Tycho and Kepler

by Kitty Ferguson

  The Austrian Hapsburg family had met in secret in April 1606 and agreed to recognize Rudolph’s younger brother Matthias as the head of the family instead of Rudolph. Two years later, Matthias led an army from Vienna into Bohemia to within a day’s march of Prague. Rudolph abdicated, ceding to Matthias the kingdom of Hungary and the archduchies of Austria and Moravia, keeping only Bohemia (including Prague), Silesia, and Lusatia for himself, with Matthias named as his successor there.

  In the spring of 1610, a few months after Kepler’s anniversary celebration, nature for a time upstaged domestic or political concerns, at least for Kepler and some of his close acquaintances. On March 15 a carriage stopped at Kepler’s door. Its passenger was Kepler’s friend Wackher von Wackenfels, who was an imperial councillor twenty years Kepler’s senior, a distant relation, and a brilliant man with many scholarly and scientific interests. Von Wackenfels stuck his head out of the carriage window to call Kepler to come down immediately, for there was stupendous news: Galileo Galilei, with his new telescope, had discovered four new planets. Von Wackenfels and Kepler were so overcome they could do little more than babble with excitement. Kepler’s enthusiasm was tinged with anxiety over whether these discoveries were planets, or moons around one of the other planets. Von Wackenfels was not sure, but Kepler said they surely must be moons, because he had established with his polyhedral theory that there could be only six planets.

  A copy of Galileo’s book2 reporting the discovery—Sidereus Nuncius (The Starry Message)—reached the emperor, who loaned it to Kepler. Kepler’s own copy arrived soon after via the Tuscan ambassador with a request that the imperial mathematician send his opinion back with the same courier before the week was out. To Kepler’s relief, he read that the “planets” were four moons orbiting Jupiter.

  Kepler’s response enthusiastically agreed with Galileo that the discovery of Jupiter’s moons supported Copernican astronomy, for it was evident that not everything in the universe was revolving around Earth. The fact that through a telescope the planets looked like disks but the stars remained points—no larger than they appeared to the naked eye—indicated that the stars were indeed as far away as Copernican theory required. As for Kepler’s theories, it no longer seemed such an oddity that Earth should have its own version of a “planet-moving force” to keep a moon orbiting. Kepler suggested that Jupiter must have intelligent inhabitants; otherwise, why should God have given Jupiter moons that would go unappreciated by any creature except the few Earth dwellers who had telescopes? Kepler was not among those fortunate Earth dwellers.

  Kepler’s letter was ready on April 19, in time for the courier’s return trip. Galileo’s response was not so rapid in coming. It took him four months, but it was effusive: “I thank you3 because you were the first one, and practically the only one, to have complete faith in my assertions.”

  There was widespread curiosity about Kepler’s reaction to Galileo’s discovery. Kepler published the letter he had written to Galileo as a thirty-five-page book, Dissertatio cum Nuncio Sidereo (Conversation with the Starry Messenger). One of the most surprising things about it was that Kepler felt called on to defend the use of the telescope as a reliable scientific instrument. Some astronomers had voiced suspicions that Galileo’s discoveries might be nothing more than artifacts of the instrument, not something really there. Kepler’s reassurance about the use of telescopes, though he had seen only inferior models in Prague and was relying on his knowledge of optics, was almost as valuable as his defense of the plausibility of Galileo’s discoveries. Kepler had hinted broadly to Galileo that he would like to have one of the telescopes Galileo was sending to important people all over Europe, but Galileo did not send him one, so, unable to confirm the discoveries himself, “plausibility” was as far as Kepler could go.

  In the late summer and early autumn, soon after receiving Galileo’s belated reply, Kepler was able to borrow a telescope Galileo had sent Elector Ernst of Cologne, duke of Bavaria. Kepler called together the mathematician Benjamin Ursinus and several other friends, and they viewed Jupiter4 from August 30 to September 9. To avoid being misled by one another, they agreed that each man would look through the telescope and, without comment, draw in chalk on a tablet what he had witnessed through the lenses. Only after everyone had done this were the drawings compared.

