Tycho and Kepler

by Kitty Ferguson

  Tycho was a complete novice when it came to talking before a crowd, but he must have been a riveting speaker, for after the lecture a professor of law, Albert Knoppert, approached him and commented, “When I heard5 your attacks on the philosophers and physicians, and even the theologians, I was afraid that you would also launch into us jurists—so afraid that I broke into a sweat!”

  The lecture crowd dispersed, but the discussion about astrology and free will continued at a leisurely meal hosted by Dançey. Hemmingsen wanted to clarify a few points—particularly that Tycho agreed that God works and acts with absolute, unrestricted freedom and that human beings also have completely free will—and then declared himself well satisfied with Tycho’s views.

  The day following this highly successful introduction, Tycho began in earnest to lecture on astronomy. He kept no notes, and probably used none, except to record that he covered the theories of the Sun and Moon “according to the models6 and parameters of Copernicus” and supplied copies of portions of the Prutenic Tables for those in the audience who could not afford to buy them.

  Tycho had not, however, rejected Ptolemaic astronomy and become a convert to Copernicanism. He explained Copernicus’s Sun-centered planetary theory, but then he proceeded to explore the possibility that Copernican theory could be interpreted in such a way as to allow Earth to stand still. In Tycho’s words, Copernican theory might be “adapted to the stability of the Earth.” A decade would have to pass before he would realize this early hope in his “Tychonic system,” but Galileo’s nemesis was waiting in the wings.

  Tycho’s attitude toward Copernicus—deep respect that fell short of acceptance of a moving Earth—would not have seemed a startling departure to the more well informed among his audience. Although Copernicus’s book De Revolutionibus, which had been published a month before that astronomer’s death in the spring of 1543 (about thirty years before Tycho’s lectures), departed radically from the prevailing Ptolemaic worldview, neither the scholarly nor the religious world had reacted negatively to it. Galileo’s clash with the pope did not occur until well after Tycho’s death. Meanwhile, a great deal of time was elapsing, and no overt, dramatic conflict was occurring among scholars or theologians about how the universe was arranged.

  Copernicus himself undoubtedly believed that his Sun-centered astronomy represented literal truth—what really was going on in the universe. He did not regard it as only a mathematical scheme that made the prediction of planetary movement simpler and more accurate. However, it was not in the mind-set of his contemporaries to assume or even to recognize that he had made any such truth claim in De Revolutionibus. Aristotle had done ancient and medieval astronomers a considerable service by drawing a line between physics and the mathematical sciences, including astronomy, in a way that could be interpreted to mean that astronomers need not search for Aristotelian “causes” for celestial motions. By Ptolemy’s day, it had become routine to invent devices such as the epicycle and equant that yielded reliable predictions, without any need to explain what might cause the planets to move in the manner prescribed by those devices. In fact, to declare that Ptolemy either did or did not think the planets literally move in the way these mechanisms had them moving would be to misunderstand him. In the absence of any remote chance of conclusive direct evidence one way or the other, there was much to be said for not belaboring that question—maybe for not even realizing the possibility of such a question. A man who worried about whether his mathematical system represented literal reality was an exception. This was not an intellectual situation confined to the ancients. A similar mind-set exists today at the leading edge of theoretical physics.

  Copernicus had no better view of the skies than Ptolemy, and his forte was not observational astronomy: Most of his observations were less accurate than those of Hellenistic and Islamic astronomers. However, Copernicus accepted the neo-Platonic idea that underlying all the complication of nature there are simple, harmonious patterns. Nothing so complexly convoluted as Ptolemaic astronomy and the mathematical wilderness needed to support it could represent truth. Copernicus’s own system, which potentially explained planetary movement in a much more spare and simple way, must, he believed, therefore be a clearer insight into reality.

