Tycho and Kepler

by Kitty Ferguson

  Tycho invited Kepler to accompany him to the imperial court. There, for the first time, he introduced him to Emperor Rudolph and proceeded to make a dramatic proposal: Tycho and Kepler would take on the prodigious task of compiling a superb new set of astronomical tables based on Tycho’s observations and more accurate than any the world had ever known. With the emperor’s gracious permission, these would be named the Rudolfine Tables. Great astronomical tables in the past, such as the Alfonsine Tables, bore the names of their royal sponsors. The Rudolfine Tables would, similarly, be a monument to Rudolph and a testament to his generous support of learning. There was nothing further required of the emperor than what he had already granted Tycho . . . except for a salary for Johannes Kepler.

  The emperor was thrilled with the idea. The paperwork for Kepler’s salary was begun posthaste.

  Tycho had previously provided for the future of his family. Now he had also provided for Kepler’s future. With this bold decision, Tycho placed all his precious observations in Kepler’s hands. Tycho’s secrets would no longer be secrets from Kepler.

  ON OCTOBER 13, 1601, only a few days after meeting with the emperor, Tycho accompanied a friend, Councillor Minckwicz, to dinner at the palace of Peter Vok Ursinus Rozmberk just a few steps from the emperor’s gate. Courtesy forbade one to rise from the table for any reason before one’s host had risen, and it was Tycho’s adherence to this simple point of etiquette, “so trivial an offense,”20 as he himself put it, that brought him to his deathbed.

  All the barriers that had kept Tycho and Kepler at a distance had now disappeared, and it is Kepler’s description that provides more intimate details about the days that followed than are available about any other episode of Tycho’s life. Kepler wrote,

  Holding his urine21 longer than was his habit, Brahe remained seated. Although he drank a little overgenerously and experienced pressure on his bladder, he felt less concern for the state of his health than for etiquette. By the time he returned home, he could not urinate any more. [Kepler here noted down the positions of the Moon, Saturn, and Mars on the night of the banquet.]

  Tycho’s own medical expertise was considerable, and he tried various remedies, but with no success. He endured five days and nights of agony, unable to sleep.

  Finally, with the most excruciating pain, he barely passed some urine. But, yet, it was blocked. Uninterrupted insomnia followed; intestinal fever; and little by little, delirium. His poor condition was made worse by his way of eating, from which he could not be deterred. On 24 October, when his delirium had subsided for a few hours, amid the prayers, tears, and efforts of his family to console him, his strength failed and he passed away very peacefully.

  At this time, then, his series of heavenly observations was interrupted, and the observations of thirty-eight years came to an end. During his last night, through the delirium in which everything was very pleasant, like a composer creating a song, Brahe repeated these words over and over again: “Let me not seem to have lived in vain.”

  It was only to God and Kepler that the prayer could have been addressed. Several years later, Kepler added to his description in a chapter of his book Astronomia Nova that when Tycho lay dying, “although he knew22 that I was of the Copernican persuasion, he asked me to present all my demonstrations in conformity with his hypothesis.”

  Tycho Brahe’s tomb in the Maria Tein church in Prague.

  Perhaps, had there been a choice, Tycho would have preferred burial on his once-beloved Hven, or in the parish church in Kågeröd on his ancestral estate where his parents and other family lay. In Prague, even at Benatky, he had always felt like a man far from home. But it is doubtful that Denmark, even had it chosen to do so, could have given Tycho a more magnificent burial than Prague did, or one that would have pleased him more. Kepler described it:

  The casket23 was draped with black cloth and decorated in gold with the Brahe coat of arms. In front of the casket were carried candlesticks, likewise adorned with his arms, and a black damask banner displaying his titles and arms, in gold. Behind the casket was led his riding horse, followed by a black taffeta banner and then another horse draped in black cloth. [Followed by men walking single file, carrying Tycho’s sword and armor.] The casket was borne by twelve imperial officials, all noblemen. Behind the casket walked Tycho’s younger son, between the Swedish count Erik Brahe and Baron Ernfried von Minckwicz, in long mourning dress. They were followed by other imperial councillors, barons, and noblemen, Tycho’s assistants and servants, then Tycho’s wife, guided by two distinguished old royal judges, and finally his three daughters, one after the other, each escorted by two noble gentlemen. Then proceeded many stately women and girls, and after them the most distinguished citizens. The chairs in the church in which the family sat all were draped with black English cloth. The streets were so full of people that those in the procession walked as if between two walls, and the church was so crowded with both nobles and commoners that one could scarcely find room in it. When the sermon was over, the banners, helmet, shields, and other arms were hung over the crypt.

  Kepler did not record where he walked in the procession. He must have been among the assistants and servants. However, he already knew that he was no longer to continue as any sort of “hired hand.”

  19

  THE BEST OF TIMES

  1601–1606

  KEPLER HAD THE news two days after Tycho’s death: Barvitius, the imperial secretary, came to tell him that the emperor had named him imperial mathematician, and he should apply for a salary immediately. The title carried with it responsibility for the care of Tycho’s instruments and manuscripts, as well as for the completion of his unfinished work, most urgently the Rudolfine Tables. The legacy was Kepler’s. Tycho’s full set of observations had fallen into his hands, the pages open at last.

  Tycho’s instruments and intellectual property were not really the emperor’s to bestow, for they belonged to Tycho’s family. So Rudolph purchased them for 20,000 florins, more than Kepler at his old Graz salary would have earned in a century. Of course, as the Brahe family were aware by now, collecting on such a promise from the emperor was no easy matter. To Kepler, the value of the observations was beyond any price.

  Following Tycho’s death and burial, the Keplers moved out of Tycho’s mansion by the wall to a house across the river from the imperial enclave, in a section of Prague known as the New Town (it dated only from the fourteenth century). For the first time since leaving Graz, they had a home of their own. Their house was across the street from the Emaus cloister, an hour’s walk from the palace. Tycho would have been pleased to have that much distance between himself and the emperor, though it was a long journey for Kepler when he had to make it.

  The first decade of the seventeenth century was a glorious time to be living in Prague, albeit an expensive one. With the court in residence, it was the center of political life in Europe, wealthy, cosmopolitan, rich in history but moving with energy into the new century. Its narrow streets echoed with many languages. Kepler called it “a gathering of nations.”1 Carriages of courtiers drove along its wider avenues and stopped at splendid houses that were not only impressively large but also exquisite in their proportions and details and furnished with treasures. Other men than Tycho and Kepler also learned that the royal coffers were unable to make good on all the promises Rudolph II made. Nevertheless, Rudolph’s interest in learning and the arts set the tone of a court and a wider community that drew many superbly productive artists and scholars to Prague. One of Rudolph’s numerous idiosyncracies was a shyness that at this time in his life caused him to shut himself away for days at a time, paying little if any attention to the activities he supposedly supported, but his name and reign are still linked with a great flowering of the arts and scholarship.

  There was no higher honor to which an astronomer could aspire than the one that was now Kepler’s. After so much despair and struggle, it seemed that he and his family were at last on their feet. Barbara Kepler had a
house of her own to manage, and in July 1602 she gave birth to a daughter, Susanna. The little family that had consisted only of three uncomfortable refugees—Johannes, Barbara, and Regina—was growing larger and looking forward to happiness and prosperity, with freedom from religious persecution.