  Kepler simultaneously followed up on ways two lenses could be combined to magnify images. That same August and September he wrote a book on the subject, Dioptrice, which was published in l6l1. Kepler warned in the preface, “I offer you,5 friendly reader, a mathematical book, that is, a book that is not so easy to understand.” His approach in Dioptrice was indeed rigorously mathematical. The book contained the first detailed optical theory of two lens systems and a new, improved telescope design, later known as the “astronomical” or “Keplerian” telescope.

  Kepler made several attempts to continue the correspondence with Galileo, but Galileo, except for a short letter seventeen years later recommending a student, apparently never wrote Kepler again. His silence may have been partly a reaction to Kepler’s first letter about the moons of Jupiter, for Kepler had not hesitated in a friendly fashion to set the historical and scientific context straight by mentioning other researchers whose imaginative thinking might have helped lead Galileo to his discoveries. Mästlin congratulated Kepler on having “pulled out Galileo’s feathers6 exceedingly well,” and since Galileo had chronic difficulties recognizing fine gradations in friendship and support, he may have seen Kepler’s letter as threatening. Later, in an appendix to a small book defending Tycho Brahe’s theories about comets, Kepler upbraided Galileo for erroneous ideas about them and pointed out that the phases of Venus that Galileo insisted were strong support for Copernicus’s model were just as much in accord with Tycho’s.

  Galileo was not a man to take well to such reproofs, but he had had little support from the scientific community and had probably never met anyone who was his scientific equal. If Kepler and Galileo had exchanged more letters and ideas, Galileo might not have gone on for the rest of his life believing that planetary orbits were circles and that the Moon had nothing to do with the movement of the tides. And Kepler would have learned about inertia.

  AT NEW YEAR 1611, Kepler gave his friend von Wackenfels a whimsical gift, a letter about why snowflakes are hexagonal. Kepler’s writing reflected his cheerful mood that early winter as he invented the puns that he put in his letter, anticipating Wackher’s laughter when he read it. Strena, a New Year’s Gift; or, On the Six-Cornered Snowflake was both a delightful bauble and a pioneering study in what would become the science of crystallography. Kepler could not know that the time spent on this letter would be the last happy hours, and Strena7 the last lighthearted achievement, of his years in Prague among such friends as von Wackenfels.

  That winter, Rudolph made a desperate and foolish move. He plotted with his cousin Archduke Leopold V, bishop of Passua, to bring an army to Bohemia. What Rudolph hoped this would accomplish is not clear. In February, while the populace of Prague waited in dread as Leopold’s undisciplined, unpaid soldiers ravaged the countryside and neared the city, tragedy struck the Keplers. Barbara was still recovering from Hungarian fever, which she had caught just before New Year, when the three children came down with smallpox. Eight-year-old Susanna and three-year-old Ludwig recovered, but six-year-old Friedrich, who had been a particular delight to Kepler, died on February 19. It had been many years since he and Barbara had lost their first two infant children. This time Barbara was less resilient. Kepler wrote that she was “wounded to the depths8 of her being by the death of the little boy who was half her heart to her.” She slipped into even deeper depression.

  Within days of Friedrich’s death, Leopold’s troops were in Prague and occupied the area surrounding the palace and the Lesser Town nearer the river. Bohemian Protestant vigilantes banded together in other parts of the city, including the area around the Keplers’ house, ostensibly for defense purposes but also to loot clois
ters and Catholic churches in the Old Town. The streets of Prague became bloody battlefields as the two groups fought for turf, while in the imperial palace the atmosphere was thick with madness and ruin. The emperor paid off Leopold’s men (the treasury, for once, was responsive), and they departed, but Rudolph’s reign was over. Preparations were made to crown Matthias king of Bohemia. Life as Kepler had known it in Prague, on both the personal and public levels, had come to an end.

  In the spring of 1611, as Kepler put out urgent feelers in an attempt to make provision for his and his family’s future, he once again looked hopefully to his native Württemberg. With a list of achievements and honors to match any in Europe, he hoped to be welcomed home with a professorship or a political appointment in the ducal court.