  However much Copernicus was motivated by a desire for greater simplicity, De Revolutionibus was not an easy book, and not many had the expertise to wade through it, but skilled mathematicians soon found that Copernicus’s math was brilliant and extremely useful. Long before controversy arose about whether or not Earth really moved and whether it or the Sun was in the center, Copernicus’s math and astronomy had proved too valuable to discard. The Copernican Prutenic Tables, which Tycho had found a little more reliable than the antiquated Alfonsine Tables based on Ptolemaic astronomy, were drawn up in Protestant Wittenberg by Melanchthon’s younger colleague, Erasmus Reinhold. Melanchthon and others of his school revered Copernicus for his mathematics and tables, and used these without worrying themselves about any deep cosmological conflict. Catholic scholars used Copernicus’s tables to calculate a new calendar, and an amicable situation continued in which the Catholic Church had no official policy regarding the arrangement of the cosmos. It had managed to stay largely clear of that debate already for at least three centuries, for Copernicus had not been the first to propose a moving Earth. There was a vague, tacit understanding that if all parties could avoid declaring that any scientific arrangement of the universe or any scriptural cosmological statements should or should not be treated as literal truth, then everything would continue smoothly. That made a great deal of sense when there was no observational evidence to argue definitively for either theory.

  The blame for Copernicus’s failure to make clear his conviction that his theory represented reality—so that no one could have misinterpreted him—lies most directly with a preface to De Revolutionibus that was added by Andreas Osiander, who was left in charge of overseeing the final stages of publication. Osiander urged Copernicus to write a preface claiming that his theory was intended to be interpreted hypothetically. Copernicus refused, so Osiander wrote it himself anonymously, implying that Copernicus had penned the warning, “Beware if you expect truth from astronomy lest you leave this field a greater fool than when you entered.”

  If Copernicus saw the preface, he was nearly on his deathbed and may not have understood what it was. It undoubtedly would have angered him, but it meshed with the mind-set of his time and was instrumental in allowing his astronomy to infiltrate the scholarly world without seeming to pose a threat. Astronomers with whom Tycho had come into contact as a student drew on both theories without feeling they were engaging in intellectual contradiction. Tycho was able to study Copernican astronomy with awe and respect, while not agreeing that Earth moved and the Sun was the center of the system. His use of both Ptolemaic and Copernican tables was not unusual.

  However, by the early 1570s, even before the nova led to any crisis of faith in Aristotelian/Ptolemaic cosmology, a few younger scholars had begun to explore more seriously some of the implications of Copernicus’s theory. If Earth was in orbit around the Sun, then observers on Earth ought to see an annual shift in the apparent positions of the stars. This was not a new idea. Since ancient times the lack of this “stellar parallax” shift had been taken, very scientifically even by modern standards, to mean that Earth did not move. Similarly, if one were in a carriage, looking out at a forest, and saw no shift of the tree trunks in relation to one another, one could be fairly certain the carriage was not moving. There was another possible explanation for the absence of stellar parallax, one that the ancients had not been willing to accept—except for Aristarchus of Samos, who had originally proposed the idea of a Sun-centered cosmos seventeen centuries before Copernicus: Suppose the stars were infinitely or nearly infinitely far away. Riding in the same carriage, but with the forest extremely distant on the horizon, one would discern practically no shift of the tree trunks in relation to one another.

  Lo
gic therefore dictated that accepting Sun-centered astronomy as literal truth meant accepting also that the universe is enormously large. With this realization, new speculation began about the distances of the planets and the stars. The nova of 1572 contributed to that speculation as well as challenging belief in the unchangeable perfection of the celestial realms.

  Copernicus had concerned himself only a little more than Ptolemy with questions about why the heavenly bodies should move as they do. How they move was puzzle enough to occupy a man for a lifetime, let alone the question of what caused them to move that way. However, it was not long before it began to seem a little absurd to suppose that the universe could be working according to both the Ptolemaic and the Copernican systems at once. In hope of preserving some semblance of logic, scholars began in earnest to explore whether it had to be one or the other and to ask whether there was a way to draw old and new ideas into a coherent picture. This was the question Tycho had decided to try to answer.

