TRYING URSUS: A REAPPRAISAL OF THE TYCHO–URSUS PRIORITY DISPUTE

JUAN D. SERRANO, Medellin, Colombia

1. INTRODUCTION

The late sixteenth century is one of the most interesting and exciting periods in the history of astronomy. It is a period of both consolidation and transition, as Richard Jarrell has appropriately described it: “consolidation of the mathematical techniques of Copernicus and transition from the purely mathematical account of planetary motions to a wider discussion of the actual nature of the universe.”1 During the years following the publication of Copernicus’s De revolutionibus in 1543, the most important developments in astronomy and cosmology took place in Northern Europe, particularly in Protestant Germany and Bohemia. Jarrell further classifies the serious practitioners of astronomy at the end of the century into three categories: “those loyal to the Ptolemaic order of the universe but willing to introduce Copernican math-ematical techniques; those who had adopted some variation of the geo-heliocentric framework; and those who were confirmed Copernicans.”2

The humanistic educational reform initiated by Philip Melanchthon encouraged the study of mathematics and astronomy, and the University of Wittenberg became a sort of Mecca for the mathematical astronomers of the time. It was the place where Georg Rheticus, Copernicus’s only direct disciple, had taught, and it became the place where Copernican astronomy was most seriously and thoroughly studied. Among those young astronomers who undertook the pilgrimage to Wittenberg was Tycho Brahe (1546–1601). But it was in Bohemia where the latter’s encounter and subse-quent brief period of collaboration with Johannes Kepler (1571–1630) took place. This was certainly one of the most significant events in the history of astronomy, for it involved an unprecedented fusion of theory and observation: Kepler was one of the ablest mathematicians in Europe and Tycho certainly the best observational astronomer.

For our present purposes, what interests us is that both Tycho and Kepler came to work together under the patronage of Emperor Rudolph II in Prague and, at the same time, both men worked under the shadow of a third character: Nicolaus Reimers Baer (1551–1600), also known by his Latin nickname of Ursus (“The Bear”), who had been the former Imperial Mathematician, a post in which first Tycho and then Kepler were to succeed him. During the last decade of the sixteenth century, Tycho and Ursus became engaged in a fierce and bitter priority dispute over the invention of the geo-heliocentric system of the world, a compromise between Ptolemaic geo-centrism and Copernican heliocentrism, and Kepler, who worked for Tycho, was also reluctantly drawn into it.

Shortly after Tycho’s first diagram of the geo-heliocentric system appeared in print,

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Ursus published a book of his own, containing a very similar cosmological arrange-ment. When Tycho learned about it, he accused Ursus of having stolen the system from him in a visit the latter made to Uraniborg, his observatory on the island of Hven. Tycho initially aired his accusation only in private correspondence, but when he decided to publish a selection of his letters, the matter went public. When Ursus in turn found himself labelled a plagiarist in print, he decided to respond, and he did so by publishing a second book attacking Tycho’s claims to priority and originality in the discovery of the geo-heliocentric system, and also attacking him personally. As a result, Tycho decided to take legal action against Ursus, on the grounds of plagiarism and defamation. When arrangements were being made for the upcoming trial, and relevant circumstantial evidence was being gathered by Tycho, Ursus died, so the trial never took place. Nonetheless, Tycho persevered in his pursuit of Ursus beyond the grave. Now that the author was gone, the enemy was his book, so Tycho launched a campaign against Ursus’s book and also planned to publish a volume of his own containing all the relevant documents, with the aim of settling matters forever: establishing his priority, restoring his honour and reputation, and refuting Ursus’s claims. For the latter task, he recruited the help of Kepler, who indeed wrote a defence of Tycho against Ursus.

FIG. 1. The Tychonic (left) and Ursine (right) geo-heliocentric systems of the world, as depicted in Tycho’s De mundi aetherei recentioribus phaenomenis (1588) and Ursus’s Fundamentum astro-nomicum (1588). In the Tychonic arrangement the orbits of the Sun and Mars intersect in order to account for the fact that, while at opposition, Mars comes closer to the Earth than the Sun, while in the Ursine arrangement there is no such intersection. Both placed the Earth at the centre of the universe, but while Tycho declared that the Earth was “a sluggish body, unfit for motion”, while Ursus attributed a diurnal rotation to it.

19Trying Ursus

Why is the Tycho–Ursus dispute important and interesting for historians and philosophers of science? It is my purpose in this study to provide my own answer to this question by examining the dispute from a different perspective: that of Ursus’s proposed trial. In what follows I shall offer a plausible construction of the trial of Ursus — which in fact never took place — based on a thorough analysis of the avail-able evidence by means of an exercise mixing counterfactual history and creative writing, with the aim of presenting and weighing the evidence on either side in a more clear and balanced way than has been done so far. Given that several versions of the geo-heliocentric system were produced around the same time,3 I think it fair to give Ursus the benefit of the doubt.

Furthermore, unlike Tycho and Kepler, Ursus remains to this day one of the less-studied characters in the history of science: none of his works has been translated into English in its entirety, no biography has been written of him,4 and we possess no portraits. His name does not even occur in the authoritative Dictionary of scientific biography. In fact, Ursus seems to have been effectively banished from the history of science. Is this perhaps an indication of how effective Tycho’s anti-Ursus campaign turned out to be in the end?

What is interesting and to my mind unique about this particular case is that, unlike more famous and thoroughly discussed priority disputes in the history of science, such as the Newton–Leibniz quarrel over the invention of calculus, the literature on the Tycho–Ursus feud is comparatively scarce. As a priority dispute it is also atypical, and therefore interesting, in another sense: that it was intended to be resolved in a courtroom; and had Ursus’s trial taken place — which surely would have happened had not the defendant died too soon — his own life would have been at stake, for according to Bohemian Law, he could have been sentenced “to be branded in infamy, and beheaded or quartered”.5 It is true that Galileo’s life would have also been at stake had not he chosen to recant, but unlike Galileo’s trial (the circumstances of which were very different), Ursus’s proposed trial has been barely studied.

The dispute also has a social side, for certainly Tycho, a Rigsraad6 of the highest Danish nobility, saw it as a matter of honour to defend himself from the attacks of a low-born former swineherd from the peasantry of Holstein, suddenly turned into Imperial Mathematician. Both Ursus and Kepler gave the dispute an epistemological twist which for Tycho apparently was only of secondary importance; for Tycho it was clearly a matter of establishing his priority and thereby destroying Ursus’s cred-ibility, while at the same time defending his honour and noble reputation, blemished by Ursus’s vicious attack on him and his household. As Gingerich and Westman put it, “the ordering of the heavens had become a matter of honor in a rivalry where priority was the prize”.7

The difference between Tycho’s and Ursus’s claims in the dispute is an important one for, as Robert Iliffe points out, “a key region for debate in priority disputes surrounds the question of whether the disputants were claiming credit for the same thing”8 which is clearly not the case here: while Tycho presented his system as his own and unique innovation, Ursus argued there was nothing new under the Sun.

20 Juan D. Serrano

Whereas Tycho is claiming for himself the priority in the discovery of the true system of the world, Ursus claims that his rival has only made a textual discovery, as Gingerich and Westman put it.9 Indeed, several geo-heliocentric arrangements coexisted at the end of the sixteenth century.10 Iliffe further remarks that in priority disputes “the problem for the worker who wishes to publish first is that it is necessary to publish proof of the method while keeping the nature of the method secret”.11 In this case, even though Tycho indeed published first, it was a mix of his unprecedented and overpowering authority as an astronomer, his social prestige, and his effective rhetorical strategies, rather than actual proofs of his “method”, that was decisive for establishing his priority claim. As Robert Westman has observed:

The legal position of Tycho was such that his only obligations were to his liege-lord, the King of Denmark, and not to the rector and senate of a university. His social position was such that he could take the economic and status privileges of nobility and confer them upon the activity of astronomical investigation. The result was a new role model and new prestige for astronomical activity. Tycho, the noble lord and vassal to the Danish king, surrounded himself with an entou-rage — not an army of knights and foot soldiers such as his family would have preferred, but of skilled mathematicians, observers and instrument builders. His weapons were not spears and arrows but his giant observing instruments and, from 1584 onward, his printing press. Tycho could publish what he liked, when he liked (except when there were paper shortages) and, more importantly, for whom he liked.12

Tycho was well aware of his position and understood only too well that if the dispute was fought in the arena of observational astronomy Ursus would be defeated, for “in this field Ursus had no skills and resources to rival Tycho’s”.13 Therefore, in addition to the the defence of his tarnished honour, Tycho framed the resolution of the dispute in terms of his competence and authority as an observational astronomer and instru-ment maker. Just as Galileo after him, Tycho had his own “instruments of credit”, to borrow the term from Mario Biagioli. But as we shall see, he was claiming triumph on a problem (the measurement of Mars’s parallax) which was insoluble by the standards of astronomical observation of the time, even by his own high standards.14