  In April that door was emphatically slammed in his face, his application denied because of his earlier admission, still on record, that he believed a Calvinist also was a “brother in Christ.” In Lutheran Württemberg this amounted to a criminal view. A man espousing it might spread his poisonous ideas among his students. Ironically, Kepler was excluded for following one of the basic tenets of Lutheranism, the principle of a “priesthood of all believers” in which it is every believer’s right to interpret the Scriptures for himself.

  Though there were other possibilities had Kepler had time and heart to pursue them, he accepted a teaching position in Linz in a school similar to the one where he had started his teaching career. It was not a university appointment of the sort one would have thought awaited a man of his stature, but he could look forward to being treated with respect, and the position of provincial mathematician was created especially for him. Since Linz was in Upper Austria, he could also retain the title of imperial mathematician. The new emperor Matthias could be expected to reconfirm that appointment. Kepler’s contract called for him to “complete the astronomical tables9 in honor of the emperor and the worshipful Austrian House, [to benefit] the entire land and also for his own fame and praise.” He was also charged with making a map of Upper Austria and “producing whatever other mathematical, philosophical, or historical studies were useful and suitable.” As he made these arrangements, Kepler clung to the hope that life for Barbara in Linz would be more like it had been in her beloved Graz. For his sake, she had endured ten years in Prague, and it was her turn to find some measure of happiness.

  In June, returning to Prague from a journey to settle matters in Linz, Kepler found Barbara again dangerously ill. Matthias’s Austrian troops, now in the city to establish peace, had brought a contagious fever. Barbara had insisted on helping nurse the sick, and she had caught the fever herself. She died on July 3.

  A portrait believed to be of Johannes Kepler, painted by Hans von Aachen around 1612.

  Kepler and what remained of his family did not make the move to Linz immediately, for Rudolph still needed him in Prague. To the last, Kepler remained loyal to his unfortunate patron, dividing his time between a grief-stricken home and a doom-stricken palace. With the political situation beyond hope, he nevertheless struggled to keep astrology as far as possible out of the heads of the gullible emperor and his closest advisers, and attempted to mislead Rudolph’s enemies by informing them, counter to what he was actually finding in his astrology, that the stars still predicted long life for Rudolph and difficulties for Matthias. Rudolph died in January 1612. Matthias renewed Kepler’s appointment as imperial mathematician but permitted him to leave Prague. In May 1612 Kepler, soon to be followed by his two children, aged eight and three, moved to Linz without the wife and mother for the sake of whose happiness Kepler had decided to go there.

  Kepler’s reputation as a Nonconformist with a tolerance for Calvinist views followed him to Linz. The Lutheran pastor asked him to recant on this issue and, when he refused, denied him the privilege of taking Communion. Though Kepler may have seemed to militant Lutherans to lean toward Calvinism, he was not a Calvinist. “It makes me heartsick,”10 he wrote, “that the three big factions have so miserably torn up the truth among themselves that I have to gather the little scraps together wherever I find them.” This devoutly Christian man, so at home with his God, found himself without a home in any earthly church and having to endure gossip in Linz about his religious plight and exclusion from Communion.

  There was, however, a brighter side to his new situation. His salary of 400 florins was actually paid regularly. Also, in 1612, Kepler finally became custodian of all Tycho’s observations. Tycho’s son Georg was by then serving as the Brahe family’s representative and proved to be a staunch ally at court when Jesuit astronomers with imperial support attempted to take Tycho’s library, instruments, and manuscripts. Kepler, with Georg’s help, kept the precious observational data.

  The first task Kepler set his mind to in Linz was not scientific. With two young children, it was essential that he remarry. That concern occupied his thoughts for a year while he considered no fewer than eleven candidates.11 As a topic of conversation in Linz, the subject of Kepler’s wooing eclipsed the subject of his religious problems. He weighed the advantages of each woman in his own mind and in letters to an acquaintance, discreetly referring to them not by name but by a number.

