Tycho and Kepler

by Kitty Ferguson

  Tycho voiced the conviction that humans were created in the image of God, by God, who put them on Earth at the center of the universe with a good view of the rest of it, so that through contemplation of the visible Creation they might learn something of the majesty and wisdom of the Creator. There was, Tycho declared, no better teacher of theology than the universe itself, and this was so even on Earth, where, with the exception of the human soul, dissolution and change held sway. It was especially true in the celestial realms—immense, unchanging—so redolent of God’s power and intellect. In view of all this potential, Tycho lamented the ignorance of most people—even those who fancied themselves as authorities—about the heavens.

  He continued by discussing his theory of meteorology, the theory that would underlie his almanac: celestial events, especially of the Moon—because it is immediately next to Earth’s atmosphere—had, he thought, a strong influence on the weather. However, one should not put too much reliance on weather prediction based on celestial events because there were also local conditions on Earth, this realm of chance and change, and these conditions varied from place to place. Therefore, his hope was not so much to predict the weather as to study the discrepancies between predicted and actual weather and find out more about how Earth and heaven were linked. Thus Tycho neatly sidestepped the danger that his predictions might not be correct. Discrepancies weren’t a problem; they were the leading edge.

  Tycho also managed, in passing, to sneer at some of his contemporary astronomers and to pay homage to Danish royalty. He said he was not one to sit snug by the fire and learn his astronomy from books and papers, and so he had decided to use his own observations rather than the Alfonsine (Ptolemaic) or Prutenic (Copernican) tables. Furthermore, because all people owe a great debt to their native land, he would use Copenhagen as his place of reference in establishing the meridian and the horizon. In other words, for purposes of this almanac, King Frederick should think of himself as the center of the universe.

  At the end of his introduction, Tycho listed the other manuscripts he had written on subjects related to the topic at hand. He had been busy. There were tables of the risings and settings of the Sun, Moon, and planets, their configurations for each day and each lunar octad, and the predicted weather for each day of the year. He urged the need for systematic meteorological observations, and then he moved on with a flourish to quote Ovid on the joys of astronomy. Finally there was a verse he had written himself, lamenting that the demands of the world, of courtly life, and the intense cold of the north disturbed the serenity that a man required for contemplating the stars.

  As work on the almanac progressed, Tycho ran into a hitch. The almanac predicted an eclipse of the Moon on December 8, 1573. Investigating the astrological significance of this event, he found that it seemed to predict the death of King Frederick. Such a prediction was a matter of national security and certainly could not be announced straightforwardly in an almanac. On the other hand, if the king died, it would be a feather in an astrologer’s cap to have predicted it. Tycho chose to clothe the announcement in rather garbled allegorical writing, which if necessary could later be interpreted, with hindsight and a little help from its author, to predict this catastrophe. (It did not occur.)

  Soon after the new year began, Tycho carried the almanac manuscript to Copenhagen along with several other manuscripts, including the one about the new star. To his astonishment, he learned that no one in Copenhagen had yet spotted the nova. He spoke of it while dining among friends at the home of Charles de Dançey, the French ambassador. Dançey thought Tycho was chiding them all for not watching the sky as closely as he was. Johannes Pratensis, who was on the faculty of medicine, sniffed that other professors at the university could not have missed anything so dramatic. Tycho held his peace and allowed the conversation to move to other matters. When a clear night came, there was the star, and it was, as Tycho had told them, not at all like a comet. Amid the excitement, Tycho brought out the manuscript he had written about the star. Pratensis urged him to publish it.

  Tycho had never published any of his manuscripts, nor was he thinking of publishing this one. Scholarly endeavors were beneath a man of his rank. A nobleman might read a book and acquire some learning thereby, but surely not write one. And if he did happen to write one, it was surely for the sport of it, not for public consumption. Tycho brushed off Pratensis’s suggestion on the grounds that his manuscript was not polished enough, that he had never intended it to be widely read.

