Post-classical history

Reforming the Heavens

The sun (or earth, depending on your point of view) around whom Europe’s astronomers turned was the Jesuit professor Christopher Clavius (1538–1612). He had been a young technical adviser to the calendar reform commission but, by the time the change actually took place, he was considerable less junior and was put in charge of explaining the new system to the Catholic world. He also composed a textbook on astronomy, organised as a commentary on Sacrobosco’s The Sphere, that he kept up to date throughout his long career.1

Astronomy at the time Clavius was active, the late sixteenth century, was in a state of turmoil. Copernicus’s radical proposal for a moving earth was initially not the main issue that was preoccupying astronomers. Their problem was rather a slew of celestial prodigies that either refused to obey the theories of Aristotle and Ptolemy or else just should not have been there at all. Clavius’s textbook makes an excellent guide to the ferment that engulfed astronomers during his lifetime.

Comets and Supernovas

Two heavenly marvels arose in the 1570s that challenged existing ideas. The first was a nova, or new star, in the constellation of Cassiopeia and the second a comet. Until these events, everyone agreed that the heavens were unchanging and the planets had always followed the same paths since the beginning of time. It was impossible for a new star to appear and as for comets, Aristotle had said that they were confined to the atmosphere.2 Medieval people found no contradiction in believing that comets and eclipses were completely natural phenomena and also signs from God.3 When Halley’s Comet appeared in 1456, anxiety about the Turkish conquest of Constantinople was still high and the Turks themselves were encamped outside Belgrade. The omen in the sky only heightened fears that catastrophe was imminent. Nonetheless, the story that Pope Callistus III (1378–1458) took the desperate measure of excommunicating the comet is just a legend.

Jerome Cardan was one of the few authorities who disagreed with Aristotle and suggested that comets might originate among the stars rather than in the atmosphere.4 But his opinions carried little weight with astronomers. Few Europeans had noticed the ‘guest stars’ in AD185, 1006 and 1054 that were carefully recorded by contemporary Chinese and Islamic astronomers. This was probably because the western belief that the heavens are fixed was completely incompatible with the appearance of new stars. On the other hand, the nova of 1572 (nova, of course, is just Latin for ‘new’) was impossible to ignore. For the most dedicated skywatchers it presented a remarkable opportunity to test the theory that the heavens never changed.

The 1572 event was what we now call a supernova – a massive star exploding with a brightness millions of times greater than our sun. Even though it occurred 7,500 light years away, the 1572 supernova could be seen in daylight and was brighter than Venus. In all, it was visible for about fifteen months although it faded rapidly after the initial explosion. Astronomers all over Europe carried out observations on it. At the time, the big question that everyone wanted answered was: where was the nova located? According to the traditional view, it should have been an atmospheric aberration, but careful observations proved that this was impossible. Over the fifteen months for which the nova was visible, astronomers made extremely careful measurements of its position relative to its neighbouring stars. Clavius collated all this information and found that the position of the nova was exactly the same for all observers. That meant that the nova had to be beyond the moon and the doctrine that the heavens could not change was proven false. Clavius himself cautiously acknowledged this in the 1585 edition of his textbook.

If it is true [that the nova is a new star] then Aristotle’s followers ought to consider how they can defend his opinion about the matter in the heavens. For perhaps it should be said that the heavens are not made of a fifth element but rather changeable bodies – albeit less corruptible than the matter here on earth … Whatever it finally turns out to be (and I do not insert my opinion into such matters) it is enough for me at present that the star we are talking about is located in the sphere of the fixed stars.5

The result was confirmed by a further nova that appeared in 1604.