Relatively little has been written on the Tycho–Ursus dispute and even less on Ursus’s trial. Only three book-length treatments of the Tycho–Ursus dispute have been written. The first, authored by the British historian of natural sciences Nicholas Jardine, contains the first English translation of Kepler’s tract Apologia pro Tychone contra Ursum, accompanied by his interpretative essays.15 In the same work, Jardine provides English translations of the relevant passages from Ursus’s Tractatus.16 Jardine has studied thoroughly the status of astronomy in the sixteenth century, identifying the variety of intellectual positions towards it, and thus showing Pierre Duhem’s dichotomy of instrumentalism v. realism to be an oversimplification. Jardine prefers to replace those anachronistic terms for the more nuanced ones of moderate and radical scepticism towards the epistemological claims of astronomy. On the one hand, there

21Trying Ursus

were those like Osiander and Ursus who believed astronomy’s goal was empirical adequacy alone (‘saving the phenomena’ as accurately as possible) and not causal explanation (as Kepler would contend). On the other hand, there were the radical sceptics such as Pontano and Frischlin, who denied the very possibility of knowledge regarding celestial phenomena. In his interpretative essays Jardine focuses on the epistemological side of the dispute, particularly on Kepler’s defence of a realist and progressive vision of astronomy as evinced in his response to the interrelated sceptical arguments of observational equivalence (the objection that several different hypoth-eses can account equally well for the same set of observed phenomena, which implies there is no single ‘correct’ hypothesis) and of insufficiency of evidence (the related objection that agreement with the observed phenomena is not sufficient to establish the ‘truth’ of an hypothesis, as hypotheses which have been regarded as ‘true’ in the past have subsequently been discarded).17 Other authors, such as Rhonda Martens and James Voelkel, also centre their attention on the epistemological import of the Apologia and its relation to the other Keplerian works, emphasizing its importance for Kepler’s more mature philosophical and scientific ideas.

The second, a posthumous book by the American classicist and historian of sci-ence Edward Rosen, is perhaps the most complete collection of material relevant for situating the Tycho–Ursus quarrel in its historical context, and it is very useful for the translations it provides of all the relevant letters exchanged not only between Tycho and Kepler, but also between them and other contemporaries such as Christoph Rothmann, Thaddeus Hayek, Michael Maestlin and the Landgrave Wilhelm IV.18 However valuable these materials are, Rosen limits himself, in my view, to presenting the evidence without engaging much in a critical discussion of it.

The third is the volume more recently prepared by Nicholas Jardine and Alain Segonds introducing La guerre des astronomes, a series of critical editions, French translations and commentaries covering documents relating to the controversy over priority in the formulation of the geo-heliocentric world system.19 This introductory volume presents an overview of the controversy and of the sources for its study.

Another especially valuable study for our understanding of the Tycho–Ursus affair deserves mention: Owen Gingerich and Robert Westman’s monograph on the German mathematical astronomer Paul Wittich.20 This not only elucidates the complex relations among the networks of astronomers in late sixteenth-century Protestant Europe, but actually offers a new interpretation of the available evidence concerning the dispute. They suggest that both Tycho and Ursus may have profitted from Wittich more than they acknowledged, that Ursus may have taken his geo-heliocentric hypothesis not from Tycho but from Wittich (as his system is actually closer to the latter’s than to Tycho’s), and that Wittich’s work may have acted as a catalyst in Tycho’s own thinking.

Not surprisingly, most of the attention in the Tycho–Ursus dispute has been drawn towards the philosophical side of it — and therefore to Kepler’s intervention in it — which is very interesting in its own right, and Kepler’s Apologia is certainly the single most important primary source we have on this. Historians and philosophers

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of science interested in the Tycho–Ursus dispute seem to agree on one thing, namely on the seminal importance that the underlying debate on the epistemological status of astronomical science, brought into view more clearly by this dispute, had for the sub-sequent development of science and the historical and philosophical reflection on it.

Regarding Ursus’s proposed trial, the evidence is yet more scarce. Besides Rosen’s and Jardine’s translations of the relevant letters in their aforementioned books, the best collection of primary sources available is an extensive study on the trial by Jardine and his collaborators, comprising nine brief papers, which appeared in two parts in the Journal for the history of astronomy. 21 This study has provided English translations of additional highly relevant primary sources for the understanding of Ursus’s proposed trial. These include a personal letter from Ursus to Emperor Rudolph II, in which he defends himself from Tycho’s charge of plagiarism;22 information about the printing of Ursus’s Tractatus;23 a translation of Ursus’s Demonstratio, a brief point-by-point summary he probably prepared for his defence;24 a translation of Tycho’s assistant Johannes Müller’s refutation of Ursus’s Demonstratio;25 and Kepler’s own point-by-point refutation of Ursus’s Demonstratio, which was probably composed around the same time as the Apologia and indeed treats basically the same points addressed at more length in the Apologia.26 Additionally, Kepler composed a summary of the Tycho–Ursus dispute at the request of Tycho’s son-in-law, Franz Tengnagel. In this document, entitled Concerning the quarrel between Lord Tycho and Ursus over hypotheses,27 Kepler not only presents an outline of the case and the points he will treat more extensively in the Apologia, but also his conclusion: Tycho is indeed the sole author of the hypotheses in question and Ursus took them from him. All these were preparatory materials intended for use in both the trial and the volume Tycho planned to publish afterwards.

Although Kepler’s main works have been translated into the English and other languages, neither Tycho’s nor Ursus’s works have been. Whilst modern compre-hensive Latin editions of the works of both Kepler and Tycho have been produced,28 Ursus’s works are rare and the information we have on his life and work is scarce. Besides the passages of the Tractatus and the Demonstratio translated by Jardine, no book-length treatment of Ursus currently exists in English.29 On the other hand, the life and work of both Kepler and Tycho has been fairly well studied. Two excellent and comprehensive scientific biographies have been written about them,30 in addition to many good-quality scholarly papers, monographs and books, among which those by Gingerich, Westman and Jardine are especially illuminating.

No comprehensive attempt has been made so far to offer a plausible rendering of Ursus’s proposed trial itself. Of course we cannot know how Ursus’s trial would have ended, but we can offer some plausible insights based on the available evidence, which is precisely what I intend in this study, with the aim of weighing the evidence for and against both Tycho and Ursus, side by side. This work consists of three parts: in the first section, I will give an account of what is known about the dispute and the proposed trial; in the second I will discuss Kepler’s testimony; and in the third I will present the trial itself, which in turn will be divided in three sub-sections: Tycho’s brief, Ursus’s brief, and the verdict.

23Trying Ursus

2. SETTING THE STAGE

The story starts in September 1584, when Danish nobleman Erik Lange and his retinue visited Uraniborg, Tycho Brahe’s astronomical observatory on the island of Hven. Among Lange’s servants was the German Nicolaus Reimers Baer, known by his Latin nickname of Ursus (“The Bear”), a self-taught mathematician from the peasantry of Holstein. During the two weeks they stayed there as Tycho’s guests, Ursus supposedly peered into Tycho’s library and private papers, among which alleg-edly was a sketch of his newly discovered system of the world, the geo-heliocentric system (a compromise between Ptolemy’s and Copernicus’ cosmological models). Being suspicious of Ursus’s behaviour, Tycho arranged for his assistants to have him searched while he slept, and they supposedly found in his pockets a handful of notes and drawings of Tycho’s instruments and the buildings at Uraniborg. But the search turned up no diagram of his world system.31

Four years later, between late 1587 and early 1588, Tycho rushed to print at his own Uraniborg press his De mundi aetherei recentioribus phaenomenis [On the recent phenomena of the aethereal world], a treatise devoted to his study of the comet that had appeared in 1577. However, as chap. 7 of Book II in this work he included a digression on his new system of the world, including the now-famous diagram of the system. As has been hinted above, the Tychonic system is a geo-heliocentric one, namely a hybrid between Ptolemaic geocentrism and Copernican heliocentrism: the Earth is stationary at the centre of the universe, with the Moon and Sun revolving concentrically about it, but with the rest of the planets revolving around the Sun. The most remarkable feature of this system is that it displays an intersection of the orbits of the Sun and Mars, something Tycho had introduced — thus demolishing the idea that solid celestial spheres existed — to account for the fact that during oppositions Mars comes closer to the Earth than the Sun, a conclusion he had supposedly arrived at from his studies on Mars’s parallax.32

In the summer of the same year (1588), a treatise entitled Fundamentum astro-nomicum [The foundation of astronomy] came out of the presses in Strasbourg, authored by Ursus. In it, besides his presentation of trigonometrical methods useful for astronomical practice, he included in chap. 5 a diagram — dedicated to Tycho’s rival observational astronomer, the Landgrave Wilhelm IV of Hesse-Kassel — of a geo-heliocentric system that was very similar to Tycho’s. It differed, however, in that unlike Tycho, Ursus attributed diurnal rotation to the Earth and the orbit of Mars completely encircled that of the Sun, thus avoiding the intersection of their orbits. In the same chapter, Ursus claimed he had first conceived this system in 1585.