  Pratensis did not give up. After Tycho had gone home to Herrevad, Pratensis sent him some reports written about the new star that had only recently reached Copenhagen from abroad with the spring thawing of the sea-lanes. When Tycho read them, claiming he did so only because he lacked anything to do while he was ill, he was distressed by the incompetence of those who had studied the star, and particularly with claims that it was a comet only about as far away as twelve to fifteen times Earth’s radius.

  Back in Copenhagen to check on work being done for him in an instrument shop, Tycho again encountered Pratensis. Only now did he admit to Pratensis (who was not a nobleman) what the real obstacle was. Pratensis didn’t take offense, and suggested Tycho consult his powerful relative Peder Oxe. Oxe raised the possibility that Tycho might publish his manuscript anonymously, but Tycho returned home again with it still under his arm. In the end, Pratensis prevailed, and a letter from him became the preface to the published book.

  Tycho’s decision to publish De Stella Nova11 was a major turning point. To his mind and the minds of his contemporaries, a life of serious scholarly pursuit and the life of a nobleman were incompatible. If he had got by so far, it was because he still seemed a young man who had not yet settled down seriously to either sort of life. However, it had in fact become far too late to follow the traditional path into knighthood, and government service—which was still not out of the question—was simply not what he wanted. Hence he was already some distance beyond the pale even before he decided to publish this book. Nevertheless its publication was, for him, another significant and overt step away from orthodoxy.

  De Stella Nova began as a quiet, short treatise, not passionate or provocative. Tycho reported his studies and his findings and reasoned about the location and the nature of the star. He also dealt with the purpose of it, a matter that greatly interested him—not merely its philosophical meaning but also its practical significance—for surely such a celestial event could not be completely irrevelant to humans. The astrological predictions turned out to be both difficult and dire, but more telling when it came to Tycho’s own future was a last addition to the manuscript before it went to press in the spring of 1573, when the star was still visible but greatly dimmed.

  Though the front of the book included a poem by one Professor Johannes Franciscus praising Tycho’s noble lineage, Tycho chose in his epilogue to cast scorn on the supposed glories of his class: feats of arms, association with kings, and the pursuit of frivolity, wine, woman, and song. He declared himself unwilling to be held back by fear of what people would think of him, for his would be the greater and immortal glory of having improved astronomy beyond anything it had been before. His lineage was among the noblest, but he himself would take pride only in what he would accomplish himself.

  Tycho concluded with an allegorical flourish. He portrayed the goddess Urania commissioning him to find the position, distance, and meaning of the new star, but that was only the beginning. He also was charged to do the same for all the other stars, plot the paths of the Sun, Moon, and planets, and discover their influences on meteorological phenomena. Who, he asked, having such a vision, could ever lower his eyes to mundane interests? Tycho sent copies of the book to friends, patrons, and scholars, but none to his family. Many years later, Johannes Kepler wrote12 that if it signified nothing else, the nova of 1572 heralded the birth of an astronomer, the great Tycho Brahe.

  IN 1945, astronomer Walter Baade made it a project to find out what the bright object was tha
t Tycho discovered in the sky over Herrevad in 1572. Studying Tycho’s observations, Baade concluded13 that it was in all probability a Type I supernova, the explosion of a “white dwarf” star. Such a cataclysm occurs when an elderly star has exhausted all its nuclear fuel and collapsed to a sphere about the size of Earth, with a mass close to the mass of the Sun. In a star that small with a mass that huge, matter is packed to almost inconceivable density—hundreds of tons per cubic inch. Most white dwarf stars are parts of “binary systems” in which two stars continually circle one another. The dwarf’s partner in the system is usually a much larger but far less massive star. As the two perform their celestial waltz, the denser dwarf star cannibalizes matter from its companion and gradually adds to its own mass. The mass limit a white dwarf can attain without collapsing and exploding under the pull of its own gravity is about 1.44 times the mass of the Sun. Once having exceeded that limit, the star rips apart in a titanic explosion. That explosion is a Type I supernova.

  Tycho knew nothing about supernovae. Knowledge about them was more than 350 years in the future. Tycho called the star nova, Latin for “new,” though modern astronomy uses the term nova for a less violent explosion.

