Tycho and Kepler

by Kitty Ferguson

  Alchemy required glassware, and a master of the Venetian art of glassmaking, Antonio de Castello Veneziano, with a retinue of assistants, arrived in Denmark, possibly on the run, for the Venetian Republic was extremely possessive of its glass industry. Steen invited them to make their home at Herrevad, adding Venetian dialect to the mix of Danish and Latin already spoken there. Soon the glassmakers were producing not only alchemy vessels for Steen and Tycho but also drinking glasses and windowpanes for King Frederick and Queen Sophie.

  One of Tycho’s first undertakings at Herrevad was to construct a new astronomical instrument, a “half-sextant” with straight walnut legs and a curved brass arc. A little later he added a larger, interchangeable sixty-degree arc. It was this sixty-degree arc that gave a “sextant” its name—probably coined by Tycho himself (see figure 3.1). Sixty degrees is one-sixth of a circle; a half-sextant has a thirty-degree arc. Sextants and half-sextants resemble slices of pie. By sighting along the two legs or sides (where the pie is “cut”)—pointing one leg toward one star and the second leg toward another, for example—it was possible to measure the angular distance between two heavenly bodies. One could similarly measure a body’s altitude above the horizon.

  Tycho was getting more from this effort than a better instrument. He was developing expertise and learning lessons that would serve him later. One conclusion Tycho reached was that in order to design and manufacture instruments capable of the precision he wanted, he would need highly skilled, specialist instrument builders working at his own facility, where he could supervise them. For the moment, he had to content himself with getting the best results he could with nonspecialist artisans under his supervision, while ordering more intricate or decorative parts, and sometimes whole instruments, from Copenhagen. What began at Herrevad was never completely realized there, but the possibilities Tycho saw unfolding at the beautiful old abbey gave him a much clearer vision of what he hoped to accomplish and how to go about it.

  In that same watershed year, 1572, when Tycho ended his European wanderings and began to pursue his interests closer to home, he met—perhaps not for the first time—a young woman named Kirsten Jørgensdatter.4 Like other key events in Tycho’s personal life, the beginning of his lifelong relationship with her is frustratingly undocumented. She was not a woman of noble lineage. (If she had been, more would be known about her.) Pierre Gassendi, Tycho’s earliest biographer, reported a description that he got firsthand from one of Tycho’s last students: Kirsten was “a woman of the people5 from Knutstorp’s village.” To this day, tradition in the Knutstorp area has it that she was a clergyman’s daughter and that her father was pastor of the Knutstorp parish church at Kågeröd, about two miles from Knutstorps Borg.

  The name Jørgensdatter indicates that Kirsten’s father’s first name was Jørgen. From 1546 to 1569 the pastor at Kågeröd was Jørgen Hansen. Most likely it was he who christened the infant Tycho and buried his twin brother. If Kirsten was his daughter, she must have spent her childhood in the half-timbered parsonage beside the little stone Kågeröd church while Tycho was growing up in the castles of his uncle and father.

  The family coats of arms of the Brahes and the Billes are carved on the arch of the Kågeröd church above the family pew. The pulpit in the church is situated not at the front but about halfway back, and the enclosed box pews have seats front and back so that if they are not too crowded, the occupants can move to face the pastor during his sermon. If Tycho sat in the family pew during his visits to Knutstorps Borg, perhaps his eyes fell on Kirsten as the congregation shifted their seats. The young daughter of the pastor would have been modestly clad, neither like a peasant girl nor like a child of the aristocracy. As she grew to be a young woman, a white lace collar and cuffs probably would have been her only less-than-somber adornments.

  Much later in the lives of Tycho and Kirsten, after two other pastors had come and gone, a Hans Jørgensen—again indicating that his father’s first name was Jørgen—was called to the Kågeröd church by the joint lords of Knutstorp, the brothers Tycho and Steen Brahe. Records show that this Hans Jørgensen visited Tycho at his island castle observatory that same year, 1591. Scholars have speculated whether he went there only to be interviewed for the position or also to visit his sister, who by that time had been living for twenty years as Tycho’s wife.

