Tycho and Kepler (47 page)

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 Jupiter
⁴
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 feathers
⁶
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.

A
T
N
EW
Y
EAR
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
Strena
⁷
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 depths
⁸
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 cloisters 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 tables
⁹
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 development 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 Vinariorum
¹²
(New Solid Geometry of Wine Barrels) himself.