Then, in 1577, a huge comet appeared in the sky. Surprisingly, Clavius hardly mentioned it in his textbook although many other astronomers scrutinised its movements. The most famous of these men was an irascible Dane called Tycho Brahe (1546–1601). Tycho was born in southern Sweden, then ruled by Denmark, to a noble family with close ties to the Danish throne. He was brought up by his uncle and sent to the university of Copenhagen at the age of thirteen. There he saw a partial eclipse of the sun which fascinated him – he was impressed not so much by the event itself as by the fact that it had been long predicted. He bought some textbooks on astronomy, including the Almagest, and set out to discover as much about the subject as possible. His education continued in Leipzig and he also took the opportunity to travel widely in Germany and Switzerland. By 1572, his father and the uncle who had brought him up were both dead.6 Tycho returned to Denmark and found that he had inherited money and so had leisure to further his research interests. When the nova exploded in early November of that year he was ready.

Like everyone else, Tycho wanted to measure the parallax of the nova. This was an attempt to calculate the distance to the nova by carefully observing how it moved against the background of the fixed stars. Unlike others, however, he used a potentially more accurate technique. The further apart the observations are made, the better the resulting parallax measurement. Clavius had correspondents throughout Europe so his observations were separated by a couple of thousand miles – from Sicily to Germany. Tycho made his observations 10,000 miles from each other without ever moving from Denmark himself. As the earth is rotating (Tycho thought that the whole sky rotated and the earth remained stationary, but the principle is the same), it followed that taking his observations twelve hours apart meant that he obtained the maximum distance between them. This would not have been an easy task at all, but Tycho was an extremely patient and skilled observer with the money to have his own custom-made instruments built for him. But even with this technique, Tycho could see no parallax of the nova at all. It was simply too far away.7

Tycho distributed copies of his work among the intelligentsia of Denmark, although it did not reach Rome where Clavius was working. This advertisement of his genius had the desired effect. The king was keen to keep this adornment to his majesty in Denmark. Tycho wanted a dedicated observatory where he could get on with his work and in 1576 the king presented him with his own island and enough money to build an entire research centre there. Over the next twenty years, the 2,000-acre island of Hven, sandwiched between Denmark and Sweden, was the scene of his life’s work.8

Tycho had already realised that the medieval astronomical tables were not accurate enough. The date given for one conjunction of Jupiter and Saturn in 1563 was out by a month.9 With mistakes this large, it was no wonder that astrologers were unable to do their job properly. He set out to complete a set of new observations of planetary movements from his base on Hven. At the same time, he could keep an eye out for any other astronomical novelties as they appeared. He did not have to wait long for the first one – the comet of 1577. Tycho again carried out his observations of the comet’s parallax and proved that it must be a heavenly phenomenon as well. He placed it beyond Venus. But unlike the nova, comets are certainly moving and the path of this one took it through the orbits of the other planets. If the planets really were carried on the surface of giant solid spheres, the comet must be passing straight through them. Peurbach’s idea of impenetrable shells was by no means universally accepted by astronomers, but Tycho’s observations of this and other comets seriously undermined the theory for those who believed it. Tycho himself took a long time to accept this implication and did not even publish his work until 1588.10

After twenty years of having free rein over the management of his research centre on Hven, Tycho fell out with the new king of Denmark. He was now so famous that he had to put up with a constant stream of visitors to his island. He had also become an expensive indulgence for the king who cut back his allowance and insisted on economies. Tycho was not happy and neither was he the sort of man to take a perceived insult lightly. In his youth, he had lost part of his nose during a duel over a trivial matter that history does not even record. Annoyed with the king, he departed in search of a new benefactor. Tycho’s reputation was such that he was quickly adopted by the richest patron of all, the Holy Roman Emperor Rudolf II (1552–1612). Together with his instruments, students and records, Tycho moved to Prague in 1599.11 He had hardly settled in when he died without ever publishing his two decades’ worth of observations. He had, however, brought out a volume of his preliminary findings in which he suggested a radical remodelling of the solar system.