  Type I supernovae happen frequently in the universe, but they are rare in any one galaxy. In our Milky Way there was one in 1006. There is no record of anyone in Europe seeing it, but in China it was called a “guest star.” Tycho’s nova was next in 1572, and in 1604 there was another, observed by Johannes Kepler. There have been no Type I supernovae in the Galaxy since. Tycho knew of no precedent except for a report of a new star in the second century B.C. from the Hellenistic astronomer Hipparchus of Nicaea. Tycho ruled out the possibility that the star that heralded Christ’s birth and led the Magi was another example, on the grounds that that star had to be much nearer to Earth to lead anyone anywhere. He was again underlining the fact that the 1572 nova did not move in relation to other stars, and hence was extremely distant, well beyond the Moon.

  Radio astronomers14 in the late twentieth century were able to identify a source of radio emission that they believe comes from what remains of the 1572 supernova. They place it exceedingly far beyond the orbit of the Moon.

  fn1 Herrevad today, still deep in the country, retains some of its medieval atmosphere, though it is now a riding school with only a small museum to recall the past. There are ancient trees, broken walls, and footings of vanished buildings, and shaded, pollen-strewn ponds that probably date from Steen Bille’s day.

  4

  HAVING THE BEST OF SEVERAL UNIVERSES

  1573–1576

  SPRING, WITH THE thawing of the sea-lanes, was the customary season for a Dane to set off on travels abroad. In spite of a new wife and the work at Herrevad, Tycho was restless again in the spring of 1573. He made plans to leave Denmark and was even thinking of making a permanent move. Details of his book’s publication delayed the journey. He still had not left by the following autumn, when Kirsten gave birth to a baby girl, also named Kirsten, on October 10.

  On November 11, Pratensis held a Martinmas feast at his lodgings in Copenhagen to mark the first anniversary of the evening when Tycho first saw the nova. One copy of the invitation still survives,1 promising sugar, almonds, chestnuts, a goose, a suckling pig, and eighty bottles of wine. They were going to drink more than they were going to eat. Although Tycho may have poured scorn on the frivolities and excesses of the Danish nobility, life among scholars was not exactly abstemious.

  Tycho did little observing in the summer and autumn of 1573, partly because of the distractions of publishing his manuscript. However, in early December he and his fourteen-year-old sister Sophie observed an eclipse of the Moon and discovered that some adjustments he had made to the Prutenic (Copernican) Tables, while calculating when the eclipse would occur, had succeeded even better than he had hoped. “I myself cannot2 sufficiently marvel over the fact that at this early age, only twenty-six, and without the aid of numerous and accurate observations of the motions of the Sun and Moon, I should have been able to obtain such precise results,” wrote Tycho with unembarrassed self-approval.

  Tycho and Sophie observed the eclipse from Herrevad with an elegant new quadrant3 built to his order by specialists in Copenhagen. It was a creation of great artistic beauty, fashioned of brass and gold. Evidently Tycho’s financial situation had improved now that he could anticipate inheriting, soon, a portion of his father’s estate.

  That inheritance notwithstanding, a painting on the side of the quadrant reemphasized that Tycho had little good to say of his noble heritage. It showed a table holding symbols of aristocratic life—scepters, coats of arms, ostentatious clothing, goblets, dice. Around the table were symbols indicating the futility of that life—a skeleton, a withered tree. On the other side of the tree the branches and roots were shown alive and abundant, and seated in its shade was a man studying a book and a celestial globe. “By Spirit we live,” declared Tycho’s inscription above the picture. “The rest belongs to death.”

  The new quadrant wasn’t large or designed to make such fine measurements as Tycho’s sextant, and it must not have been very useful, for he made few observations with it. However, Tycho clearly felt that instruments made of metals rather than wood represented the future. The quadrant was a first experiment with a relatively inexpensive model and also an exquisite trophy to celebrate his new life as an astronomer and his break with the traditions of nobility.

  Figure 4.1: The brass-and-gold quadrant. The painting that repudiated Tycho’s aristocratic heritage is in the circle marked K and L. The drawing is from Astronomiae Instauratae Mechanica.