  Tycho was one of those younger scholars who believed that astronomy had to be more than brilliant mathematical constructs, that its goal could and should be to reveal what is really happening. He also recognized that Copernicus thought he had revealed that. Nevertheless, Tycho felt obliged to reject Copernicus’s idea of a moving Earth. Because Tycho regarded Copernicus as the greatest mathematician of the century, this rejection, rather than being a sign of closed-mindedness, was a symptom of his independence and self-confidence as a scholar. He was not being antiscientific or militantly ignorant. The only way to prove that Earth was not standing still was to find stellar parallax, and that Tycho could not do. There was simply no physical evidence to show that Copernicus was right.

  Earlier, when he was a student in Leipzig, Tycho had come to agree with Copernicus that Ptolemy’s use of the equant was offensive, and it seemed true that Copernican astronomy had removed the need for this theoretical point around which a planet appeared to move without changing its speed. So Tycho had begun to search for a way to eliminate the use of the equant without capitulating entirely to Copernican astronomy. Because he did not use notes for his Copenhagen lectures, it is only possible to surmise from later developments what he might have said. He had declared his intention to expound “the motions of the planets7 according to the models and parameters of Copernicus, but reducing everything to the stability of the Earth,” thus avoiding both “the mathematical absurdity of Ptolemy and the physical absurdity of Copernicus.” The result toward which he was moving at the time of the lectures was a mechanism resembling figure 4.2, although he may not yet have had this scheme firmly in mind.

  IN THE SPRING of 1575, when Tycho’s father’s estate was finally fully settled and Tycho came into an annual income of about 650 dalers, approximately twice the annual stipend of a senior professor at the University of Copenhagen, he abruptly canceled the remainder of his lectures. Once again he had decided to go abroad, this time in style, accompanied by servants and a baggage train. Kirsten stayed at home in Denmark.

  Tycho went with the blessing of King Frederick and, in a sense, as Frederick’s emissary. The king had grandiose plans to transform the dark medieval fortress that guarded the entrance to the Øresund at Elsinore (immortalized in Shakespeare’s Hamlet) into a Renaissance castle. Finding Europe’s best architects, hydraulics engineers, decorators, smiths, sculptors, painters, weavers, and myriad other experts required some scouting, and this was a task for which Tycho was well suited. Tycho’s personal reason for going was quite different and would not have pleased Frederick. He was looking for a place to put down roots and pursue his own scholarly passions on a more extensive scale than he could at Herrevad.

  Figure 4.2: One of Tycho’s first attempts to explain planetary motion without using an equant but leaving Earth as the unmoving center of the system: He moved the center of motion to a point some distance from Earth. He also added a miniepicycle. The planet rides on the miniepicycle, which wheels along centered on the epicycle, which in turn wheels along centered on the deferent—which is centered on a point some distance from Earth.

  Tycho stopped first that spring in Kassel with Wilhelm IV, Landgrave of Hesse, who was an avid astronomer himself. Wilhelm had commissioned many astronomical instruments, clockworks, and gadgets, and he conducted something like an academy for the advancement of astronomy. Overburdened with affairs of state, he had recently found little opportunity to make astronomical observations, but when Tycho arrived he set aside all other activities. Wilhelm agreed with Tycho that the nova had been beyond the Moon, though he thought he had detected a bit of parallax. He commemorated Tycho’s visit by having a small picture of Tycho painted in the background in his next official portrait.

  From Kassel, Tycho continued to Frankfurt to add to his personal library at the famous Frankfurt Book Fair, a long-lived trade fair that was a staple of the medieval intellectual world and still occurs annually. Venturing farther south, he revisited Basel. Peder Oxe, Charles Dançey, Steen Bille, and Johannes Pratensis all had studied at the University of Basel. Tycho was favorably impressed with the mild climate and easy access to scholars in France, Germany, and Italy.