Recall that Ptolemy and his followers, such as Clavius, believed that all the planets, as well as the sun and moon, orbited the earth. Tycho realised that his observations made this impossible. He continued to place the earth stationary at the centre of the universe with the sun and moon going around it. But the other five planets – Mercury, Venus, Mars, Jupiter and Saturn – he had orbiting the sun instead. This arrangement had one major drawback. In order to get the model to match his observations, Tycho had to accept that the orbit of the sun intersected that of Mars. This was impossible if the heavens consisted of a set of solid spheres as Peurbach believed. Tycho initially despaired, but then he realised that he already had the data to reject solid spheres from his work on the comet of 1577.12

Clavius, however, stuck firmly to the solid spheres of Ptolemy. He rejected Tycho’s model although he did praise him as ‘a distinguished astronomer’.13 His reaction to Copernicus, whom he called ‘that eminent restorer of astronomy in our era’, was more complicated.14 Clavius rejected the idea that the earth moved, but he was happy to use Copernicus’s mathematical ideas, suitably adapted, to amend his Ptolemaic picture of the universe. Clavius’s textbook goes to the trouble to refute Copernicus at considerable length but does not bother even to mention Tycho’s alternative.15

The Magnetic Earth

Although Clavius’s discussion of Copernicus’s theory was unusually full, he was not the only astronomer to refer to it. In England, whether or not the earth moved was a matter debated at Oxford University by the 1570s and the question is also discussed briefly in English-language astronomy textbooks from this period. None of this meant that there were many people who agreed with Copernicus. Before 1600, hardly anyone did. But one of those few was an English doctor called William Gilbert (1544–1603).

Gilbert had entered St John’s College, Cambridge in 1558. At this time, St John’s was the most mathematical of Oxbridge colleges and even had a paid examiner in the subject. We know that Gilbert himself held this position in 1565 because he had to sign his name in the college’s register. He does not appear to have rated mathematics as a useful adjunct to science and went on to earn a medical degree in 1569.16 In the 1570s he built up his practice and entered the London College of Physicians by 1581. This meant that he was one of the top doctors in the city – wealthy, well connected and with leisure to carry out experiments.

His intellectual development is difficult to pin down because we do not have many documents from his early years. We do know that he rejected the materialist philosophy of Aristotle in favour of something much more like the magical worldview that we discussed in earlier chapters. Gilbert started off with some notes attacking Aristotle’s views on the weather, but soon the occult property of magnetism drew his interest. He set out to investigate it as closely as he could.

We have already noted how the compass had appeared in Europe during the twelfth century and was an indispensable aid to navigation. That it worked by magnetism was not doubted. This had led some scholars to suggest that it pointed north because there was a giant magnetic mountain in the Arctic region that attracted compass needles. Others, like Peter the Pilgrim, had thought that compasses pointed towards the celestial pole and that they aligned themselves with a heavenly magnet. In the sixteenth century, these questions began to have an urgent practical significance, because as European explorers moved further away from familiar waters they found that their compasses started doing strange things. Magnetic north is inclined at about 11° to true north. Thus, the deviation between true north and the direction in which a compass points is not the same from place to place, a phenomenon known as ‘variation’. Worse still, variation was not regular because, as we now know, the earth’s magnetic field is not regular either.17

We have also seen how medieval thinking about magnetism was limited by the fact that it did not cohere with Aristotle’s materialistic philosophy. No one could deny that magnetism existed, but explanations for occult properties still tended to be marginalised as magical. That is why natural philosophers failed to take forward the work of Peter the Pilgrim. His letter on magnetism was printed in 1558 in an edition by Jean Taisnier (1508–62). Taisnier, although a priest and noted scholar in his own right, passed off Peter’s work as his own in one of the most notorious cases of plagiarism of the sixteenth century.18 The remarkable thing is that he thought that the knowledge of medieval scholarship was now so slight that he would be able to get away with such a blatant act of intellectual larceny.

Gilbert knew Peter’s treatise well and based some of his own experiments with a spherical magnet on its suggestions. Even so, he dismissed the earlier writer as ‘fairly learned for his time’ and did not acknowledge the source from which he had borrowed his ideas.19 That said, Gilbert took matters much further than Peter and coupled his experiments with some intriguing theoretical suggestions. He showed that he could model compass variation with his spherical magnet. This meant that the earth was possibly a giant magnet too, suffused with the occult property of magnetism.