  Adding strength to the local tradition that Kirsten was the pastor’s daughter, not a peasant girl, is the fact that though casual liaisons between noblemen and peasant girls were not unusual, lifetime alliances were. Tycho’s relationship with Kirsten Jørgensdatter was no mere youthful dalliance. It is much more likely that he would have chosen as his companion for life the daughter of an educated clergyman, with a family background and upbringing not quite so drastically different from Tycho’s own as a peasant woman’s would have been.

  If Kirsten was Jørgen Hansen’s daughter, her position in society was indeed considerably above that of a peasant, but there was still a daunting chasm between her station in life and Tycho’s. The nearest he had ever come to experiencing her world was when he lived with a clergyman during his school days, and that clergyman had probably been a bishop, certainly not a humble pastor. Kirsten would have grown up in obscurity in a thatched-roof cottage, working with her hands in the kitchen, house, and garden, and probably never traveling more than a few miles from the village of Kågeröd. So far apart were their worlds that Inger Oxe and Beate Bille would have had difficulty imagining what Kirsten’s daily life was like, as Kirsten would have had difficulty imagining theirs.

  However well Tycho and Kirsten would manage to bridge the gulf between their different upbringings, that gulf, as it was formally imposed by Danish society, law, and tradition, could not be bridged. Tycho the nobleman and Kirsten the commoner could not legally become man and wife.

  There was an alternative that was considered neither scandalous nor sinful. The earliest Danish law codes of ancient Jutland had recognized the legality of slegfred marriages; that is, common-law or morganatic marriages. Under the ancient laws, which were still in force, a woman who was a commoner, who lived openly as a wife in a nobleman’s house for three winters, dining, drinking, and sleeping with him and carrying the keys to his house, was his wife. Originally, among the polygamous Vikings, slegfred had meant a wife of secondary status, but in Tycho’s day it had lost that connotation. When Tycho and Kirsten began their relationship, the courts had just recently reaffirmed that the offspring of such a marriage were not bastards but slegfred children. However, they and their mother remained commoners, no matter how nobly born the father was, and the children could not inherit their father’s estates. None of the expectations and rights of a nobleman’s sons and daughters applied to them.

  Tycho was well aware that his choice of Kirsten strongly reinforced his image as a young man who was willing to flout convention, and that it would likely have drastic consequences for his future and for his descendants. The reputation and influence of his powerful extended family was also at stake. Family honor and alliances through marriage were of enormous importance to Tycho’s relatives. Their reactions to his choice were, predictably, not enthusiastic. The only advantage for them was that the family inheritance would not have to be so widely shared. A few were actually sympathetic.

  Surprisingly, Tycho’s morganatic alliance with Kirsten did not dim his hopes at court. King Frederick had reason to be understanding. He, like Tycho, had fallen in love with a woman beneath his station, Anne Hardenberg, a noblewoman but not of royal blood. Frederick’s father, King Christian III, had forbidden their marriage. After the old king’s death, Anne had continued to live within the royal family as a part of the queen mother’s court, and King Frederick had refused to have anything to do with negotiations for a different bride.

  However, when Frederick finally announced that he would enter into a morganatic marriage with Anne, even though their children could never succeed to the throne, the opposition among the nobility at home and abroad w
as so vigorous that he in the end agreed to give up Anne and marry his fourteen-year-old cousin, Princess Sophie of Mecklenburg. Their marriage took place in July 1572, during the time when Tycho was falling in love and sealing his relationship with Kirsten Jørgensdatter. Frederick invited all the Danish nobility, commanding them to dress in new court attire and ride their best horses, and each to accompany himself with two squires and a page. Tycho—not being a warrior knight—had no squires and pages of his own and on such occasions had to borrow them from someone else in the family. Because Kirsten was not a member of the nobility, it was unthinkable that she could accompany Tycho to a royal celebration.