  The years he lived in Prague were indeed golden years for Kepler, the peak years of his life, with many friendships, respect he richly deserved, and splendid scientific accomplishments, but the difficulty in collecting his salary cast a pall over them. Not until five months after Kepler’s appointment did he receive the first payment, and he continued to encounter obstacles collecting even a pitifully small portion of what he was owed. He made such a pest of himself with the royal treasury that a nasty note about him is still appended to the treasury records. When persistence failed, and it almost always did, the Keplers fell back on meager revenues trickling in from Barbara’s property and whatever extra compensation Johannes could scrape up here and there. The Keplers did not, however, live in poverty. Their home was “simply run,”2 as Kepler put it, but the lifestyle they managed to maintain was comfortable and appropriate for a man in his position. Kepler’s wardrobe included fashionable attire with a standing lace collar, the expected work clothing at court and when an imperial mathematician appeared in public.

  Barbara Kepler did not find Prague nearly so congenial as her husband did. What is known about her comes almost entirely from Kepler’s letters, and in these when he mentioned her he most often wrote in her defense, placing the blame on himself that their marriage was not happy.3 Though their lifestyle in Prague was similar to the way Barbara had lived when she was a young woman, she had few skills in simple household economy and, to judge from a few of Kepler’s more candid letters, opted instead for a miserliness perhaps born of fear of sinking into true poverty. Whether her overzealous attempts to economize were well-meant self-sacrifice or a kind of self-martyrdom thrown in the face of her husband is uncertain. She chose, for instance, to cut back severely on her own clothing budget, at the risk of becoming an embarrassment, in order to spend everything on her children.

  While Kepler flourished, Barbara grew melancholy and was bitter about real and imagined differences between her life and the lives of the women she saw around her. Her husband wrote that she had neither “the heart nor the means”4 to make herself better known in Prague society, a plight she may have lamented as much as he. She did, however, make a good impression on some who met her. Contemporary descriptions call her lovely in appearance, polite, respectable, modest, pious, and generous toward the poor.

  At home, where she was burdened with unpleasant economies and a husband often buried in his studies, the more sympathetic Barbara seems not to have been much in evidence. In one of his later letters Kepler described Barbara as “weak, annoying,5 solitary, melancholic” and “fat, confused, and simple-minded.” She immersed herself in prayer books, yet for all her piety she could not curb an ugly temper. Kepler also was not unfailingly placid and long-suffering, and of their relationship he admitted, “There was much biting6 and getting angry, but it never came to any hostility . . . both of us well knew how our hearts felt toward each other.” The quarrels would end when Kepler saw that something he had said had deeply hurt Barbara. Overcome with guilt, he would stop immediately. But “not much love7 befell” him.

  Barbara did not understand astronomy, and though she had followed him into exile because of his conscience, and both of them were deeply religious, there was little if any discussion of religion between them. Kepler, either because he thought his mature, complicated faith would disturb his wife’s simpler beliefs, or because these were not matters to discuss with a woman, always spoke in Latin and avoided German (the language she understood) when he conversed about religion with visitors to their home.

  Meanwhile, though Kepler may have felt he lacked for love from Barbara, he was deeply and widely loved in Prague. Both as imperial mathematician and as a private individual, he received attention and appreciation of a sort he had never known before and had always, shamefacedly, longed for. The emperor himself kept up with Kepler’s scientific work, and visiting dignitaries and royalty sought his company. He had many devoted personal friends, from the highest court officials to simple uneducated people whose uninformed opinions about astronomy and astrology he seemed genuinely to value as a spur to his own thinking. His old friend Hoffmann, who had first brought him to Prague, remained close and provided him with two astronomical instruments, for Tycho’s instruments remained locked away, unavailable, waiting for the emperor to make good on the promised payment to the Brahes. Kepler also kept up a lively correspondence—for he was an engaging letter writer—with numerous acquaintances and scholars.