Word soon reached Tycho in Denmark of the publication of Ursus’s book, and of the fact that Ursus himself had been visiting the Landgrave’s observatory in Kassel, and sharing information about Tycho’s instruments and world system with his main rival in the quest for accuracy in astronomical observation and instrument-building ability. A mechanical model of the geo-heliocentric system based on Ursus’s account had even been built by Joost Bürgi (Byrgius), the Landgrave’s mechanician. Tycho

24 Juan D. Serrano

was enraged. He immediately rushed to denounce Ursus as a plagiarist, first in private correspondence with friends and colleagues,33 claiming that Ursus had stolen the system from him during the visit that the latter had made to Uraniborg in 1584, and then in print. In order to establish his priority, Tycho decided to publish his Epistolae astronomicae [Astronomical letters], the intended first part of which appeared in 1596. This volume contained a selection of his correspondence with his astronomical colleagues, including the exchange between himself and Christoph Rothmann, the Court Mathematician to Landgrave Wilhelm IV.

In the meantime, Ursus had been promoted to the position of Imperial Mathemati-cian to Emperor Rudolph II in Prague, a post he held from 1591 up to his death in 1600. Now it was Ursus’s turn to get enraged when, upon seeing Tycho’s book, he found himself referred to as a plagiarist and a “dirty scoundrel”.34 In consequence, Ursus decided to respond in like manner, by publishing a book entitled De hypoth-esibus astronomicis tractatus [Treatise on astronomical hypotheses], which appeared in Prague in 1597, shortly before Tycho himself moved there to work under Emperor Rudolph II. Ursus’s book is more of a personal attack on Tycho than a properly astronomical treatise. What makes it interesting is that in defending himself against the charges of plagiarism, Ursus launched an attack not only on Tycho’s person and claim to originality in the invention of his system, but also on the validity of astro-nomical hypotheses as representations of the true form of the universe. Not only did he contend there that Tycho’s (and his) geo-heliocentric ideas had been anticipated in Antiquity by Apollonius of Perga and Martianus Capella, and more recently by Nicolaus Copernicus, but he also argued, as Andreas Osiander did in his anonymous preface to Copernicus’s De revolutionibus, that astronomical hypotheses were merely convenient fictions devised for the sole purpose of ‘saving the phenomena’. Truth about the arrangement of the universe or celestial motions, therefore, was not the province of astronomers. This explains the peculiar twist the dispute was to take with the intervention of Kepler, who took Ursus’s claims seriously indeed and launched into an investigation of the history of astronomical hypotheses from the ancients up to Tycho, in order to disprove Ursus’s allegations.

Given the agressive and insulting language Ursus chose to employ against Tycho and his supporters in his Tractatus, “repaying them in their own coin”, as Launert and Jardine observe,35 and that the insults extended to Tycho’s noble household, the latter decided take legal action against Ursus.

Tycho’s accusation was twofold. Firstly, there was the charge of theft and pla-giarism. Tycho alleged that Ursus had stolen his system of the world — whether by physically removing a preliminary sketch of the system from his library or by committing it either to his notes or his memory — while on his visit to Uraniborg, and then had published it as his own in his Fundamentum astronomicum of 1588. Secondly, Tycho accused Ursus of defamation and civil damages, for his vicious personal attack on Tycho and his family in his De hypothesibus astronomicis trac-tatus of 1597. Essentially, Tycho’s goal in bringing Ursus to justice was to establish his priority as discoverer on the geo-heliocentric system of the world and restore his

25Trying Ursus

and his family’s noble reputation and honour.On Ursus’s side, more than claiming his priority over the discovery of the geo-

heliocentric system, he clearly sought to undermine Tycho’s claims, namely to show that, far from being an original invention or discovery of Tycho’s, as the latter presented it, the germs of it had been laid down long ago by Apollonius of Perga, Martianus Capella and Nicolaus Copernicus, and that Tycho, in claiming his priority, had either not read these authors or had chosen not to give them credit. Aside from these astronomical considerations, Ursus also had personal motives against Tycho, who allegedly had mocked him while at Uraniborg and then had both labelled him a plagiarist and insulted him in print.

Enter Johannes Kepler. By the end of the sixteenth century, religious upheaval was mounting in Graz, where Kepler held the post as District Mathematician at the local Protestant School. The Counter-Reformation measures were gaining force and Protestants had either to convert to Catholicism or leave the city.36 In the midst of all this, Kepler had published his first work, the Mysterium cosmographicum (1596), where he presented his polyhedral hypothesis, and had sent copies to both Tycho and Ursus, hoping to get their approval. Ursus had never replied, but Tycho encouraged him to join his staff at Uraniborg. Soon, however, King Frederick II was dead and Tycho himself was forced to leave Denmark, as the new King, Christian IV, was unwilling to support his researches. Tycho moved temporarily to Germany and from there to Bohemia, where he was welcomed by Emperor Rudolph II. Tycho moved his instruments and staff to Benatky, nearby Prague, the site he had chosen for his new observatory. As these events unfolded, Ursus had published his Tractatus, which contained a letter full of praise Kepler had written him,37 printed without Kepler’s consent. This elicited Tycho’s doubts as to which side of the ongoing dispute Kepler was on, so he wrote to Kepler asking what his position was.38 In reply, he got a long letter from Kepler,39 who was not even aware of Ursus’s book and the printing of his letter therein, excusing himself and condemning Ursus’s behaviour. Once he knew Kepler was on his side, Tycho wrote to Kepler again,40 absolving him and reiterating his invitation to join him, now in Bohemia. As things happened, there were delays in the exchange between Tycho and Kepler, and Tycho’s invitation crossed paths with Kepler as he travelled with his family to Bohemia. In February 1600, Tycho and Kepler finally met, and one of the first assignments Tycho gave Kepler was help-ing him with the ‘mathematical’ part of his campaign against Ursus, a task Kepler undertook with some reluctance.

In August 1600, just as arrangements were being made for his imminent trial, Ursus died, and with his death the dispute took a new turn. Now that the author was gone the enemy was the book, so Tycho turned his energies to the collection of all the relevant documentary evidence with the aim of assembling an ‘anti-Ursus’ volume intended for publication. In a later letter to Kepler, Tycho not only spelled out his enterprise, but also made clear how he intended him to participate in this project: “I have decided therefore to publish this entire proceeding, together with the com-missioner’s decision and His Imperial Majesty’s Decree, in a special book which

26 Juan D. Serrano

will expand to a moderate size…. In the second part of the book I shall reply to the questions that are mathematical and concern the hypotheses. That is why I should also like to have your views of these matters soon.” 41 Tycho specifically stated that his goal was not the destruction of the person of Ursus, as much as of his book (the Tractatus), which was indeed physically destroyed almost in its entirety: “This is not merely a personal action, but also a real action on account of the publication of the book which speaks forever in place of its author. Therefore I must proceed against it.”42 And then he adds: “To me, however, the question was not so much destroying his person, which everybody knows was clownish and vainglorious, but rather his book, stuffed full of so many insults and lies, and restoring the glory and reputation of myself and my associates. God willing, this may still happen.”43

Concerning the legal arrangements advanced for Ursus’s trial, Tycho managed to obtain from the Emperor himself both the appointment of four Imperial Commis-sioners — two Barons and two doctors of jurisprudence — to judge the case44 and the commission of the Archbishop of Prague, Zbigneus Berka de Duba, “to seek out all copies [of Ursus’s Tractactus] for burning and to punish the printer”.45 In a letter to his friend Georg Rollenhagen, Tycho relates: “The Most Reverend Archbishop of Prague has already been charged by the Emperor to notify the printer, have all the copies sought out, as many as are found in Prague, and have them consigned to the flames.”46 For as it turned out, Ursus’s book had also been published without imprimatur and Ursus had even challengingly written on the title page Pragae Bohemorum Apud Autorem: Absque Omni Privilegio [“In Prague of Bohemia, at the House of the Author, Without Any Licence”].47 According to Jardine, Segonds and Mosley, Ursus’s trial would have been what was called an Aktenversendung (“Send-ing of files”), a legal procedure laid down in the Constitutio criminalis Carolina of 1532, “whereby a trial could be delegated to a tribunal for judgment on the basis of affidavited documentary testimonials”.48

In the same letter to Rollenhagen, Tycho not only reveals these details about the proposed trial, but also hints at the possible sentence in store for Ursus: “Had the author lived a while longer, he would have been sentenced, as I learned from the commissioners, to be branded in infamy, and beheaded or quartered according to Bohemian Law.”49 Furthermore, he says he sent two doctors of jurisprudence and a notary to Ursus, already lying on his deathbed, “to ask whether he was willing to retract that malicious publication, chockful of insults”, but he “would not admit most of them, even though they were found spelled out on the pages indicated in his publication” and “he refused to recant and submitted everything to the decision of the judges”. So Tycho managed to obtain from the Emperor the appointment of the four commissioners, “but the defendant died”.50

What Tycho did not suspect then was that he himself had a little more than a year to live. Once Ursus died, he was appointed the new Imperial Mathematician, a post he held up to his death in October 1601. Upon Tycho’s death, Kepler inherited the post in turn, and so the whole case fell into his hands.

