long he resisted, writing that the truth of nature, which he had rejected and chased away, came back in by stealth through the back door in a disguise he was willing to accept. There is a habit of mind here that a Christian reader can borrow whole. Kepler had a beautiful theory and a stubborn fact, and he let the fact win, because the world is what God actually made rather than what a tidy mind expects him to have made. Eight minutes of arc were not enough to see with the naked eye. They were enough to overturn two thousand years.

(Q) "Now, because they could not have been ignored, these eight minutes alone will have led the way to the reformation of all of astronomy, and have constituted the material for a great part of the present work." + Source: Kepler, Astronomia Nova, ch. 19 (Donahue translation). He has just finished admitting that his best circular model fails at two points in Mars’ orbit by eight minutes of arc, and that he has no way of blaming the discrepancy on Tycho Brahe’s instruments. A few lines earlier he calls Tycho a gift of divine kindness and says it is fitting to acknowledge and honour the benefit with a thankful mind (T1).

(T2) The sun does not merely sit at the centre; it reaches out and moves the planets. + Kepler noticed something his predecessors had recorded without thinking much about it: the further out a planet lies, the slower it goes. Mercury races, Saturn crawls. On the old picture there was nothing to explain there, because each planet was carried by its own turning sphere and that was simply how the spheres turned. Kepler asked the childlike question. Why should distance from the sun make any difference to a planet’s speed, unless the sun is doing something to it? His answer was that a force streams out of the turning sun like the spokes of a wheel and drags the planets round with it, thinning as it spreads into the wider and wider spaces further out, which is why the distant ones lag. The details are wrong, and it is worth being exact about how. He took the force to be a kind of magnetism, and he had it shoving each planet sideways, along the direction of travel. What really holds a planet is a pull straight inward, toward the sun, bending a path that would otherwise carry on in a straight line and leave the solar system behind. Kepler imagined a sun that sweeps; the sun actually tugs. What is right, and what changed everything, is the shape of the question: a planet moves because something is making it move, and the astronomer’s job is to find out what. He said so on the title page of the Astronomia Nova, which announces a new astronomy based on causes, or celestial physics. He also saw further than his own machinery allowed. In the introduction to that book he sets out a definition of weight that is startling to read: heavy bodies are not seeking the centre of the universe, as Aristotle had taught, but seeking each other, and the pull runs both ways between them. He applies it to the tides and gets the moon right. Nearly eighty years later Isaac Newton would supply the law Kepler was reaching for.

(Q) "Gravity is a mutual affection between cognate bodies towards union or conjunction (similar in kind to the magnetic virtue), so that the earth attracts a stone much rather than the stone seeks the earth." + Source: Kepler, Astronomia Nova, Introduction (W. W. Bryant’s translation, 1920). Cognate bodies are bodies of one kind, a stone and the earth it fell from. The point of the sentence is the word "mutual." On the older view a stone falls because earthy matter belongs at the centre of the world and is going home. On Kepler’s view the stone and the earth are pulling on one another, and the earth wins only because it is vastly the bigger of the two (T2).

(T3) The length of a planet’s year is fixed by its distance from the sun, in one ratio that holds for all of them. + This is the law that made Kepler’s name among mathematicians, and it is easier to feel than to state. Measure everything in earth units, which is how it is usually done: the earth’s year counts as one year, and the earth’s distance from the sun counts as one unit of distance. Now take a planet, any planet at all. Square the time it takes to go once round the sun, take the cube of its average distance from the sun, and divide the first number by the second. For the earth both numbers are one, so the answer is one. Jupiter sits about five and two-tenths units out and takes just under twelve years to come round: that year squared comes to roughly a hundred and forty, five and two-tenths cubed comes to roughly a hundred and forty as well, and dividing one by the other puts you back at one. Do it again for Mars, for Saturn, for the rest. Six planets, one ratio, no exceptions. What makes this different from the nested solids of his youth is that it is not a pattern he liked the look of. It is a relation that generates numbers, which can be checked against the sky and can fail. It did not fail. And it hands you something for free: measure how long a new planet takes to go round, and you know how far away it is without going there. That is how the map of the solar system was drawn. Kepler put the law in a book about musical harmony, and readers have found that eccentric ever since. He did not. He had believed since Graz that the spacing of the heavens was laid out on a plan, and here at last was the plan, expressed in the same kind of ratio that makes two notes sound well together. He gives the dates of the discovery in the book itself, including the day in March when he got the arithmetic wrong and threw the answer away.

(Q) "But it is absolutely certain and exact that the ratio which exists between the periodic times of any two planets is precisely the ratio of the 3/2th power of the mean distances." + Source: Kepler, The Harmonies of the World, Book V, ch. 3 (Wallis translation). The three halves power is the same statement as squaring the years and cubing the distances, written the way a seventeenth-century mathematician wrote it. A planet’s mean distance is just its average distance from the sun, halfway between its nearest approach and its furthest remove, which is the one figure that stands in for a whole ellipse. A few lines earlier Kepler dates the discovery precisely: conceived on 8 March 1618, "unfelicitously submitted to calculation and rejected as false," and then "summoned back on the 15th of May," when it survived everything he could throw at it (T3).