Gilbert’s painstaking work has become a paragon of experimental science. It is no surprise that his experiments were geared towards the investigation of an occult property ignored by Aristotle. The laboratory tradition that inspired Gilbert belonged to alchemists and magicians, not bookish natural philosophers. The ancient Greek prejudice against men who used their hands to make a living still caused university men to sneer at artisans, just as doctors looked down on surgeons. Gilbert took the laborious experimental techniques handed down by generations of alchemists, stripped out their bogus metaphysical speculations and emphasised their virtues of patience, manual skill and meticulous record-keeping. He was well aware that experiments could be extremely hard work. He repeated the same procedure over and over again, explored every possible modification and warned his readers not to despair if they could not repeat his results on their first attempt. Finally, he carefully distinguished magnetism from static electricity and kept his empirical data segregated from theory. This last point is important because Gilbert’s theoretical ideas are considerably less impressive than his experiments. The earth, he believed, was not only a giant magnet, it was alive. Magnetism was the soul of the earth, its ‘astral magnetic mind’.20 Unlike Aristotle, for whom the earth was dead, inert and still, Gilbert saw it as moving, active and aware. He thought that the earth turned itself so that all of its surface would be warmed by the sun’s rays. The whole solar system was an arrangement of magnetic beings that were propelled through space by the force of magnetism.

Published in 1600, Gilbert’s book On the Magnet covered his investigations and discoveries in exhaustive detail. It was the first work of natural philosophy by an Englishman to achieve European renown since those of the Merton Calculators. Gilbert was also one of the few convinced Copernicans of the sixteenth century. By ignoring his debt to the medieval Peter the Pilgrim while emphasising his experiments and Copernicanism, historians have portrayed Gilbert as a thoroughly modern man of science. However, we can only maintain this picture if we ignore his place in the tradition of occult properties and his own overarching philosophical position. To be fair, he only wrote explicitly about all of this in his later treatise The New Philosophy, which never enjoyed anything like the popularity of On the Magnet. However, the clues are there, even in the earlier book.

Gilbert’s theories especially appealed to another inheritor of the European medieval tradition who used them extensively in his own cosmological speculations. That man was an assistant of Tycho Brahe called Johann Kepler.

Kepler’s Early Career

Born in 1571 near Stuttgart in Germany, Johann Kepler was initially raised by his grandfather while his father, a soldier, was away on campaign. The comings and goings of his parents meant that he had a disjointed youth with little money available. However, he was a bright child and managed to win a scholarship to a secondary school and thence to the university of Tübingen. His eventual aim was to become a priest.21 Religion was the central theme of Kepler’s life – both his own beliefs and the social climate in which he lived. He was an unusually devout man at a time when strong religious views were commonplace.

In the aftermath of the Protestant Reformation, the religious geography of the Holy Roman Empire was a mess. Germany, which made up the bulk of the Empire, was a patchwork of statelets each independently ruled but owing nominal allegiance to the Holy Roman Emperor. In 1600, this was Rudolf II whose seat was in Prague. It was Rudolf who had offered Tycho Brahe a new job when the cantankerous astronomer left Hven.

Each ruler determined the religion of his state, as well as how rigorously he would enforce the local orthodoxy. Although basically a follower of Luther, Kepler did not subscribe to all the official doctrines of the Lutheran church. This made him potentially unwelcome in both Catholic and Lutheran jurisdictions, although in fact he spent most of his life employed by Catholic rulers who found him too useful to persecute.