  As imperial mathematician, Kepler was expected to produce calendars—as he had in Graz—and to give Rudolph advice based on astrology. Kepler had become less and less fond of casting horoscopes, an activity he now described as “unpleasant”8 and “begrimed” work, which should nevertheless not be “smothered.” He began applying a new, more scientific approach, attempting to trace whatever appeared to be established from experience back to causes and physical links, and in 1605 he stopped producing prognostications entirely. His advice to Rudolph often came in the form of essays, not all directly related to astrology, for Rudolph had got into the habit of asking Kepler’s predecessor Tycho for many kinds of advice. One essay was an opinion about a dispute between the Republic of Venice and Pope Paul V about a pump without valves that Kepler had invented. Another had to do with Galileo’s discoveries with the telescope.

  When it came to Kepler’s scholarly work, it was antagonism with the Brahe family that unexpectedly determined his research agenda. At the time of Tycho’s death and funeral, Tengnagel, Elisabeth, and Tycho’s eldest son were away. Kirsten was distraught with grief, and of the rest of the family only Magdalene and Tycho’s younger son Georg were in Prague. Kepler did not wait to consult them or find out how financial matters would be settled. He took charge of Tycho’s astronomical observations. During the year after Tycho’s death, he reveled in the freedom to consult them whenever he wished, without Tycho to snatch them away and accuse him of being too inquisitive.

  Tengnagel returned to Prague in October 1602 and found that there had been almost no payment from the royal treasury for the instruments, observational logs, and manuscripts. He also could not discern that Kepler had made any progress on completing the manuscripts. Through machinations at court, Tengnagel contrived to have the task of composing the Rudolfine Tables transferred to himself, at double Kepler’s salary, though Kepler continued to be imperial mathematician. To Kepler’s way of thinking, he had made plenty of progress, and it was inconceivable that he should relinquish the Mars data when he was so close to answers. He handed over most of the material, but not the Mars observations. Those he secretly kept, thinking it improbable that Tengnagel would actually consult the observations himself and notice that something was missing.

  In the course of this unpleasantness between Kepler and Tengnagel, the emperor in the autumn of 1602 inquired of Kepler what he planned to publish in the near future to justify his employment as imperial mathematician. Kepler made some rapid decisions. Taking stock of his unfinished works, he promised two books: First, within eight weeks, by Christmas, he would complete Astronomiae Pars Optica (The Optical Part of Astronomy). That, he thought, was nearly finished already, the fruit of the summer he had spent in Graz prior to his family’s expulsion. Second, he informed the emperor, by the next Easter (1603) he would complete “Commentaries on the Theory of Mars.” He had been working on Mars, albeit with many interruptions, since he had first joined Tycho at Benatky.

  Kepler was being overly optimistic about completing Astronomiae Pars Optica by Christmas. He was still the same man who at age twenty-six had written that his eagerness led him “to think of a lot9 of things as easy, which proved difficult and time-consuming in the carrying out” and who “in writing [would] continually st
art thinking about new things.” He had already begun to consider many elements of optics that were relevant to astronomy besides those he had been investigating in Graz—for instance, the extent to which light is refracted as it enters the atmosphere, the question that had plagued Tycho. Kepler put his mind to this problem without complete success, for he used erroneous data. Also, he decided there should be exhaustive treatment in the book of eclipses and the sizes and distances of the Sun and Moon. He anticipated no great problem there, for he was currently producing a treatise on the subject. Soon he decided, however, to keep that material separate and not include it. But he needed to delve into the function of the human eye.

  In this area, Kepler met great success. The question of how the eye works was not new, and there were theories that attempted to explain it, but Kepler’s previous optical analyses gave him the background to discover, after rigorous calculation, that the old theories were wrong. Applying his idea of light rays, he was the first to realize that the image of the outside world is not captured in the fluid of the eyeball. It is, rather, projected by a lens in the eye onto the surface of the retina. Working like a “pencil of light,” as he called it, the light rays “draw” the image on the retina. Kepler discovered that the image is upside down and backward on the retina. He was not able to explain how the mind compensates for this, but he did arrive at a precise understanding of the way in which differently shaped eyeglasses could correct nearsightedness and farsightedness. This was a particularly relevant question for him, because he wore spectacles himself.