What he didn’t understand was the wider world—that Tübingen and the Lutheran world in general felt themselves to be under siege, that in their growing orthodoxy the faculty of the Stift were not particularly interested in new approaches. New ideas are always dangerous in unsettled times, and for every good idea that comes along, an army of resistance emerges from the dust to squash it back down. Kepler’s refusal to condemn the Calvinists over their doctrine of Communion made him suspect. His questions about the Lutheran ubiquity doctrine, which held that believers could receive the Body and Blood of Christ in Communion because Christ, being God, was everywhere, an idea that was a central part of the Formula of Concord, turned into a problem. Then he discovered Copernicus.
His philosophy professor at the university was Vitus Müller, who along with Martin Crucius taught him Aristotle and staged many of the students’ public disputations. In his first years at Tübingen, Kepler read the works of Nicholas of Cusa and found the Catholic heretic’s geometrical mysticism to be similar to his own. Crucius eventually led Kepler through the labyrinth, one by one introducing him to the philosophers who would dominate his thinking throughout his life—to Plato and the Neoplatonists and through them to Pythagoras. The secrets of the universe were in the shapes of things, in the geometry of the universe, and in that geometry there were harmony, order, and perfect reason. Throughout his life, Kepler would seek that harmony, and if he could never find it on earth, he would find it in the sky. He was impelled to do this, for it was central to his intellectual life. A boy who had grown up in a chaotic family in a world where Christians preached hell to one another from their churches on opposite sides of the town square, where old women were regularly accused of witchcraft, and where emperors and princes sent armies tramping across people’s farms and bloodying innocent peasants’ wheatfields could either despair of all order and all civility or spend his life looking, as Kepler did, for a place where the universe made music.
Crucius was enamored of the Greeks, because they were the first, the inventors, the originators of the Western intellectual world. Whatever we think, they thought it first. For Crucius, anything not found in Aristotle could not be true.12 Kepler accepted this for some time, so much so that Crucius asked him to collaborate on his magnum opus, a commentary on Homer, to help him interpret the astrological and astronomical allusions in the poems, but the work was never quite satisfying to Kepler. His own inclinations were too different from his teacher’s. Although both men were industrious and addicted to detail, Crucius was a gleaner, a gatherer of notions, while Kepler was a separator, splitting ideas into their purist form, skimming off the dross to find the gold.
Eventually this led Kepler closer to his final mentor, Michael Mästlin, who of all of Kepler’s teachers had the greatest influence on his life. Mästlin introduced Kepler to the Copernican universe, and for the young Kepler it seemed as if a new window had opened in his mind. Typical of Kepler, he instantly began taking the Copernican position in his university debates and getting himself into trouble:
In Tübingen, as I listened attentively to the lectures of the famous Magister Michael Mästlin, I saw how awkward in so many ways the customary notion of the structure of the universe had become. I was delighted, therefore, by Copernicus, whom Mästlin often mentioned in his talks, and I not only frequently promoted his views in the students’ debates, but also wrote a careful disputation concerning the thesis that the first motion [the revolution of the sphere of the fixed stars] comes from the rotation of the earth. I also set to work assigning to the earth on grounds of physics, or perhaps metaphysics, the motion of the sun across the sky, just as Copernicus had done on grounds of mathematics. To this end, I have bit by bit—in part out of Mästlin’s lecture, and in part out of my own thoughts—gathered together all the mathematical advantages that Copernicus has over Ptolemy.13
Meanwhile, as Kepler gushed about his new discovery and defended Copernicus as if he were a besieged city, Mästlin, the man who started Kepler along this track, stood by nonplussed. Mästlin was a short man, taciturn and introverted, who kept his more dramatic emotions calmly sealed away. He rarely smiled and yet was rarely angry. He had a high forehead and a narrow jaw, which made him look slightly bulbous. His hair was black, worn short, and he sported a bushy goatee, ubiquitous in the seventeenth century. He had made his fame by showing that a new star, a nova that appeared in 1572, had actually been a distant object and not some trick of the atmosphere. Conservative by nature, he was unwilling to part with Ptolemy completely, though he was aware of the growing complexity of the geocentric system, and he knew that even before Copernicus pressure had been building against the system for some time.