  Tycho’s earlier career choices had been unorthodox and had led him into astronomy. Now, unwittingly and many years before anyone could predict how it would all end, he had made a decision that would have repercussions far beyond his own lifetime, his own children, and the borders of Denmark. When Tycho took Kirsten Jørgensdatter as his life partner and began to sire children by her, he set his feet much more firmly on the path that would lead to Prague and to Johannes Kepler.

  A plaque on an exterior wall at Herrevad commemorates an astronomical event late that same year that also gave Tycho a powerful push toward that future. The plaque announces: HERE TYCHO BRAHE, ON THE EVENING OF NOVEMBER 11, 1572, DISCOVERED A “NEW STAR.”

  It was a clear autumn evening after several days of overcast skies. Tycho, now nearly twenty-six, was walking back to supper from his alchemy laboratory, glancing up at the familiar darkening sky as he went. To his astonishment, right over his head, near the three stars that make up the right-hand half of the W of the constellation Cassiopeia, there was a star he had never seen before. “I knew perfectly well—6 for from my youth I have known all the stars in the sky, something which one can learn without difficulty—that no star had ever before existed in that place in the heavens,” Tycho wrote, “not even the very tiniest, to say nothing of a star of such striking clarity.” It was brighter than any other star or planet in the sky.

  Not quite trusting his eyes, and wanting witnesses to what he was seeing, Tycho called his servants and then stopped some peasants who were passing nearby. These people had not spent nights studying the stars as Tycho had, but they dutifully craned their necks to gaze up beyond the trees and the darkening walls, trying to oblige their noble companion by giving him an opinion as to whether or not this really was something new. They were not able to confirm that this star had not been there before, but they did agree, when Tycho called their attention to Venus, that the new star was brighter even than that bright planet. “I doubted no longer,”7 reported Tycho. “In truth, it was the greatest wonder that has ever shown itself in the whole of nature since the beginning of the world, or in any case as great as [when the] Sun was stopped by Joshua’s prayers.”

  Tycho realized that the star’s position in relation to the zodiac meant it could not be a planet, and, though he had never seen a comet, he knew from his reading that a comet has a tail and a fuzzy appearance. This had neither. However, the real test of whether it was a comet was whether it moved in relation to the other stars. Finding whether it did took several nights of watching, armed with his cross staff. Tycho could not discern any change of position. This was no comet. Though more observation and calculation were needed to make certain, Tycho was also fairly confident that the new star was not closer to Earth than the Moon’s orbit—a dramatic conclusion in the context of the astronomy he knew. “Let all philosophers,8 new as well as ancient, be silent! Let the very theologians, interpreters of the divine mysteries, be silent! Let the mathematicians, describers of the heavenly bodies, be silent!” he exclaimed.

  Aristotle’s ancient cosmology, which insisted that change could occur only in the region closer to Earth than the Moon’s orbit (the “sublunar” region), was still gospel among most scholars. Tycho had never declared himself an avid follower of Aristotelian cosmology, but he had not escaped its influence. However, it was in Tycho’s nature to want to test things out for himself. To do that, he had to try to measure the parallax shift of the new star, or nova, as he dubbed it.

  Parallax shift is the apparent shift of an object against the background when observed from different viewing positions. The simplest demonstration is by holding one finger up in front of your face, focusing on the distance, and closing first one eye and then the other. The finger appears to shift from side to side against the background. Your two eyes are the two “viewing positions.” The shift is a parallax shift. The further away you hold your finger from your face, the smaller the shift.