  During the summer of 1574 Tycho moved to Copenhagen. There is no record of Kirsten and his daughter accompanying him, nor any to show whether Kirsten was “keeping the keys” of another house in accordance with Jutish law. Knutstorp was Tycho’s widowed mother’s domain, Herrevad his Uncle Steen’s and Aunt Kirstine’s. But Kirsten was expecting their second child. The baby girl was born before the end of the year and christened Magdalene.

  Tycho’s status within the Copenhagen scholarly community had been transformed with the publication of his book. De Stella Nova was a serious professional credential and established him as something of an authority in astronomy. Before the summer ended, there was talk of his delivering lectures at the university, even though he had never acquired an M.A. degree or intended to have an academic career.

  There had been obstacles to a nobleman publishing a book. There were even greater obstacles to his lecturing at the university. Not only his own image and self-image were at stake but the status of others as well. Since the Reformation, church and university had become the domains of an educated middle class—usually better educated than the nobility—who served as bishops, lesser clergy, teachers, professors, and scholars. During Tycho’s lifetime and for many years thereafter no nobleman in Denmark served in these capacities. An unwritten rule of society in sixteenth century Europe was that people, knowing their places, stayed in them and did not trespass on other territory. It was almost as unthinkable for a nobleman to move down the ladder as for a commoner to move up. It was not exactly forbidden; it just did not happen.

  Someone thought of a way around this obstacle: A number of noble university students signed a petition and presented it to the king, who then added his request to theirs, inviting Tycho to lecture to these students of his own social class. The lectures were also open to anyone else who chose to attend. After that matter was settled, it did not take long to schedule the lectures.

  The first, held on September 23, was an hour-long formal introduction to the series. Two years earlier, Tycho had revealed his philosophy and beliefs in the introduction and epilogue of his almanac. Now, in 1574, approaching age twenty-eight, he was still a religious man, deeply influenced by the religious traditions of his time, continuing to interpret human activity within a context of religious belief. He spoke of Seth and Moses before moving on to Hipparchus, Ptolemy, and Copernicus. As he had d
one in his almanac, he argued for the value of astronomy in liberating human minds from worldly matters and turning them toward heaven, as well as for providing calendars and predicting the weather. That last usefulness had been called into question because of so many failed predictions. However, Tycho pointed out that the Sun was responsible for the seasons of the year, and many believed (though not everyone in his time—Galileo was to be a notable exception) that the Moon influenced the tides. To Tycho it seemed reasonable to think that the stars also had to have something to do with the turbulence of weather and other weather patterns.

  Discussing the use of astronomy for horoscopes was a touchier matter. Since Augustine of Hippo in the fourth and fifth centuries, nearly all Christian theologians, including Luther, had opposed the use of astrology to predict human events. Tycho felt obliged to rebut this long tradition and to come down on the side of Luther’s disciple Melanchthon. When Tycho began this part of the lecture, nearly all in the room turned their heads to look at Niels Hemmingsen. That elderly theologian and follower (usually) of Melanchthon had recently switched his allegiance and attacked Melanchthon’s favorable views about astrology. Hemmingsen smiled and tipped his academic hat. Then the attention reverted to Tycho.

  Tycho took this hat tipping as a delightful challenge. Philippist theology (the theology of Philipp Melanchthon) had a strong hold at the university. Tycho hoped to show in what way his own views meshed with this popular thinking and how they overcame Hemmingsen’s objections to it. The heart of Tycho’s argument was that the stars influenced individual lives and human events but did not determine them. He insisted that “there is something in man4 that has been raised above all the stars.” God had endowed human beings with free will, and created man so that he can “overcome any malevolent inclinations whatsoever from the stars if he wills to do so.” Tycho recommended good education, discipline, and other desirable human activities, for in them—in rising above a “brutish life”—lay the best possibility of deflecting the influence of the stars. Tycho would never lose his faith in astrology, interpreted as he did in this lecture, even much later when he was focusing almost exclusively on astronomy.