27Trying Ursus

3. KEPLER’S TESTIMONY

3.1. The Apologia

In addition to the relevant letters exchanged between Tycho, Ursus, Kepler and their contemporaries (most of the which, translated into English, are contained in the above-cited sources), the single most important document is, of course, Kepler’s unfinished Apologia pro Tychone contra Ursum.51 The composition of the Apologia was one of the first tasks Tycho gave Kepler as his new employee. Kepler said that he had voluntarily lifted the burden of writing it from Tycho’s shoulders, but of course the latter pressed him to fulfil his promise.52 This treatise, intended as the ‘mathematical’ part of Tycho’s legal brief, to be included in Tycho’s projected ‘anti-Ursus’ volume, was composed between the end of 1600 and the beginning of 1601 in Prague.53 It was temporarily abandoned due to Kepler’s return to Graz to settle business affairs in April 1601, and dropped definitively after Tycho’s death in October of that year. However, in a later letter to his friend and fellow astronomer David Fabricius, Kepler clearly stated his intentions to expand, finish and publish the Apologia: “I have writ-ten against Ursus. But I am not satisfied. I must first look at Proclus and Ibn Rushd on the history of hypotheses. I shall publish [this material] some time, when that can be done with less ill-will than now. After all, Ursus was my predecessor [as Imperial Mathematician].”54 These intentions notwithstanding, Kepler’s Apologia remained unpublished until 1858, when Christian Frisch, the editor of Kepler’s works, included it in his Joannis Kepleri astronomi opera omnia, and more recently it was re-published in the modern edition of Kepler’s complete works.55

As was mentioned before, Kepler was supposed to write a defence of his new patron’s priority in the discovery of the geo-heliocentric system. Not only was this an unpleasant task for Kepler, but it was made worse by the fact that Ursus had pub-lished in his Tractatus a letter Kepler had written to him a couple of years before, containing the phrase “I admire your hypotheses”.56 This assertion, and his praise of Ursus’s abilities as a mathematician, although intended by Kepler only to elicit a commentary from the Imperial Mathematician about his first work, the Mysterium cosmographicum, could be a source of trouble with his new employer. Whose side in the dispute was Kepler taking? Given that Kepler’s letter had been published without his consent, and his overpraise of Ursus expressed in it made the initial approach between him and Tycho uneasy (as the dispute was already underway), Kepler had as many personal motives to write against Ursus as to defend Tycho when he set to write the Apologia; and indeed at its outset he presents himself as the judge in the dispute. However, Kepler was never interested in the priority issue itself — his brief discussion of it was confined to the introductory exordium and he deliberately refrained from addressing it in the remainder of Apologia — but instead he restricted his discussion to the philosophical, historiographical, scientific and even philological (to use Kepler’s own word) aspects raised by Ursus. This is what makes the Apologia so interesting for the history and philosophy of science: whilst Kepler was supposed to defend Tycho’s honour, he ended up rather defending the epistemic legitimacy

28 Juan D. Serrano

of astronomical hypotheses, as well as the progress of astronomical knowledge and the astronomer’s competence to share in the quest for the truth about the universe (which up to that point had been confined to the territory of theologians and natural philosophers).

Indeed, the Apologia has attracted the attention of philosophers and historians of science mainly because of the reflections on epistemology, scientific methodology and the historical development of astronomy it contains. Max Caspar, Kepler’s foremost biographer and translator of his main works into the German language, observes that the Apologia “is of great interest, especially in respect to methodology, since in it Kepler explained the notion of the astronomical hypothesis and rejected the interpreta-tion that as such it is only a matter of obtaining correct conclusions by calculation”.57 Rosen says its interest lies, on the one hand, in its discussion of scientific method and the nature of astronomical hypotheses, and on the other, in its discussion of the history of astronomy,58 and calls the Apologia “the earliest exploration of the history and logical structure of astronomical hypotheses”.59 Jardine calls it no less than “the birth of history and philosophy of science as a distinctive mode of reflection on the status of natural science”,60 and adds that “had it been published, it would probably have stood on a par with other seminal works on scientific methodology such as Descartes’ Discourse on Method and Bacon’s Novum Organum”.61 More recently, Martens has called the Apologia “an early modern treatise on realism and Kepler’s most epistemologically focused work”,62 and Voelkel describes it as “a fine summary of Kepler’s philosophy of astronomy”.63 However, only Westman has called attention to a no less interesting aspect of the Apologia: “While Kepler’s treatise has been praised, and rightly so, for its perceptive and forward-looking analysis of methodo-logical realism and its refutation of scepticism, I think that we must also see it as a development, and explicit articulation, of the Copernican tradition of defending the rights of a mathematical astronomer to make new claims in natural philosophy.”64

As Jardine has pointed out, Kepler composed his Apologia as a Ciceronian judicial oration, in which he assumed the role of judge in the dispute and treated astronomi-cal hypotheses, rather than his patron Tycho, as the ‘client’ to be defended.65 The extant text of the Apologia consists of four parts or chapters. In the first part, Kepler presents his definition of astronomical hypotheses and his requirements for their legitimacy, as well as a refutation of Ursus’s views on hypotheses as laid out in his Tractatus. In the second part, Kepler provides a brief history of the development of astronomical hypotheses, from Thales up to Tycho, with the aim of showing that there has been theoretical progress in astronomy. The last two parts are devoted to a detailed refutation of Ursus’s claim that the geo-heliocentric hypothesis was already expounded in the works of Apollonius of Perga, Martianus Capella and Nicolaus Copernicus, which if proven true, would undermine Tycho’s claim to originality and priority. After discussing Apollonius and Capella and when Kepler is about to move to the discussion of Copernicus, the Apologia suddenly breaks off. Tycho’s death apparently released Kepler from the burden of finishing his defence (although he intended to finish it, nonetheless, as he said in his letter to Fabricius), and Kepler

29Trying Ursus

now turned his attention to the astronomical research he is better known for, namely his work on the orbit of Mars, which was to lead him to the Astronomia nova and the first two laws of planetary motion.

3.2. Kepler’s Verdict

As Schofield points out, “Kepler judged the case on the mathematical rather than the circumstantial evidence”.66 In fact, at the very outset of the Apologia Kepler declares he will confine himself to the ‘mathematical’ part of the dispute and leave the legal action for Tycho to carry out.67 Though that was precisely the task Tycho had given him, certainly the latter would have desired Kepler to make more use of the circumstantial evidence he had collected.

However, Kepler’s preliminary conclusion, based on the circumstantial evidence from both Ursus’s book and Tycho’s report, was sharply summarized in a letter to his friend Herwart von Hohenburg, Chancellor of Bavaria:

Ursus declares: 1, that he derived his hypotheses from elsewhere; 2, that he found it set out in Copernicus and attributed to Pergaeus [Apollonius]; and 3, that it is the same as that which Tycho holds. So he admits that he copied it from someone else, which is just what Tycho claims.

It should be noted that whoever revealed Ursus’s hypotheses to him (lack-ing a little, which it is easy to make good) clearly held the same hypotheses as Tycho. But who is he? Ursus says he is Copernicus, and through him Pergaeus, and no one else besides. So no one apart from Pergaeus in Copernicus or Tycho is the author of the Ursine hypotheses. But I declare that neither Pergaeus nor Copernicus was the author of the Ursine hypotheses. So Tycho alone, and no one else, was the one who showed Ursus the way to his hypotheses. The confirmation of the case lies in a grasp of the true meaning of Copernicus — Ursus himself provides the rest by his confession.68

It would be anachronistic to include Kepler’s Apologia in the trial, for it simply would have been unavailable to the commissioners: it was composed around a year after his report to Tengnagel, dated March 1600. By that time Ursus was dead and Tycho had succeeded him as Imperial Mathematician. In spite of Jardine’s claim that the Apologia marked “the birth of history and philosophy of science” — a claim which is debatable in its own right, since the Apologia did not see the light until 1858 — the truth is that for the purpose it was intended (to serve as the ‘scientific’ part of Tycho’s never-published volume on the trial) it was entirely irrelevant, despite the cogency of the arguments Kepler deploys there against Ursus and the sceptics in general. In the Apologia not only does Kepler reassert that Ursus indeed took his hypotheses from Tycho (a conclusion he had announced in his earlier report to Tengnagel), but he clearly asserts the priority of Tycho over the geo-heliocentric system:

Ursus pretends that the other transformation was carried out by Apollonius in such a way that whereas before the Earth (the centre of the fixed stars) was carried

30 Juan D. Serrano

round with an annual motion about the immobile Sun, the centre of the planets, now the Sun and the centre of the planets go round the Earth and the centre of the fixed stars, which are at rest, in the same period of a year. And, indeed, this form of hypotheses was invented and established by Tycho Brahe, for reasons widely expounded by that author and from motives which are not to be belittled (and Ursus concocted these things in order to harass him).69

Furthermore, Kepler not only credits Tycho with having produced novel hypotheses; he goes on to assert their superiority over the ancient ones: “Through the hypoth-eses which he justly claims as his own Tycho has provided us with a more perfect astronomy than they left behind.”70

Although Kepler did not doubt that Tycho was the author and inventor of these hypotheses, in other places he appears to cast some doubt on his originality. For instance, in a letter to his friend Herwart von Hohenburg, Chancellor of Bavaria, Kepler says: “It could well be that he discovered them [his hypotheses] of his own accord, not guided by Copernicus’s opinion, but that nevertheless he constructed a world that can easily be derived by alteration of the Copernican opinion.”71 Here Kepler seems to be making a concession to Ursus, and even more emphatically so in section 18 of his refutation of Ursus’s Demonstratio: “Against Maestlin (who is, however, fairer to Copernicus than Ursus, and spoke not in order to disparage him but to exalt his invention), against Tycho, against Ursus, I say: it was at the outset less possible for Copernicus to dig out the opinion of Aristarchus concerning the five planets from the very few words that Archimedes utters about the Earth, Sun and fixed stars than it was for Tycho to make his hypotheses out of the Copernican ones.”72 Thus, when not constrained by the deference due to his patron, Kepler expressed the possibility that Tycho could have derived his geo-heliocentric arrangement from the Copernican one, which was precisely Ursus’s charge (note that in the quoted pas-sage from his refutation, Kepler seems more concerned to defend Copernicus than Tycho). As is well known, Kepler was a committed Copernican and never believed the Tychonic arrangement was the true portrayal of the universe, but in the Apologia he had to conceal his true inner feelings.