  Though scholars had understood for millennia the mathematical principles of such a shift and of the way it grows smaller with distance, determining the distance to a star by parallax was still impossible in Tycho’s day. Powerful enough telescopes would not exist until the 1830s. However, astronomers had known since ancient times that the Moon does have a parallax shift, that two observers a distance apart on the face of the Earth see the Moon in two different positions against the background stars. An observer could even stay in the same place and allow the daily rotation of the celestial sphere (or the rotation of the Earth, if he believed Copernicus) to change his “viewing position” for him. If he did that (as Tycho had done), he saw for himself that the Moon did indeed have a parallax shift. A star or other object as close as the Moon, or closer, would also have a parallax shift. And so, although it was not possible for Tycho to find the distance to the new star by means of a parallax measurement, it was possible for him to conclude that if this new object in the sky did not show any parallax shift against background stars, while the Moon did, it could not possibly be nearer than the Moon.

  Tycho was not the only person who noticed the nova, and not all agreed that it was further away than the Moon’s orbit. Some actually made observations and were convinced that the new star was below the Moon, even though they could discover no parallax. Some insisted it was a comet. Others conceded that it was farther away than the Moon but argued that it was not really new, or that it was not a real change in brightness that caused it gradually to dim after its first appearance.

  Evidently, Tycho was the only scholar capable of seeing beyond Aristotelian assumptions. He measured the angular distance between the nova and another star, and repeated the measurement several hours later. Though the sky as a whole had moved, the distance between the two stars had not changed. Tycho performed this test more than once, measuring the angular distance between the nova and not one but several other stars. Again and again he found no change, no parallax shift. Because this result ran so counter to current science and philosophy, Tycho decided to come at the problem from another direction. He had already calculated, indirectly—by comparison with other stars—how far the nova was above the celestial equator in terms of angular distance (its declination). Now, as a double check, he measured the declination directly, which required finding the maximum height the star went above the horizon at Herrevad. If the nova had never gone higher in the sky than sixty degrees above the horizon at Herrevad, he could have used his sextant, which measured sixty degrees. However, the star went much higher. It reached its highest point at Herrevad only six degrees from the zenith (the point of sky directly above Tycho’s head).

  Tycho’s way of solving this problem, obvious as it seems in retrospect, was a true innovation. He turned the sextant around,9 set it in a north window, recorded the star’s lower culmination—that is, how far above the horizon it was at its lowest point—and calculated the star’s declination from that. He was so pleased with this simple ingenuity that he included a drawing of the sextant in that position much later in his book celebrating his instruments (figure 3.1).

  Before a month had passed, Tycho had completed these tests to his own satisfaction and was confident enough to send his findings and his conclusions—that the star was not a comet and not in the sublunar region—to a few friends. He was not so sure of himself as to send them to scholars in the more learned circles at the Uni
versity of Copenhagen.

  Figure 3.1: One of the first instruments Tycho designed was the sextant, used for measuring altitudes of bodies above the horizon, their azimuth (distance from the meridian), or their distances from one another. Two legs were joined with a hinge (right) so that the end of one moved along the arc (left) by means of a screw (E). There were sights at C and K. This drawing from Astronomiae Instauratae Mechanica (probably of Tycho’s second sextant) shows it set in the north-facing window to observe the nova at its lower culmination. He put the hinge end (I) close to his eye and found the star through the two sights.

  As his twenty-sixth birthday approached, rather than spend more time and effort reporting his study of the star, Tycho assigned himself an entirely different project that had to be completed before the end of the year or not at all: putting together an astrological meteorological almanac for 1573. If the predictions in it were to have any impact, it was necessary to complete it before that year began.

  Tycho’s introduction to his almanac was in the form of an “oration”10 that provides a window into the mind of this eloquent, thoughtful, and well-educated young man. He began it in traditional fashion by invoking Urania, astronomy’s muse in classical mythology, and proceeded to describe the universe as being Earth, seas, Sun, Moon, stars, animals, vegetables, and minerals, and the Creator as incorporeal, immense, eternal, incomprehensible, and omnipresent but not located in any single place. It was a good Lutheran beginning.