At Tübingen, Kepler received the standard mathematical education that Lutheran universities offered. The astronomy professor who taught Kepler was called Michael Maestlin (1550–1631), who was part of the international correspondence network that included Tycho and Clavius. As well as the official classes, Maestlin kept his best students, including Kepler, informed about the latest happenings in astronomy. From his professor, he learnt about Copernicus’s hypothesis of a moving earth as well as the comet and supernova that had challenged received wisdom.22 Kepler was so obviously gifted as a mathematician that the university offered him a job teaching the subject at one of its satellite seminaries in Austria for a couple of years before he started his theological training. As it turned out, he never did return to theology because of his disagreements over Lutheran dogma.

Kepler was a theorist and not an observer. His eyesight had been damaged by a childhood illness, which made him completely unsuited to skywatching. As a mathematician, however, he had very few equals and could successfully manipulate vast amounts of data. In 1596 he published his first book, The Mystery of the Universe. For Kepler, the most important fact about the world was that God had created it. Like Copernicus, he was convinced that the structure of the heavens had to reflect the perfection of its creator. This perfection, he thought, would reveal itself best through the precision of geometry.

Also like Copernicus, Kepler put the sun at the centre of the universe. He then arranged the planets around the sun, but with the size of their orbits determined by the five basic solids – the tetrahedron (four triangular sides), cube, octahedron (eight triangular sides), dodecahedron (twelve pentagons) and icosahedron (twenty triangular sides).23 In his model, Kepler imagined that the orbit of Mercury would just fit within an octahedron and that the orbit of Venus, the next planet out, would just fit outside it. The orbit of Venus, in turn, was encased by an icosahedron outside of which orbited the earth. Next he placed a dodecahedron, then a triangle and finally a cube, with a planet between each one. Saturn, the most distant planet, he believed orbited outside the cube. The neatness of this arrangement seemed to provide additional evidence of the providence of God but was not precisely correct. For Kepler, even a small error rendered the model not quite good enough. No one else was convinced by his idea of using the five regular solids either, although the book did receive some good reviews.

14. Kepler’s first model of the universe based on the five basic solids from The Mystery of the Universe (1596)

To complete his work and find the perfect fit for the planets’ paths, Kepler needed the astronomical data that Tycho Brahe had devoted a lifetime to collecting. Tycho himself was notorious for guarding his figures but after some troublesome negotiations, Rudolf II accepted Kepler as Tycho’s official assistant. They were to collate all the data and prepare it for publication as the Rudolphine Planetary Tables.24

The working relationship between Kepler and Tycho was not good but within six months, the older man was on his deathbed. During a lucid moment before a fever carried him away, Tycho handed over the project of publishing the tables to his assistant. Shortly afterwards, the Emperor appointed Kepler as his new imperial mathematician.25 The real treasure, Tycho’s twenty years of observations, remained the property of the Brahe family, who caused endless trouble for Kepler with demands for money and a say in how the tables were published. After 25 years, the conditions attached to the use of the observations could still not be agreed and so Kepler went ahead and published them anyway.26

Kepler’s Model of the Solar System

In the next few years, Kepler finally cracked the problem of how the planets move, although no one took much notice. Now that he had access to Tycho’s observations, Kepler could test his ideas with data of unprecedented accuracy. Concentrating on the orbit of Mars, Kepler found that his best model, based on Copernicus and including eccentrics and epicycles, was out by eight minutes of arc, which is about two fifteenths of a degree.27 Kepler was nothing if not pedantic and an eight-minute error would not do. So important was this small difference to Kepler’s success that he later called it a ‘good deed of God’s’.28

We can summarise the system of astronomy that had been inherited from the ancient Greeks with three axioms: the immutability of the heavens, circular orbits and uniform motion. Thanks to the supernova of 1572, immutability had already been dismissed by Clavius and Tycho. Many sixteenth-century natural philosophers also thought that the other two axioms were either untrue or irrelevant. A rising tide of scepticism made out that it was impossible to accurately map the planets and certainly to know how they really moved. Astronomy was just a matter of trying to construct the best mathematical model of the planets’ observed paths across the sky.