And yet no one had any proof. The weight of observation had been growing, seeding dissatisfaction among a number of astronomers, but such a momentous change would require a wealth of evidence. Copernicus himself had been afraid to publish until after his death. By Kepler’s day, there were at least four distinct models of the universe floating through the intellectual air that were whispered about or heatedly discussed by clusters of students in the smoky dark corners of beer halls. First, there was the official cosmos, the geocentric, finite universe of Aristotle and Ptolemy, reiterated by St. Thomas Aquinas. Then there was the infinite cosmos of Nicholas of Cusa, with God at the everlasting, omnipresent center. Third, there was the “heliostatic” universe of Copernicus, in which the planets, including the earth, orbited the sun, which was fixed in place. And finally there was the model resurrected by Tycho Brahe, first discussed by Plato’s student Heracleides Ponticus, in which the sun orbited the earth and the planets orbited the sun.14 Each of these systems had its supporters, and each had its detractors.
Surprisingly, the difficulty in adapting to the new placement of the earth in relation to the other heavenly bodies was not primarily that it spelled the downfall of human dignity. Later generations seemed to think that the geocentric model promoted the dignity of humanity’s place in the universe, as the apple of God’s eye, while the Copernican system turned this around and set the earth spinning meaninglessly through a meaningless universe. This is not quite accurate, for Aristotle never thought of the earth as a special place or the apple of anybody’s eye. The earth occupied the lowest position in the cosmos, where all things chaotic and all things corruptible eventually settled. The world beneath the sphere of the moon was the privy of the universe, where living things came into existence and then died away, where sooner or later all life returned to rot.15 Only the heavens were eternal; only the heavens were divine. Redefining the earth as a planet, as Copernicus did, actually set the earth into the heavens with the other planets and raised property values all around.
Modern people, however, cannot fully understand or appreciate the Aristotelian model without also seeing it within its wider metaphysical framework. When Plato looked at the human person, he saw a paradox. First, there was the body—material, corruptible, an instrument for use on the material earth. Then there was the soul—immaterial, incorruptible, sharing in some fashion with the eternal and divine Ideas through its capacity for reason. What it could know, even vaguely, it could share in. These two halves of the human person were incommensurable, blind to one another. They spoke different languages, perceived the universe in different ways. Aristotle accepted this dichotomy and accepted their union within the human person, but admitted that it was one of the greatest mysteries of all. Somehow, these two parts were joined together by an intermediary substance, an apparatus so subtle that it was corporeal and incorporeal at the same time, so subtle that it approached the immaterial soul in essence, shimmering in the dark, and yet was still a body, able to join with the flesh and pass along the commands of reason. This substance, this astral body, the pro–ton organon, the primary instrument relating soul to body, was made of the same stuff, the same spirit, pneuma, that made the stars. The spirit that moved the heavens, that moved the stars across the sky, was the same spirit that raised the human arm in greeting
or moved the human leg in walking. The vast sky was not dead, then, but subtly, luminously alive, and we in our gross flesh were in some small way cosmic beings.16
Perhaps finally the Aristotelian universe was simply familiar. And it was beautiful, if a bit creaky. It did a good job synthesizing the appearances, the phenomena that people actually saw when they looked into the sky, and it looked reasonably similar to the Babylonian cosmos of Genesis. It was comfortable. It worked well enough for weekdays and sometimes on Sunday. The problem was that it was getting too complicated, and no mathematician liked that.