  From Basel Tycho traveled south once again and eventually reached Venice, where more than thirty years later Galileo would astound the Venetian doge and senate with a new invention, a telescope. Tycho was invited to learned gatherings that were part of the lives of rulers of the Venetian republic and which also included scholars from the university in nearby Padua. As both a nobleman and a scholar, he was in his element. In the interest of discovering experts for King Frederick, Tycho probably viewed in person some of the new villas designed by Andrea Palladio in the Veneto, the mainland near Venice. These architectural gems, whether he saw pictures of them or went there himself, made a lasting impression on him.

  When summer ended and before the snows closed the passes through the Alps, Tycho journeyed north again to Innsbruck and then to Augsburg, where he had previously spent such enjoyable and profitable months. At Regensburg, not far downriver from Augsburg, he attended the coronation of Rudolph of Hapsburg, a ruler destined to play a significant role in his future. Rudolph was being crowned “King of the Romans,” which made him heir apparent to the Holy Roman Empire.

  Having considered numerous possibilities during his journey, Tycho decided he would8 emigrate from Denmark and settle in Basel. On the way home to put his plan in motion, Tycho’s final stop on the Continent was in Wittenberg. There he found university and city astir with news of a religious conflict, an early eruption of a division among Protestants that would later cause great unhappiness for Johannes Kepler and make working for Tycho Brahe his only option.

  Twenty years earlier, the 1555 Peace of Augsburg had ended a period of widespread religious and political upheaval—or at least much of Europe had hoped it would. According to the treaty, only Catholicism and Lutheranism would be tolerated within the Holy Roman Empire. The present trouble had begun when Augustus, elector of Saxony, discovered that many theologians at Wittenberg were secret followers of another Protestant reformer, John Calvin. Augustus took strong measures. He imprisoned the errant theologians, including Melanchthon’s son-in-law. When Tycho reached Wittenberg, they were still in prison, and friction had developed between Augustus and King Frederick of Denmark.

  Denmark had not been party to the religious articles of the Peace of Augsburg, and Niels Hemmingsen, the elder scholar who had tipped his hat at Tycho’s lecture, had been instrumental in achieving a consensus in Denmark that made no sharp distinction between Lutheranism and Calvinism. Hemmingsen also had published a work that gave a Calvinist interpretation to the Eucharist, and a similar work had appeared anonymously in Wittenberg. Worse for Denmark, all the imprisoned theologians in Wittenberg admitted that they had obtained their Calvinist ideas from Hemmingsen while attending the festivities surrounding King Frederick’s wedding in 1572. Hemmingsen appears to have been an exceedingly busy and effective proselytizer who did not pause for royal holidays. />
  Since the trail of clues led directly to Copenhagen, Elector Augustus sent a complaint to King Frederick, who immediately summoned to his castle all Copenhagen pastors, all endowed professors of the University of Copenhagen (including Hemmingsen and Pratensis), and the bishop of Roskilde to answer the elector’s charges. Peder Oxe was one of the three commissioners who examined them. Hemmingsen presented an eloquent defense of the peace and unity of belief and religious practice of the Danish church, describing German theologians as leaping about like cooks trying to please the palate of whatever noble they served. If Denmark paid attention to them, the result would be similar confusion. The difficulty was not immediately settled, but Peder Oxe told Hemmingsen privately not to worry, and matters quieted down.

  By the end of December 1575, Tycho had returned to Denmark and was preparing to liquidate his assets for the move to Basel. The court had moved to Sorø Abbey for Christmas. King Frederick relished the rich food, mead, Rhenish wine, and ale of the abbot of Sorø, as well as the learned, amusing conversation around the abbot’s table. Tycho went there to pay his respects to the king, report on his journey, and visit his aunt and foster mother Inger Oxe, who was at this time the noblewoman in charge of Queen Sophie’s chamber.

  Tycho had just turned twenty-nine and was an experienced courtier, polished by his travels and attendance at many courts. Garbed appropriately with flowing cape, feathered hat, and sword, he was an imposing figure, barrel-chested, elegant, and of distinctly noble bearing. His eyes were light-colored, and his hair, beard, and substantial mustache were reddish blond. In portraits, his false nose looks a fairly successful imitation, close to flesh-colored—though an astute portrait painter would have made it so in any case.