Kepler rejected the defeatism of the sceptics although his reasons were religious rather than scientific. As far as he was concerned, the heavens must reflect their maker. ‘For a long time, I wanted to be a theologian’, he wrote, ‘now however, behold how through my effort God is being celebrated through astronomy.’29 As the Bible itself states: ‘The heavens declare the glory of God; and the firmament shows his handiwork.’30 There was no imprecision about God and he did not make eight-minute mistakes. Nor was he the capricious sort who would make the heavens into an unsolvable puzzle. If the paths of the planets were ordained by God, then they must be simple and elegant. In keeping with his faith, Kepler was absolutely unwilling to abandon the axiom of uniform motion. But the need for the planets to move in circles was a Greek addition to that basic principle and he could drop it without compromising his belief in God’s fidelity.

Copernicus had also derived his astronomical ideas from his theology. However, for all his calculations, he had failed to achieve total empirical accuracy because he was committed to movement in circles. Now Kepler completed the chain between religion and science. His ideas about God provided his hypothesis, he had the mathematical ability to turn his ideas into a system and, at last, Tycho’s data meant he could check to see if his system was actually true.

And so, after a great deal of work, Kepler found exactly how the planets move. Like Copernicus, he realised that a planet’s orbit could not centre on the sun and that its uniform motion was not its absolute speed. In fact, planets move faster when they are close to the sun than when they are further way. Kepler found that the axis of a planet’s orbit swept through an equal area in any given time. This was the uniformity that he was looking for. His greatest insight was that orbits are not circles, or even based on circles, but ellipses. An ellipse is a kind of oval that has two focuses. The sun is always found at one of them. Kepler later found a mathematical relationship between the length of time it takes planets to orbit the sun and their distance from it. These three rules – that orbits are ellipses, that the axis of an orbit sweeps through a uniform area and that the period of time an orbit takes is related to its size – are now called Kepler’s Laws.31 With them, astronomers could work out anything they wanted to about the movements of the planets.

Publicising his ideas was not Kepler’s strongest point. He wrote voluminous tomes but they are practically unreadable. His laws are not hard to explain but you would never guess this from his convoluted explanations. Part of the trouble was with Kepler’s religion. He saw his science as a religious duty and wrote as if it was a complicated piece of theology. His notebooks are even worse. Sheet after sheet of calculations are punctuated with mystical speculation and prayers. Nevertheless, it remains true that Kepler cracked the mystery of the planets’ movements because of his faith in God’s creative power.

The difficulty of Kepler’s work meant that it took a long time to penetrate Europe’s consciousness. His theories only became accepted later in the seventeenth century, not so much because of his books but because of what he did with Tycho’s data. In 1627, he finally published the new astronomical tables of planetary positions that he had promised Tycho he would complete. Their accuracy was way beyond anything previously seen and they quickly became the standard for astrologers all over Europe (astrology, of course, being the main market). It was this success that sold Kepler’s laws to the public. His tables, based on his theories, were found to predict the future positions of the planets so precisely that it seemed inconceivable that they did not reflect reality.32Gradually, it became accepted that the planets, including the earth, did indeed orbit the sun in elliptical orbits.

Of course, Kepler was fallible. He stuck with his hypothesis about the regular polygons even though they did not fit the data. And he adopted Gilbert’s theory that magnetism provided the motive power of the solar system.33 We now know that it is the force of gravity and not magnetism that keeps the planets in their tracks, but at least Kepler realised that there had to be something holding it all together.

Explaining Vision

Kepler’s achievements went beyond the field of astronomy. He also solved one of the central problems of optics. We saw in chapter 9 how Roger Bacon had combined Greek and Arabic optical ideas in the thirteenth century. Although he had not really advanced beyond the theories of the Islamic scholar Alhazen, his work had been incorporated into Witelo’s Ten Books on Perspective which was the main university textbook by Kepler’s time. It set out the flawed medieval theory that only non-refracted rays hitting the eye head-on are impressed on the retina and contribute to what we see. In 1604, Kepler published his Supplement to Witelo that amended the theory to what we believe today. Rather than non-refracted rays being ignored, Kepler said that the lens in the eye bent all the rays from a particular point so that they all ended up at the same spot on the retina. Perpendicular rays were not bent at all, but the lens refracted all the others exactly as much as necessary in order to produce a focused image.34