  Number one was an experienced homemaker about his own age, but her breath stank. Number two was her daughter. She was too immature and accustomed to luxury. Numbers three and four were upstaged by number five, a serious, loving woman, whose humility, frugality, diligence, independent mind, and fondness for his children impressed him. However, her family was less respectable than number four and her dowry smaller. Kepler’s friends told him he would be marrying beneath his station. So he favored three again, then four, but four had grown tired of waiting. Number six was immature and conceited, though there was a certain nobility about her. Back to number five. But then some friends suggested number seven, who was a noblewoman. When he failed to make up his mind immediately, she rejected him. Number eight was unsure whether she wanted to marry a man excluded from Communion. Number nine had a lung disease. Number ten was ugly and fat. Number eleven was offered and then the offer withdrawn because of her youth. Finally Kepler cast aside consideration of status, family opinion, dowry, and improvement of his social rank through marriage and declared that God had led him back to choose number five—Susanna Reuttinger.

  The daughter of a cabinetmaker, Susanna was twenty-four years old, seventeen years younger than Kepler and only a year older than his stepdaughter Regina. She had been orphaned at an early age and lived most of her life as the ward of a baroness whose husband was one of Kepler’s patrons in Linz. Kepler’s stepdaughter Regina thought that Susanna was too young to be a good mother to his two young children, and there were some comments in Linz about the age difference, but Kepler had made up his mind. He loved Susanna, and he trusted her. They were married on October 30, 1613. The next summer Susanna gave birth to a daughter, Margarethe Regina, named after Kepler’s stepdaughter.

  Kepler’s scholarly work slowed down during the time he wooed and chose among the eleven women, but he did not abandon it entirely. Once again, it was not the long-languishing Rudolfine Tables that he worked on primarily, nor was it the map that was in his contract. Instead, new inspiration came from an unlikely source. Traveling on the Danube, Kepler saw many differently shaped wine barrels on the riverbanks and became intrigued with the problem of how to express their volumes. Because of an unusually good wine harvest, he decided to install some wine casks at home and learned in the process that Austrian wine merchants measured only the diagonal length of a barrel, disregarding its shape. Kepler’s book on the subject was not a huge popular success, and his superiors in Linz were not impressed that their mathematician was thinking about wine barrels rather than the Rudolfine Tables and the map. However, Kepler’s study of the wine barrels did satisfy him that the old, simple way of measuring was adequate for Austrian wine casks. More significant for the future, the mathematics that Kepler developed in the process became an important step in the history of the devel
opment of integral calculus. Another unexpected side advantage was that when Kepler failed to find an interested publisher, he brought a printer, Johannes Planck, to Linz and published Nova Stereometria Doliorum Vinariorum12 (New Solid Geometry of Wine Barrels) himself.

  When Kepler began producing calendars again in 1616, Planck printed those too. The production of the calendars was, according to Kepler, “a little more honorable13 than begging.” It was his way of raising money for the next publishing effort he had in mind, for which there had been many requests, a textbook that would make the discoveries he had written about in Astronomia Nova more accessible to nonexpert readers, to “the low schoolbenches,” as he put it. The first three volumes of the seven that would comprise Epitome Astronomiae Copernicanae (The Epitome of Copernican Astronomy) were ready for Planck in 1615, but the final pages of the seventh did not come off the press until 1621. By this time Kepler had added enormously to Copernican astronomy, and there was as much if not more of Kepler in the book than of Copernicus. Epitome was an influential book, read all over Europe.

  Meanwhile, however, after completing the first three volumes of Epitome, Kepler turned to producing an immediate moneymaker, an Ephemeris for 1618. An ephemeris consisted of tables giving the position of each planet for every day of the year, and such a book was an invaluable reference for both navigators and astrologers. Kepler knew that after the Rudolfine Tables were published, anyone who was not lazy would be able to calculate planetary positions without using an ephemeris, and sales on those would drop. If he wanted to profit from the sale of ephemerides (plural of ephemeris), now was the time. Printers normally did not have enough numbers on hand to print an ephemeris, and Kepler purchased his own set of numerical type for Planck to use. Kepler’s confidence about producing these ephemerides year after year is evidence that he had made substantial progress studying the orbits of the other planets besides Mars and Earth.