It is also well known that Kepler was not a disinterested and impartial party in the dispute, and Kepler himself admitted this openly.73 First, because as a Protestant, his post as District Mathematician in Graz was under threat due to the Counter-Reformation measures alluded to in the previous section, a situation that motivated him to go to Prague to seek Tycho’s patronage, and the Apologia was one of the first tasks Tycho assigned him. He was writing for Tycho. Second, because Kepler himself had also been a victim of Ursus, who first involved him in the dispute by printing in his Tractatus, without Kepler’s consent, a letter the latter wrote praising him when the dispute was already underway.74

Having said this, we might ask: if his Apologia would have been irrelevant for the commission’s verdict on Ursus, why was Kepler’s involvement in the dispute important at all? Paradoxically, what is most important and interesting about Kepler’s

31Trying Ursus

intervention is, I think, what is not directly relevant for the trial itself, but indeed highly relevant for the advancement and consolidation of astronomy as a physical science.

This material is mostly contained in the first chapter of the Apologia, significantly titled “What an astronomical hypothesis is”. In countering Ursus’s scepticism about the possibility of ever attaining true or even probable causal knowledge in astronomy, Kepler launches into an exploration of the development of astronomical hypotheses back into Antiquity. He starts with the origins of the term ‘hypothesis’ itself, tracing its usage from geometry, through logic and then to astronomy. Then he offers his own definition of astronomical hypotheses: “We thereby designate a certain totality of the views of some notable practitioner, from which totality he demonstrates the entire basis of the heavenly motions. All the premises, both physical and geometri-cal, that are adopted in the entire work undertaken by that astronomer are included in that totality.”75 Against Ursus, who had subscribed to Andreas Osiander’s view (that astronomers should be content only with describing and predicting, not with explaining), Kepler advances the view that the aim of astronomical hypotheses is nothing less than a portrayal of the true form of the universe. In fact, further in the chapter he defines a body of astronomical hypotheses as “the portrayal of the [true] form of the universe, derived from observations”.76

Kepler identified several components that astronomical hypotheses should have. First of all, they have both geometrical and astronomical components, but mere geo-metrical hypotheses should not be conflated with properly astronomical hypotheses. He illustrates the difference with an example: “Thus if some astronomer says that the path of the Moon describes an oval shape, it is an astronomical hypothesis. But when he shows by what circles a drawing of this sort of oval can be constructed, he uses geometrical hypotheses.”77 And further on he elaborates: “To set down in books the apparent paths of the planets and the record of their motions is especially the task of the practical and mechanical part of astronomy: to discover their true and genuine paths is (despite Osiander’s and Patricius’s futile protests) the task of contemplative astronomy; while to say by what circles and lines correct images of those true motions may be depicted on paper is the concern of the inferior tribunal of geometers.”78

According to Kepler, geometry and arithmetic — and Kepler adds physics — are truly the “wings” of astronomy, but a knowledge of these essential tools alone (such as Ursus possessed) does not suffice to make a good astronomer, whom Kepler likens to a good architect.79 Hence, erroneous opinions such as those of Osiander and Ursus stem out of conflating both kinds of hypotheses. For Kepler, in order to count as properly astronomical, an hypothesis should comply with certain requirements, which can be summarized in five points:

1. Be true (represent or portray the true form of the universe).2. Be consistent (both internally and with other hypotheses, even from other disciplines).3. Have unique physical consequences (which is what distinguishes it from other empirically equivalent hypotheses).

32 Juan D. Serrano

4. Have predictive and retrodictive power (‘save the phenomena’ as accurately as possible).5. Have explanatory power (provide what Kepler calls causas probabiles; not just predict and retrodict, but also explain).

Even though false hypotheses can occasionally and fortuitously lead to true causes, as Ursus claims, Kepler points out that it only happens by mistake and false hypotheses always betray themselves in the end. It is not just the agreement with observations, but the “physical considerations” and the agreement with other disciplines external to astronomy, in addition to the aesthetical considerations, that are decisive when determining the superiority of a hypothesis over its rivals. This is quite interesting, for at the same time Kepler is drawing a distinction between geometrical and astro-nomical hypotheses, he is blurring the disciplinary boundaries between mathematical astronomy and physics or natural philosophy. This is best exemplified with one of the Apologia’s most important passages, which effectively marks a departure from such a neat disciplinary distinction:

The astronomer ought not to be excluded from the community of philosophers who inquire into the nature of things. One who predicts as accurately as possible the movements and positions of the stars performs the task of the astronomer well. But one who, in addition to this, also employs true opinions about the form of the universe performs it better and is worthy of greater praise. The former, indeed, draws conclusions that are true as far as what is observed is concerned; the latter not only does justice in his conclusions to what is seen, but also, as was explained above, in drawing conclusions embraces the inmost form of nature.80

The astronomer, therefore, should not be content with “mythologizing” (that is, devising fictitious hypotheses and pretending to demonstrate true phenomena by means of false causes, as Osiander and Ursus had suggested), but should philosophize, and Kepler will reassert this conviction in the Astronomia nova.81

In the the remainder of the Apologia, Kepler sets out to show that Ursus’s alleged examples of “astronomical hypotheses” (and anticipations of Tycho’s) are found wanting on the grounds of his five requirements. As a matter of fact, Kepler argues in the third chapter, Apollonius was no astronomer, but just a geometer,82 and he is not concerned with the dispositions and magnitudes of the planetary orbs, but only with geometrical demonstration. Instead of providing a cosmological system, Apollonius merely offers a geometrical procedure for determining the stationary points of specific planets, but he is not concerned with the planetary system as a whole. Regarding Capella, Kepler argues in the fourth chapter that the view Ursus has attributed to him (that the inferior planets revolve around the Sun) is not exclusive to him, but was shared by Plato, Pliny and Vitruvius in the past — and in any case they were not astronomers. In astronomy, it clearly does not suffice to produce ingenious specula-tions and to invent new hypotheses. More importantly, these hypotheses should be elaborated and demonstrated with a keen eye to details, and verified by observation.

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Addressing Ursus in his refutation to the latter’s Demonstratio, Kepler argues:

If you or anyone else at all cobble together some speculation, it is not customary for it to be declared the hypotheses of an author and to be considered among their number. To succeed, a book must show genius. This honour is given to masters of the art. Nor if you produce a true [speculation] and do so first will it belong to you, who attempt nothing further, rather than the master astronomer, who refines it with demonstrations and puts it into practice.83

Hence, not anyone who just puts forth some ingenious speculation deserves to be called an astronomer, but only the one who also elaborates and substantiates his insight on mathematical, geometrical and physical grounds, and Kepler illustrates this point with the examples of Aristarchus and Copernicus. As is well known, Aristarchus was the pioneer of the heliocentric hypothesis, according to which the Earth revolves around a central and stationary Sun. But from Aristarchus all that survived is a pas-sage from Archimedes in his Sand reckoner informing us about this Aristarchean insight. What in the hands of Aristarchus, as far as we know, had only been a bril-liant conjecture, “Copernicus enriched by the addition of his entire astronomy”,84 providing a solid grounding on detailed demonstrations. By the same token, even if Ursus displayed some ingenuity in proposing a new system — if he didn’t take the idea from Tycho — that required justification is absent, or at least insufficient.

Finally, against Ursus, Kepler argues that there is also cumulative progress in astronomy. It is true that some ancient hypotheses have been discarded, but some have also prevailed. And the requirements he set forth could serve as a guide for determining the superiority of a given hypothesis over its rivals. In addition, while acknowledging the brilliance of some ancient astronomers such as Ptolemy, Kepler takes Ursus to task for claiming that ancient astronomy was superior. Not true, Kepler argues, as modern hypotheses (including Tycho’s) provide “a better grasp of the causes of astronomical phenomena”, thus leading to a “more perfect astronomy”.85

By introducing his game-changing new definition and criteria for astronomical hypotheses — which strikes us as close to the modern concept of a scientific theory — Kepler effectively laid down the methodological foundations for a new astronomical science, which was so fruitfully exemplified in his later works. At the same time, by introducing physical considerations and demanding that astronomy should not limit itself to empirical adequacy but also provide causal explanations of phenomena, he blurred the once-neat disciplinary boundary between natural philosophy or physics and mathematical astronomy. Although he never did so, Kepler intended to finish and publish his Apologia for reasons deeper than establishing his patron’s claim to priority, as Rosen suggests at the end of his book:

To eradicate mistaken beliefs; to persuade those who are in a position, whether by virtue of private wealth or political power, to support genuine research, vili-fied by the jealous or by the incompetent; and to disseminate the truth — these are the noble purposes, far transcending the personal motives, for which Kepler

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wrote his Defence of Tycho against Ursus. When he started, he intended to finish it, as is shown by the choice of his word pertexuisse, which means “to continue a weaving operation right through to the end”. But the weaving of the seamless robe of the Defence was interrupted just short of the end.86

Now that we have the necessary threads for Ursus’s trial, we can start our own weav-ing operation.