Spectacles had now been around for three centuries and the way they worked was well understood. What is special about the lens in our eyes is that it grows thicker and thinner to focus on objects at varying distances. Kepler also realised, as a corollary of his theory of vision, that we actually see everything upside down. Luckily, our brains rectify the image that appears on our retinas. An experiment quickly confirmed this result using a bull’s eye lens in a camera obscura. This too produced an inverted image.

Kepler’s interest in light had much to do with its central position in astronomy. Tycho had pioneered optical techniques in his observations and Kepler could also draw upon the medieval mathematical tradition. His insights later went towards an improved design of the telescope which, like the eye, produced an inverted image. However, his fascination with light was also related to the role it played in religious speculation. Roger Bacon had been convinced that light was the emanation of God and Kepler agreed. It was, after all, the first of God’s creations. ‘Let there be light’, he had said. And there was light.

Following these scientific triumphs, Kepler’s life fell apart. In 1611, all three of his children caught smallpox and his six-year-old son died. He was followed to the grave by his grieving mother, Kepler’s wife, within months.35 At the same time, he lost his comfortable situation as imperial astronomer when Rudolf was deposed as ruler of Prague. The Emperor died the following year.

Although Rudolf’s successor as Emperor was a zealous Catholic, the Protestant Kepler initially kept his job as imperial mathematician as a result of his fame and skill as an astrologer. Kepler’s precise views on astrology are opaque but he certainly found the vulgar practice of casting horoscopes for every occasion unpleasant work.36 However, astrology was where the money was and he went through the motions so that he could support himself and his family. Politics and religion continued to buffet him throughout his life and he was never able to settle down in one place for long.

In 1615, another tragedy blighted his family when his mother was accused of witchcraft. Kepler travelled to her home town of Leonberg to defend her, but the case dragged on and on as the accusers vainly tried to find enough evidence to make the charges stick. She could not be tortured unless strong evidence was produced against her and eventually in 1620 the case was ruled inconclusive. Even then, she was kept imprisoned for another year and died shortly after her release in October 1621.37

Kepler and the Occult

There seems little doubt that Kepler’s religious beliefs supplied him with scientific ideas as well simply acting as an inspiration. We have already seen how the central concept of placing the sun at the centre of the universe came from occult sources such as Marsilio Ficino and the Hermetic corpus. Magicians like John Dee thought that mathematics offered a key to understanding the natural world, while Nicholas of Cusa had made exactly the same observation in his theological work On Learned Ignorance. All these various influences, swirling around in the sixteenth century, make picking out the story of modern science very difficult. Polymaths were able to keep a foot in more than one camp. Ficino was both priest and magician, Cardano a physician, mathematician and astrologer, Gilbert both natural philosopher and occult theorist.

However, this kind of dual facility was becoming increasingly difficult to maintain as the seventeenth century dawned. As an intellectual pursuit, magic became marginalised. Part of the reason that it was less acceptable was that it had become less plausible. Protestants took a much more sceptical attitude towards religious miracles than medieval Catholics had, and this spilt over into how they thought about the occult.38 Furthermore, as the Counter-Reformation progressed, the Inquisition became increasingly worried about the heretical ideas that the pagan Hermetic corpus had spawned. Ficino’s heirs, as we will see in the next chapter, had to watch their step or incur the wrath of the Church. As for the witch trials themselves, they too reflected a decline in the belief in magic, at least among the upper classes. If people had stopped believing in magic, no one yet doubted the existence of the devil. It was not credible that witches could have gained occult powers by themselves, and so they must instead have acquired them from a diabolic source. Witches went from being old wise women to instruments of Satan. The resulting panic swept up Kepler’s mother and cost the lives of as many as 60,000 people over the two centuries up to 1700.39