(T4) Scripture describes the heavens as the eye sees them, and does not settle questions of astronomy. + Kepler knew exactly what would be thrown at a book that set the earth in motion, so he dealt with it in the introduction to the Astronomia Nova before anyone could throw it. Joshua prayed for the sun to stand still and it stood still (Joshua 10:12-13). The psalmist sings that God set the earth on its foundations so that it should never be moved (Psalm 104:5). If those sentences are astronomy, Copernicus is finished. Kepler’s answer is the one Augustine had already given in principle and the one most readers use without noticing. When Scripture touches on ordinary things it is not teaching a science lesson about them; it speaks about them the way people speak, so that people can understand. Joshua wanted the daylight to last, and he asked for it in the words anyone standing in a valley would use. The psalmist is praising the God who holds the world steady, and the steadiness he means is a moral and providential one, not a claim about orbital mechanics. A weather forecaster who tells you the sun will rise at six is not thereby a Ptolemaic astronomer, someone who holds that the sun really does ride round a motionless earth, and neither was the psalmist. None of this makes Scripture a lesser authority. Kepler is drawing a line around what a text is for. In the same passage he goes after astronomers who took the heavens to be a machine with nobody behind it, and he lets his own purpose show: the point of the whole exercise is to know the mind of God as far as a creature can. He simply declined to make the Holy Spirit an eyewitness to something the Holy Spirit had not undertaken to report.

(Q) "Now the holy scriptures, too, when treating common things (concerning which it is not their purpose to instruct humanity), speak with humans in the human manner, in order to be understood by them." + Source: Kepler, Astronomia Nova, Introduction (Donahue translation). The sentence opens several pages of close reading in which Kepler walks through Joshua 10 and Psalm 104 line by line. He was writing in 1609, six years before Galileo set out much the same argument in his letter to the Grand Duchess Christina, and twenty-four years before Galileo’s trial (T4).

(T5) God built the world on geometry, and made minds that can read it, which is why studying the sky is a form of praise. + Here is the conviction underneath everything else Kepler did, and it is a theological one. Geometry, he held, is not a human invention but something eternal in the mind of God, and God used it as the pattern when he made the world. That is half the claim. The other half is about us: a creature made in the image of God (Genesis 1:27) shares, in a small way, in the thoughts of the one who made it, which is why a man with paper and patience can work out what the heavens are doing. Kepler says in the Harmonice Mundi that this share in geometry is one of the reasons we are called God’s image. Take that seriously and the consequences follow quickly. The world will be intelligible, because a mind laid it out. It will be intelligible to us, because we were made to read it. And the reading will be worth doing for its own sake, as an act of homage rather than a means to a harvest or a horoscope. Writing to his patron Herwart von Hohenburg in 1598, Kepler called astronomers priests of the most high God in respect of the book of nature, whose business is the glory of the Creator rather than credit for their own cleverness. The proof that he meant it is in the books. A modern scientific paper ends with acknowledgements and a bibliography. Kepler ends the fifth book of the Harmonice Mundi with a prayer, asking pardon in case the beauty of the work has made him vain, and asking that his demonstrations serve God’s glory and the good of souls and get in the way of neither. Then he goes back to the geometry.

(Q) "O Thou Who dost by the light of nature promote in us the desire for the light of grace, that by its means Thou mayest transport us into the light of glory, I give thanks to Thee, O Lord Creator, Who hast delighted me with Thy makings and in the works of Thy hands have I exulted." + Source: Kepler, The Harmonies of the World, Book V, ch. 9 (Wallis translation). He is putting down his instruments at the end of the book, and says so: his eyes and hands are lifted from the tablet of demonstrations toward the heavens. The three lights are a piece of ordinary Lutheran teaching. Nature shows that there is a God, grace brings a sinner to him, glory is the seeing of him face to face, and Kepler is asking that the first lead him to the second and the second to the third (T5).

(Q) "The die is cast, and I am writing the book ... Let it await its reader for a hundred years, if God Himself has been ready for His contemplator for six thousand years." + Source: Kepler, The Harmonies of the World, Book V, Proem (Wallis translation). A proem is a preface, the few pages an author sets at the head of a book before the work itself begins. Writing at Linz in 1618, with the war starting around him and few readers in sight, Kepler says he is taking the mathematics of the pagans the way Israel took the gold of Egypt on the way out (Exodus 12:35-36): "I am stealing the golden vessels of the Egyptians, in order to build of them a temple for my God, far from the territory of Egypt." The hundred years turned out to be about seventy. Newton was born twelve years after Kepler died.