Twenty-first-century people often imagine that the Copernican controversy was about science against the church, but the reality was far more complex. Science as we know it did not yet exist, and the church, Protestant and Catholic alike, was in fact the normal place for intellectual discussion. Nevertheless, society itself was changing, all too fast for most people. As if under a magnet, complex social forces aligned, at once pushing Copernicus forward and shoving him back. Friends and enemies of the sun-centered universe gathered in colleges and cathedrals across Europe and shouted at one another, something that Copernicus himself described: “Since the newness of the hypotheses of this work—which sets the earth in motion and puts an immovable sun at the center of the universe—has already received a great deal of publicity, I have no doubt that certain of the savants have taken grave offense and think it wrong to raise any disturbance among liberal disciplines which have had the right set-up for a long time now.”17
The changing cosmology scared people, affronted their sense of reason, a sense that derived its rationality, its worthiness for belief, from the single idea that God had set an order to things, an order to the heavens and an order to the earth. In the heavens, the stars and planets sailed on by force of the perfect divine will, the perfect divine law, which constrained them into circular orbits, the most perfect of all shapes. This was so, because it was the way things ought to be, the best of all possible worlds. On earth, the world was likewise ordered by divine will into church and state, into hierarchies of religious and political power. Even when Luther rebelled against the order of the church, he did so by relying on the order of the state, by turning the princes and dukes into “emergency bishops,” so that the order of the whole might be maintained. Beyond this was chaos.
Who could imagine such a state? The medieval world that lingered in Kepler’s day, although ancient and creaking, leaking oil and all too often rolling over the lives of peasants, overflowed with providence, with God’s special care for trudging mortals doomed to die. What could possibly replace that? No wonder astronomers stepped lightly, not only out of fear of the church, though that fear was well founded. They stepped lightly out of fear of unraveling the cosmos, of pulling on the wrong string and having the whole order of the universe collapse at their feet. Nevertheless, the generation of astronomers before Kepler and Galileo, the generation of Michael Mästlin and Christoph Clavius, could see the value of the Copernican system. Its elegance and simplicity appealed to their reason as much as it did to Kepler’s and Galileo’s. Although Clavius was more conservative and more philosophically subtle, basing his resistance to Copernicus on the differences between reasonable hypotheses and proven fact, Mästlin was more willing to dig at the root of the problem. His criticisms of Aristotle were dangerous and he knew it, so he broadcast them carefully. Aristotle was the systematizer who had set Greek reason to the Babylonian cosmology of the Bible, and long use of his work to build a Christian cosmology had nearly identified him with Scripture itself. To reject Aristotle and his astronomical interpreter Ptolemy was to remove the cotter pins holding together the structure of rational Christianity. Mästlin never challenged Aristotle’s value head on, but picked at the philosopher’s mistakes from behind, showing how, contrary to common belief, his system did not support observation, but rather complicated it.
In Mästlin’s lectures, the lectures that so influenced the life of Johannes Kepler, he traced the path that Aristotle had taken to create his system. “What happens to fire when it is lit?” he asked. “It rises.” In this, Aristotle too was following a long tradition set down by the Pre-Socratics, that heavy things fall and light things rise, just as air and water and earth mixed in a glass jar eventually settle with the earth at the bottom, the air at the top, and the water in between. Heavy things separate from light things and wet things from dry things, hot things from cool things, and light things from dark things. Therefore, said Aristotle, the earth must be at the center of the universe, for, as anyone can see, the earth is heavy and the air, which extends outward into the sky, is light. In Aristotle’s universe, and therefore in Ptolemy’s, the heavens are made of clear crystal globes that gear against each other and groan in mystical harmonies. The spheres sing a song that perhaps only God can hear properly.