4. THE TRIAL

This is the report of the Imperial Commission appointed by Emperor Rudolph II to examine the accusations made by Lord Tycho Brahe of Knudstrup, Danish noble-man and Rigsraad,87 former Governor of the island of Hven and Court Astronomer and Mathematician to the deceased King Frederick II of Denmark, against Nicolaus Reimers Baer “Ursus” of Dithmarschen, current holder of the post of Imperial Mathematician at Emperor Rudolph’s court, as well as the latter’s defence against those accusations. The four appointed commissioners were Barons Christopher von Schleunitz and Ernfried von Minckwitz, assisted by two Doctors of Jurisprudence,88 and the Commission met at the Imperial Capital of Prague in order to settle the case, based on the files that Brahe and Ursus sent us and following the procedure laid down in the Constitutio criminalis Carolina of 1532.

These files include: a copy of the defendant’s book, De hypothesibus astronomicis tractatus, published in Prague in 1597, where the prosecutor has marked the specific offensive passages;89 a copy of Brahe’s Epistolae astronomicae, published at the author’s own press in 1596; copies of the notarised testimony of Erik Lange’s sec-retary, Michael Walther; and copies of the following relevant letters: a letter written by Brahe’s former assistant at Uraniborg, Christen Hansen, testifying on Ursus’s behaviour at Uraniborg; a letter addressed by Ursus to the Emperor, defending himself from Brahe’s accusations;90 and a letter from Brahe to his former teacher Brucaeus on the issue of Mars’s parallax, sent to us by an anonymous student; the account of the Brahe–Ursus dispute composed in March 1600 by Master Johannes Kepler, District Mathematician of Graz and Brahe’s current assistant, addressed to Franz Tengnagel, Brahe’s son-in-law;91 a pamphlet that Ursus prepared for his defence92 and the responses to it by Brahe’s assistants Müller and Kepler; and the testimony of Zbigneus Berka de Duba, current Archbishop of Prague.

The plaintiff, Tycho Brahe, claims the following: that Nicolaus Ursus stole the idea of his system of the world from him while he was visiting Brahe’s observa-tory, Uraniborg, in the autumn of 1584, under his patron Erik Lange. That then he published it under his own name, without giving due credit to its creator, in his book Fundamentum astronomicum, which appeared in Strasbourg in 1588. And that, not content with having plagiarised him, he launched into a vicious attack against his noble person and family in print, in his later book De astronomicis hypothesibus tractatus, which appeared in this city in 1597, be it noted, without imprimatur.

The defendant, Nicolaus Reimers Baer, otherwise known as Ursus, denies the

35Trying Ursus

claim that he stole Brahe’s system of the world and claims that this system was first espoused in the writings of Apollonius of Perga, Martianus Capella and, more recently, of Nicolaus Copernicus. Furthermore, he claims that even if it were Brahe’s own idea, it is not proper for an astronomer to claim to know the truth about the arrange-ment of the universe, which can come only from Holy Scriptures. He argues that the astronomer only devises fictitious hypotheses to account for celestial appearances, and therefore Brahe’s claims to truth are empty. In addition, he claims that Brahe has also slandered his person in print, in his book Epistolae astronomicae, published in Denmark in 1596, where he is called a plagiarist and insulted.

These are the disputants’ respective claims. What follows is our analysis of their respective briefs.

4.1. Brahe’s Brief

Tycho Brahe is currently recognized as one of the foremost astronomers in Europe, having built his own instruments and observing facilities (Uraniborg and Stjerneborg, on the island of Hven) and made systematic observations of the planets for nearly three decades.93 For this campaign he relied on the support of Frederick II, King of the Realm of Denmark. It is said that in his competence as a star observer and instru-ment builder he is unrivalled. As far as we know, only the Landgrave Wilhelm IV of Hesse-Kassel, has also built his own observatory and some instruments in Kassel.

Brahe claims he arrived at the idea of his geo-heliocentric system of the world, the one that he contends Ursus stole from him, from his measurements of the paral-lax of the planet Mars, which was one of Brahe’s long-term projects at Uraniborg. When observed against the background of the fixed and more distant stars at different positions with respect to Earth, the red planet should exhibit a little angular displace-ment, proportional to its varying distance from the Earth. The closer it is, the larger this displacement, or parallax, will be. While at opposition to the Sun (when Mars is 180 degrees from it), according to both the Copernican and Tychonic systems, Mars comes closer to the Earth (by around one-third of the Sun’s distance)94 and therefore should exhibit a larger parallax than that predicted by Ptolemy. However, this angle of parallax is still very small and therefore very difficult to measure.

Establishing the issue of Mars’s parallax is important on two counts. Firstly, because this measurement would mean a way to knowing which system (Ptolemy’s or Copernicus/Brahe) is better at saving the phenomena. Secondly, and most importantly for our present purpose, because this is the very basis on which Brahe has claimed to have conceived his system of the world.

In his correspondence, he reports that he conducted observations to this end during the Martian oppositions of 1582–83, 1585 and 1587, but his claims concerning the measurement in question are ambiguous: in some of the letters he published in his Epistolarum astronomicarum liber primus (1596) he claims to have started the observations directed towards this purpose and achieved his goal (namely, to have found the expected larger Martian parallax) as early as 1582.95

36 Juan D. Serrano

However, elsewhere he admits that he was not able to find the parallax at that stage of his researches. An anonymous pupil of Brucaeus has sent us from Rostock a copy of a letter Brahe wrote to his former teacher in 1584, where Brahe plainly confesses he was not able to find the parallax in 1582 and that therefore Copernicus’s system has to be discarded.96 Thus, the evidence from Brahe’s letters is not conclusive, and even contradictory. Moreover, as perhaps no one in Europe has better instruments than his, it is difficult to establish beyond doubt the truth of his claims. In any case, all competent astronomers agree that a greater parallax for Mars — due to the fact that at opposition it comes closer to Earth than does the Sun — is a prediction of both the Copernican and Tychonic systems, so even if a larger parallax for Mars were suc-cessfully measured, that alone would not establish the truth of the Tychonic system of the world: at best we would be faced here with a choice between Copernicus’s and Brahe’s systems.

At this point, it is also relevant to mention that it is also known that some rough sketches of geo-heliocentric systems existed as early as 1578,97 namely those made by the German mathematician Paul Wittich of Breslau, with whom Brahe came into personal contact when Wittich visited Uraniborg in the summer of 1580.98 Wit-tich’s diagrams showed geometrical transformations from heliocentric (Copernican) arrangements into geo-heliocentric ones. It is possible, therefore, that Brahe may have seen Wittich’s sketches and discussed these ideas with him. It is possible also that Ursus may have come into contact with Wittich at some point during both men’s wanderings through the learned circles of Germany.99 We cannot know if Wittich was a source of inspiration for both Brahe and Ursus, but that is certainly a possibility.

As has been pointed out above, in Brahe’s system the orbit of Mars intersects that of the Sun, therefore allowing Mars to come closer to the Earth than the Sun while at opposition. In Ursus’s system, there is no such intersection: the orbit of Mars completely encircles that of the Sun, so geometrically it is not possible for Mars to come closer to the Earth than the Sun, just as is the case in Ptolemy’s and Wittich’s systems. Hence, in the Ursine system Mars comes at the same distance from Earth as the Sun while at opposition. Moreover, Ursus sets the Earth in diurnal rotation, whereas Brahe regards the Earth unfit for motion. In conclusion, Ursus’s system is closer to that of Wittich than to that of Brahe’s.