Despite the cross-pollination that had occurred between them, magic and science also suffered from an estrangement. The Hermetic corpus was exposed as a fake and magicians lost the cachet that they had enjoyed when they seemed to be peddling genuine ancient wisdom. The work of Gilbert had shown how an occult property like magnetism could yield to scientific analysis, but the magical worldview itself could provide no testable hypotheses. Finally, although men like Dee and Ficino had been fascinated by numbers, the separation between their numerology and practical mathematics became ever clearer. A remarkable document of the divorce between the fantastic and the mundane is the pamphlet war between Kepler and an English alchemist called Robert Fludd (1574–1637).

With his flair for self-publicity, Robert Fludd was the pre-eminent London astrologer of the seventeenth century. Born near the Domesday village of Otham in Kent where his family had lived for generations, he received his degree from Oxford in about 1598 and travelled Europe training to be a doctor. On his return to England, he started to practise medicine and joined the influential College of Physicians in London on his second attempt.40 However, during his time abroad, Fludd had imbibed the teaching of Paracelsus, which he mixed with mysticism and the quest for ancient knowledge. In 1616, a series of pamphlets called the Rosicrucian Manifestoes appeared in Paris. These were a forgery, perpetrated by a German Lutheran.41 The documents allegedly revealed the existence of a secret society called the Brotherhood of the Rose Cross that had guarded wisdom through the centuries and would soon make itself known. The brotherhood, which claimed to include many of the most learned and influential men of the time, would bring about a reform of society and usher in a new era of peace. This caused a good deal of excitement, especially in Germany, and provided a shot in the arm for the whole mystical movement. Inevitably, for someone like Fludd, who believed implicitly in forgotten secrets, the Rosicrucian deception was completely convincing. He fell headlong for the hoax and wrote his own pamphlets defending the brotherhood.42 He even hinted that he was a member of the imaginary secret society himself.

In his pamphlets, Fludd insisted that his mystical speculations were both true science and true religion. It is true that he praised the use of mathematics to uncover the secrets of nature. But there is nothing scientific about these ramblings. At base, they are the purest gobbledegook. This contrasts with Kepler’s work, which read like a stream of consciousness but contained nuggets of epoch-making science. That Kepler knew what he was doing and Fludd did not have a clue comes through very clearly in the tracts that they wrote against each other. Kepler explained that when he used mathematics it was to describe how the universe really was in ways that could be empirically tested. Fludd, he claimed, had no solid grounds for his own mystical associations between numbers and physical things, and neither could he provide demonstrations of his ideas in action. According to Kepler, he dealt only in ‘enigmas and hermetism’. Needless to say Fludd vehemently disagreed, calling Kepler ‘vulgar’.43

Ultimately, history has vindicated Kepler. The magical systems of the Renaissance were so flexible and complicated that they could be twisted to fit anything. Thus, they explained nothing. Kepler also struck a serious blow against the holistic picture of the world favoured by magicians. His planetary model was intended to refute the idea that the universe was somehow alive. ‘My goal is to show that the heavenly machine is not a kind of divine living being’, he explained, ‘but similar to clockwork.’44

Despite his failure to convince as a natural philosopher, Fludd was a very successful physician and his magical works continued to be read for decades after his death. His family set up an impressive memorial to him in a church near Otham. Fludd still stares out from the wall of the porch of the church, a quill in his hand. His bald pate and handlebar moustache (sadly damaged like the rest of the decaying monument) make him an unintentionally comical figure.

Kepler died in 1630. He had solved two of the greatest scientific problems of the Middle Ages – how the planets moved and how we can see. He did so driven by a relentless Christian faith and working in the medieval traditions of the universities. In putting Witelo’s name in the title of his book on optics, Kepler was not afraid to admit to his sources. The same cannot be said for his contemporary Galileo Galilei. His achievements were just as great as Kepler’s, but Galileo was a great deal more circumspect about where he was getting his ideas from.

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