We are steeped in hundreds of years of Copernicanism, so we cannot see another way. In this, we are like the churchmen who condemned Galileo and the theological faculty at Tübingen who worried over Mästlin’s lectures and later shook their heads over Kepler. But Ptolemy and his followers were not fools. They never tried to explain the heavens mechanically, but only to come up with a systematic account of all the phenomena that any observer could see by looking. The earth had to be the center of the universe, they argued, because otherwise the sky would seem different looking north and looking south or looking east and looking west, but it isn’t—it is the same in all directions.18 The stars are different, of course, but the spheres appear to be the same distance from the earth no matter which way you look. Ptolemy assumed that the sky was an actual physical sphere, and not endless space, but that assumption came to him from Aristotle. Moreover, Ptolemy argued, the earth cannot move from its central position, because all things move toward the center of the universe as the heavy things sort themselves out from the light things. The earth, therefore, is a massive, bulky, weighty thing, while the heavens are light and ephemeral. So which makes more sense—that the adipose earth move from place to place or even spin, or that the aerie heavens, luminous and eternal, sailing through the sky as they do by the action of heavenly spirits, move? What later seemed so costly to Copernicus—that the entire universe would turn around the earth—seemed reasonable and natural to the followers of Ptolemy, based upon the relative weight of the two systems.
Aristotle and Ptolemy had invented a system anyone could see at work in a glass jar, watching light things separate out from heavy. Then they could step outside and watch the heavens waltz across the sky like gods in evening wear, and they would say, “Of course!” But then there was this problem—the dances of the planets, the “wanderers” that didn’t follow the expected pattern. They were eccentric, sweeping across the dance floor in arcs of perfect circles as expected, but during the time of their “opposition,” when they were on the other side of the sun from the earth, they stopped, hand-jived backward, then reversed course and waltzed on. Ptolemy explained this strange retrograde motion by inventing circles, epicycles, whose center moves along a larger circle, called the deferent, whose center was the earth. Moderns are often stymied at this idea, wondering what the planets are orbiting in their epicycles, what points of gravity were holding the epicycles together, keeping the planets moving round and round like dancers, but although everyone could see gravity at work on the earth—heavy things fall and light things rise—they could not imagine it for the heavens. These little circles were descriptive of the appearances, each planet circling through its epicycles, all inside the planet’s crystalline sphere, in the space between the inner and outer walls. The spheres themselves rotated, moving the planets across the sky, but inside the sphere the planets circled on. The presence of epicycles in the system alone wasn’t the problem, however. The problem surfaced when, with increased observation, Ptolemaic astronomers needed to add more and more epicycles to the system to keep it working. For a long time, they required only twenty-seven epicycles, but by Kepler’s day, they needed nearly seventy—f
ar too complicated.
Kepler saw this at once. Though he loved theology and Scripture, though he could read in four languages (German, Latin, Hebrew, and Greek) and write in two (German and Latin), he found himself drawn toward the study of the heavens. He had always been good at mathematics, and since the day his mother took him to see the comet back in 1577, his mind and heart had been pulled toward the stars. There was beauty, majesty, and grace there. There was harmony—he could feel it—and he longed for simplicity. Real harmony, he reasoned, can never be that complicated.
Eventually, Mästlin loaned him his own copy of Copernicus’s De Revolutionibus Orbium Coelestium, On the Revolutions of Heavenly Spheres. This was quite an act of trust, for the book was rare and a little dangerous. Mästlin told Kepler how Copernicus had delayed publication, worried about the effect of his own ideas, and how on May 24, 1543, a friend put a newly arrived copy of the book into Copernicus’s hand as he lay on his deathbed. In the preface, Mästlin said, Copernicus had stated that his ideas were merely a useful means for calculating celestial events, a mathematical hypothesis. This was the same argument that the Jesuit Clavius used when discussing Copernicus with Galileo. But that is not how the rest of the book reads. Many astronomers and mathematicians suspected that Copernicus believed what he wrote and that he wrote the preface out of timidity.19 In truth, Andreas Osiander, whom Copernicus put in charge of the book’s publication, wrote the preface, and he did this without the permission of the author, which would likely have angered Copernicus, had he not been on his deathbed. The preface had the positive effect, however, of making Copernicus’s book more palatable, so that men like Clavius, Mästlin, and Brahe could study it without fear, since the preface itself said that Copernicus’s system was meant only as a device for calculation.20
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