Let us turn now to the witnesses’ testimonies for Brahe. It is a fact, attested by the disputants themselves and by at least two witnesses, that Ursus was in Uraniborg in September of 1584, accompanying his patron Erik Lange on a visit to Brahe’s observatory. What happened there is less clear, but we have examined those testimo-nies. Both Brahe and his witnesses, Walther and Hansen, claim that Ursus behaved suspiciously while there, apparently making notes and drawings of the buildings and the instruments, and browsing through the library. All of them agree on that. Brahe specifically claims that Ursus may have found an early diagram of his system (which he now considers “defective” because it did not include the intersection of the Martian and solar orbs) showing the orbit of Mars completely encircling that of the Sun, just like the one he published in his Fundamentum astronomicum. Brahe says Ursus may

37Trying Ursus

have copied it or committed it to memory. Both Brahe and Walther say Brahe arranged with his assistants to have Ursus searched while he slept, but they found no diagram of the geo-heliocentric system in Ursus’s pockets, so this is not proven. Concerning Hansen’s testimony — which unlike Walther’s is not notarised — we conclude it is not reliable. Hansen was not at Uraniborg at the time of Ursus’s visit, but was rather told these things by Brahe’s other assistants at a later time,100 so he can hardly count as a witness and thus his account is clearly based on hearsay. Walther’s testimony, though notarised, tells that Ursus himself told Walther that he had browsed through Brahe’s books.101 However, browsing through the library is one thing and stealing a sketch, another. Unfortunately, Lange’s testimony is not available to us.102

Is it impossible that Ursus, who despite his humble origins possessed some math-ematical ability, may have conceived of a geo-heliocentric arrangement independently (an idea others like Wittich, Rothmann and Roeslin toyed with within the same period), which would make him innocent of the charges of theft and plagiarism? Certainly not. Arriving at a compromise between Ptolemaic geocentrism and Copernican helio-centrism is desirable for astronomers and enjoys appeal at the moment because of its synthesis of ancient and modern astronomical science,103 and its advantage of saving the appearances as well as Copernicus, while not contradicting Holy Scriptures. Of course, it is also possible that Ursus may have drawn some inspiration from having seen a diagram by Brahe at Uraniborg, but it is also equally true that Brahe may have drawn some inspiration from having seen Wittich’s sketches and/or discussed these ideas with him when Wittich was at Uraniborg. In consequence, Brahe’s charges of theft and plagiarism are not proven.

It has come to our attention that Brahe also entertained parallel qualms about priority and credit with Helisaeus Roeslin and particularly with the Scot Duncan Lid-dell, a former student of Brucaeus in Rostock. Even though both Roeslin and Liddell openly advocated the geo-heliocentric system, neither of them actually claimed prior-ity, exclusive credit or even independent discovery for the idea. Nevertheless, Liddell in particular aroused Brahe’s unjustified rage. Brahe took his former friend Liddell to task for teaching ideas both had privately discussed in Liddell’s visits to Uraniborg in 1587–88, and for presenting them as his own. But in fact the circumstantial evidence suggests that Liddell openly expressed admiration for Brahe as an astronomer and when he lectured on the geo-heliocentric system in Germany he gave due credit to him. In addition, Liddell made contributions of his own to the mathematical details of planetary models within the geo-heliocentric framework. In this case, it is clear that Brahe’s suspicions were unfounded and his reaction exaggerated.104

Let us turn now to Brahe’s charges of defamation. On this the Commission has found Ursus, as is clear that the language he employs in his Tractatus is rather inap-propriate for a treatise of this sort, turning away from properly astronomical, math-ematical and philosophical arguments to an unfounded and agressive attack on Brahe’s person, as well as on his wife and daughter, such as when he says, in responding to Brahe’s charge of plagiarism: “The word plagium applies to persons, and strictly to a wife or a daughter. But Tycho never married or had a wife. And his daughter,

38 Juan D. Serrano

though the most nobly-born of girls, was not yet nubile at the time I was there and so not of much use to me for the usual purpose. But I don’t know whether or not the merry crew of friends who were with me had dealings with Tycho’s concubine or his kitchen-maid.”105 Ursus’s offences are further evinced by the dozen passages the prosecutor marked for us in his copy of the Tractatus.106 As if employing this unac-ceptable language in a printed work were not enough, the work was printed without the due license and privilege of the Emperor. For this offence, the Archbishop of Prague has already ordered all the extant copies of the book in question to be confis-cated and burned, and the anonymous printer to be punished accordingly, if found. So not only is Ursus guilty of defamation against Brahe and his family, but also of breaching the law by having printed a book without the required imprimatur (it should be noted, however, that although Ursus’s book was finally published without imprimatur, we have evidence that he wrote a letter to the Emperor asking for the Imperial Privilege for his book, but as far as we know that letter was not answered and the privilege never granted).107

4.2. Ursus’s Brief

Nicolaus Reimers Baer, more widely known as “Ursus”, currently holds the post of Imperial Mathematician in the court of Emperor Rudolph II. He comes from the peasantry of Holstein and taught himself mathematics at a young age, which took him from being a swineherd to working as a surveyor at the service of Heinrich Rantzau, Governor of Holstein, and in 1591 to the post of Imperial Mathematician. He has authored a Latin grammar, a work on surveying and the first German translation of Copernicus’s De revolutionibus. Despite his acquaintance with mathematics, he lacks the material resources and technical expertise that Brahe possesses and that would enable him to make accurate astronomical observations. Although he has visited the observatories of both Brahe and the Landgrave Wilhelm IV, he has not built any instruments or performed astronomical observations himself. Clearly, then, he could not have come to his system of the world through the same avenue Brahe claims he did (that is, through careful measurements of Mars’s parallax). Therefore, if the system he published in Fundamentum astronomicum is his own creation, he came upon it through his theoretical acquaintance with astronomy.

In chap. 5 of his book, where he presents the diagram of his world system dedi-cated to Landgrave Wilhelm IV, he also states that he came upon his idea in 1585, while in Pomerania. Apart from his own assertion, there is no additional evidence supporting this claim, but it is interesting to note that if Ursus had really stolen the system from Brahe and knowingly wanted to cover himself from Brahe’s accusa-tion of intellectual theft, he would likely have dated his discovery before 1584, that is, prior to his visit to Uraniborg.108 Therefore, it remains a possibility that Ursus independently conceived a very similar idea (though not entirely equal, as has been explained in Brahe’s brief).

Ursus’s counter-claims are ambiguous. On the one hand, he says in his De

39Trying Ursus

hypothesibus astronomicis tractatus109 that astronomers cannot expect to know any-thing certain from astronomy (the true arrangement of the universe, the true causes of celestial motions) but only devise fictictious hypotheses to account for the phe-nomena. But as several different hypotheses can be conceived to account for a single phenomenon, there is no way to establish which one is true. Moreover, he says that since past astronomical hypotheses have turned out to be false, there is no reason to expect that any of the current ones are necessarily truer. If this is so, Brahe’s claims to have come upon the one true system of the world are empty.

On the other hand, Ursus also claims that Brahe’s hypotheses are not really his, but that he either took them from Apollonius of Perga, Martianus Capella or Nico-laus Copernicus, or did not read those authors carefully. In the Tractatus he says that Brahe has just revived those ancient hypotheses, “published them as his own and impudently tries to pass them off as his own work, either through ignorance or knavery”.110 But as Kepler argues in his report to Franz Tengnagel, Brahe’s son-in-law, there is no explicit formulation of the geo-heliocentric hypothesis in any of the works of those authors. According to him, Apollonius’s hypotheses are not astronomical but geometrical; Capella was no astronomer at all and took his hypothesis, the Capellan system,111 from Plato, Pliny and Vitruvius. Regarding Copernicus, he says Ursus has distorted (or misunderstood) the sense of some passages in De revolutionibus so as to give the impression that Copernicus is talking about the geo-heliocentric system. Thus, it is probably Ursus who misread those authors.112

We said earlier that Ursus’s claims are ambiguous because he says he is neither claiming his priority over those hypotheses, nor asserting their truth. Curiously, however, in the title page of his De hypothesibus astronomicis tractatus, he advertises his hypotheses thus: “A Validation and Defence of the Astronomical Hypotheses Invented, Propounded, and Published by Himself, against Certain Persons who Dare to Claim these Hypotheses for Themselves Imprudently or rather Criminally.”113 Note that it does not say “invented, propounded and published” by Apollonius, Capella or Copernicus, but by Ursus himself. It is also clear that the reference to “certain persons who dare to claim these hypotheses for themselves” is nothing but a reference to Brahe’s claims. In the same title page, Ursus quotes a biblical passage from Hosea, which reads “I will meet them as a bear, that is deprived of her whelps”, implying he is being deprived of his hypotheses. So, after all, he appears indeed to be claim-ing priority over those hypotheses, and if he charges Brahe with appropriating for himself the hypotheses of previous authors, Ursus seems to be doing just the same. Nevertheless, the bulk of Ursus’s argument intends to show that Brahe’s hypotheses are not really novel and were already set forth by previous authors, rather than to establish his own priority.

Another example of the ambiguity of Ursus’s claims is his statement that “hypoth-eses do not err in the least if they contradict the commonly held principles of other arts and disciplines, or, indeed, even if they contradict the infallible and certain authority of the Sacred Scriptures”.114 Why, then, in the same title page of his book does he present his work as “Also a Demonstration of These Hypotheses on the

40 Juan D. Serrano

Basis of Holy Scripture”,115 and why he does quote passages from the Bible to show his system is consistent with it?116 If astronomical hypotheses are not required to be consistent with the Truth established by higher disciplines, why bother to show that they are consistent with the Holy Scriptures?

Yet a third ambiguity concerning Ursus’s claims arises from his apparent admission of Brahe’s charges in Folio F, Note 20 of the same book. There Ursus says, referring to Brahe’s accusation that he has stolen his system: Sit furtum, sed Philosophicum: disce in posterum rem tuam custodire (“Let it be a theft, but a philosophical one. Learn to safeguard your things hereafter”).117 Both Brahe and Kepler, taking this literally as a confession, concluded that Ursus has declared himself guilty of Brahe’s charges of theft and plagiarism. However, this remark allows for an alternative reading other than straightforward admission of intellectual theft, namely as the not uncommon rhetorical strategy of concessio, whereby the accused grants the opponent’s charge just to show his case cannot be proven.118 Indeed, given the general tone of the Tractatus, it is likely that Ursus intended it as such, so no definitive conclusion can be derived just from that note, as Brahe and Kepler did.