What Christian Thinkers Made of Him

(†) In his own lifetime both churches shut a door on him, and he went on writing. + Kepler was a convinced Lutheran who was refused the Lord’s Supper by Lutheran pastors for the last eighteen years of his life. He had been ejected from Graz for refusing to become a Catholic, so nobody could accuse him of softness; the trouble was that he would not sign a statement he thought went beyond what Scripture said, and his church required the signature. He tried for years to get the ruling reversed, arguing that the Supper should be common ground among Christians rather than a fence, and he lost. Rome closed the other door. The Epitome of Copernican Astronomy, his textbook of the new system, went onto the Index of Prohibited Books on 28 February 1619, a few months after the first part appeared and three years after Rome had suspended Copernicus’ own book pending correction. The Index was the official list of books a Catholic was not to read without permission, and a title put on it stayed on it until somebody troubled to take it off. Kepler’s remained there, along with the other heliocentric works, the books that set the sun rather than the earth at the centre, until the nineteenth century. He was not made bitter by any of it, so far as the record shows. He kept the creed, kept the prayers in his books, and kept asking to be let back to the Table. It is worth putting that beside the familiar story in which the churches of the seventeenth century simply crushed inquiry. What actually happened to Kepler is both sadder and more ordinary: he was squeezed by two confessional machines in a century of religious war, and he neither renounced his faith nor stopped his work.

(†) Newton took his three rules and showed that one law produces all of them. + For most of the century after Kepler died, his ellipses were treated as a useful calculating device by men who did not accept his physics. Then in 1687 Isaac Newton published the Principia and the situation reversed. Given one law, that every body attracts every other with a force falling off as the square of the distance, Kepler’s three rules are not assumptions at all. They are consequences. The ellipse, the equal areas, the ratio of years to distances: each drops out of the mathematics. That is about as complete a vindication as a scientist ever receives, and it turned Kepler’s laws into the standing example of what a law of nature looks like. Notice that the phrase itself is a theological inheritance. A law implies a lawgiver, and the seventeenth century used the word deliberately: Kepler, Descartes, Boyle and Newton all understood the regularities of nature as ordinances laid down by God, in the way a king’s decree governs a kingdom. The usage has outlived the belief. Physicists still speak of laws. The practical reach is easy to underrate. Kepler’s Rudolphine Tables let astronomers predict a transit of Mercury across the face of the sun in 1631, the year after he died, and Pierre Gassendi watched it happen. The same three rules put a spacecraft into orbit around Mars today.

(†) Christians ever since have pointed to him as the man who showed that the search for causes can be worship. + There is a popular story in which religion and science have been at war since the beginning, with the church always in the way. Kepler is one of the hardest facts for that story to digest, because his faith is not a private opinion he held alongside his work. It is the reason the work took the shape it did. He expected the heavens to be orderly because they were made; he expected to be able to follow the order because he was made; and he wrote prayers into the middle of his proofs because he thought the whole activity was praise. Take the theology out of Kepler and the astronomy has no motive left. Historians of science have made that argument in a stronger form. Alfred North Whitehead, lecturing in 1925, traced the modern conviction that nature is thoroughly law-abiding back to the medieval insistence on the rationality of God, and Stanley Jaki, a Benedictine priest and physicist, spent a career pressing the same case, tracing the birth of modern science to the Christian doctrine of creation. The argument can be overdone, and it is not a proof of Christianity. What it does establish is that the first generations of modern scientists were not defying their faith when they went looking for laws. They were acting on it. One caution belongs here. Kepler is constantly quoted as having said he was "thinking God’s thoughts after him," and the sentence has never been found in anything he wrote. It is a fair summary of his outlook and a false quotation, and he is well enough supplied with real ones that nobody needs the invented version. Psalm 19:1 says what he thought, and he knew it: "The heavens declare the glory of God, and the sky above proclaims his handiwork."

(†) Where his own record falls short: he kept casting horoscopes, and he read providence into a pattern that was not there. + Two things in Kepler’s own work will not survive a Christian reading, and both are his, not his readers’. - Astrology. He cast horoscopes for forty years, for emperors and for a general, and he defended the practice in print. Casting a horoscope meant drawing up the exact arrangement of the sky at the hour of someone’s birth and reading a character, and often a future, out of the figure. Two things can be said for him, and they are both true. He attacked the popular sort with real contempt and wanted the whole business reformed, and the work paid part of his salary; he said himself that the wise mother, astronomy, would starve if the foolish daughter earned nothing. But he did not treat it merely as a wage. He held, and argued in the Tertius Interveniens of 1610 and in the Harmonice Mundi, that the angles between planets stamp something on the soul at birth. Scripture puts that among the practices Israel was to have nothing to do with, and names the stargazers as counsellors who cannot save (Deuteronomy 18:10-12; Isaiah 47:13). The right of a Christian to study what God has made does not extend to reading fates in it. - Forcing the design. The five solids of the Mysterium Cosmographicum were a guess about God’s intentions that the numbers seemed to bless, and Kepler never let it go. He reissued the book in 1621, a quarter of a century and three laws later, with notes defending the scheme. A guess of that kind is not wicked, and his own honesty about the eight minutes of arc (T1) is the cure for it. But it is the recurring temptation of religious science, and he fell into it: deciding in advance what the Creator must have found beautiful, and then reading the creation to agree. Neither failing is hidden, and neither needs to be excused. Kepler is more useful as he actually was, a brilliant and superstitious and painfully honest man who kept correcting himself in public, than as the tidy saint of science that later admirers have sometimes made of him.