More interesting to us is a note in Ursus’s Demonstratio,119 a document he pre-pared for his defence, at the end of which he clearly repeats his contention that those hypotheses “are ancient and not new, still less recently invented”.120 However, in a note in the “Little Appendix” to this document, Ursus states “that Mars does indeed pass right through the orb of the Sun is evident”, among other things “from the fact that the parallax of Mars in opposition is greater than that of the Sun”.121 Here we have Ursus effectively making an amendment to his system as originally published in the Fundamentum astronomicum, but Ursus could not possibly have reached this conclusion by himself, lacking the sophisticated instruments required to make those measurements, which posed a challenge even for Brahe, with all his instruments and competence as an observer. Clearly Ursus took at least this emendation to his system from Brahe, for there was no way for him, without an observational basis, to reach the conclusion that while at opposition Mars comes closer to the Earth than does the Sun, and Brahe’s is the only version of the geo-heliocentric system in which the orbits of Mars and the Sun intersect (neither in Wittich’s, Rothmann’s or Roeslin’s versions of the system is this the case).

4.3. The Verdict

This Imperial Commission has concluded, on the basis of a careful examination and consideration of the disputants’ respective briefs:

Ursus indeed stole his geo-heliocentric system of the world, as published in his Fundamentum astronomicum of 1588, from Tycho Brahe. The possibility remains that Ursus conceived his very similar system independently, just as did other authors around the same period.

41Trying Ursus

geo-heliocentric system before Brahe did, but only that he did so independently.

the latter still retains his priority by having published it first.

measurement of Mars’s parallax in order to establish his priority, something which was unnecessary as, again, he had published first.

parallax measurements, and not the converse, as he maintains. Neither the Tychonic nor the Ursine system seems to have been derived from astronomical observations, but rather from theory alone.

-section of the solar and Martian orbs) from any observations of his own, but rather from Brahe. But the published version of Ursus’s system (1588) does not include this amendment.

Both disputants could have profitted, without acknowledging it, from a third person, namely Paul Wittich.

Both disputants — and not only Ursus — slandered and abused each other’s honour and reputation in print, and used inappropriate language against each other in their respective writings.

De hypothesibus astro-nomicis tractatus of 1597, it was never granted to him and Ursus breached the law in printing a book without imprimatur. Not only we have confirmed that he is guilty of having committed this offence, but the order has already been given that he be punished for this, by having his book confiscated and burned.

5. CONCLUSION

The Tycho–Ursus dispute is important and interesting because it illustrates the com-plex interplay of authority, rhetoric, propriety of knowledge claims, social status and justification that characterized the interactions between astronomers in the period. Focusing on Ursus’s proposed trial has allowed us to conclude that the evidence presented against him by Tycho is insufficient to prove Ursus indeed plagiarised him, and that the historical verdict on Ursus was decided on the basis of Tycho’s and Kepler’s judgements, and on Tycho’s considerable authority as an astronomer, rather than on a real and thorough examination of the circumstantial evidence. It seems that some of what has been said about this dispute so far has taken the Tychonic and Keplerian verdicts on it for granted, namely that Ursus was indeed guilty of plagia-rism. By ‘trying’ Ursus we hope to have shown that Tycho’s case was not as solid as both himself and posterity thought, and that Kepler’s intervention — which in any case would have been irrelevant to decide the issue had the trial taken place, as his defence was composed later — gave the dispute a different twist, and, for good or bad, it became the standard historical account and verdict on Ursus. It was never actually proven that Ursus indeed plagiarised Tycho’s ideas.

42 Juan D. Serrano

Furthermore, rather than an actual claim to priority in the discovery of the geo-heliocentric system, Ursus’s claim should most appropriately be seen as only a claim to independent discovery. It may well be that Ursus arrived at his system of the world without having stolen it from Tycho, even if he did not have the astronomical competence to substantiate his claims, and therefore that he may have been innocent of the charge of plagiarism. But as has been shown, for all the observational and technical competence Tycho possesed, he was in no position to justify his claims on an observational but only on a theoretical basis.

ACKNOWLEDGEMENTS

I wish to thank Neil Tarrant from the London Centre for the History of Science, Technology and Medicine (Imperial College/UCL), my friend Oleskiy Kurka and an anonymous referee for their careful reading of my draft and their helpful remarks, suggestions and corrections.

REFERENCES

1. R. Jarrell, “The contemporaries of Tycho Brahe”, in R. Taton and C. Wilson (eds), Planetary astronomy from the Renaissance to the rise of astrophysics, Part A: Tycho Brahe to Newton (Cambridge, 1989), 22–32, p. 22.

2. Ibid., 31.3. See C. Schofield, Tychonic and semi-Tychonic world systems (New York, 1981), 118–19, and C.

Schofield, “The Tychonic and semi-Tychonic world systems”, in Taton and Wilson (eds), op. cit. (ref. 1), 33–44.

4. Only recently a book-length study of Ursus has been published (in German): D. Launert, Nicolaus Reimers: Günstling Rantzhaus — Brahes Feind, Leben und Werk (Munich, 1999).

5. Letter from Tycho Brahe to Georg Rollenhagen, 26 September 1600. Quoted in both E. Rosen, Three Imperial Mathematicians: Kepler trapped between Tycho Brahe and Ursus (New York, 1986), 307, and in V. Thoren, The Lord of Uraniborg (Cambridge, 1990), 454.

6. That is, a member of the Council of the Kingdom of Denmark.7. O. Gingerich and R. Westman, “The Wittich connection: Conflict and priority in late sixteenth-

century cosmology”, Transactions of the American Philosophical Society, lxxviii/ 7 (Philadelphia, 1988), 1–76, p. 50.

8. R. Iliffe, “In the warehouse: Privacy, property and priority in the early Royal Society”, History of science, xxx (1992), 29–62, p. 30.

9. Gingerich and Westman, “The Wittich connection” (see ref. 7), 63.10. See Schofield, Tychonic and semi-Tychonic world systems (ref. 3), 108–67, and Schofield, “The

Tychonic and semi-Tychonic world systems” (ref. 3), 33–44.11. Iliffe, “In the warehouse” (ref. 8), 52.12. R. Westman, “The astronomer’s role in the sixteeenth century: A preliminary study”, History of

science, xviii (1980), 105–47, pp. 124–5. See also V. Thoren, “Tycho Brahe as the dean of a renaissance research institute”, in M. Osler and P. Farber (eds), Religion, science and worldview: Essays in honor of Richard S. Westfall (Cambridge, 1985), 275–95.

13. N. Jardine and K. Harloe, “Kepler’s refutation of Ursus’s Demonstratio”, in N. Jardine et al., “Tycho v. Ursus: The build-up to a trial”, Journal for the history of astronomy, xxxvi (2005), 81–106 (Part 1) and 125–65 (Part 2), pp. 151–65, see p. 152.

14. Gingerich and Westman, “The Wittich connection” (ref. 7), 71. See also O. Gingerich and J. Voelkel, “Tycho Brahe’s Copernican campaign”, Journal for the history of astronomy, xxix (1998), 1–34.

43Trying Ursus

15. N. Jardine, The birth of history and philosophy of science: Kepler’s A defence of Tycho against Ursus with essays on its provenance and significance (Cambridge, 1984; hereafter: Birth of HPS).

16. Jardine, “Ursus’ Tractatus”, in Birth of HPS (ref. 15), 29–57.17. Ursus’s argument is essentially what the philosopher of science Larry Laudan has called “pessimistic

meta-induction”.18. Rosen, Three Imperial Mathematicians (ref. 5).19. N. Jardine and A. Segonds, La guerre des astronomes: La querelle au sujet de l’origine du système

géo-héliocentrique à la fin du XVIe siècle. Vol. i, Introduction (Paris, 2008). Vol. ii/1, Le Contra Ursum de Jean Kepler: Introduction et textes préparatoires (Paris, 2008) is a critical edition of preparatory texts for Kepler’s Apologia, together with essays on its persuasive strategies and recent interpretations. Vol. ii/2, Le Contra Ursum de Jean Kepler: Édition critique, traduction et notes (Paris, 2008) is a critical edition of the Apologia, with Latin-French translation and notes. Subsequent volumes (in preparation) will cover the relevant works of Tycho, Ursus and Roeslin.

20. Gingerich and Westman, “The Wittich connection” (ref. 7).21. Jardine et al., “Tycho v. Ursus” (ref. 13).22. D. Launert, “A letter of complaint against Tycho from Ursus to the Emperor (1597)”, in Jardine et

al., “Tycho v. Ursus” (ref. 13), 89–92.23. D. Launert and N. Jardine, “The production of Ursus’s De astronomicis hypothesibus (Prague,

1597)”, in Jardine et al., “Tycho v. Ursus” (ref. 13), 92–5.24. D. Launert, N. Jardine and A. Segonds, “Ursus’s anonymous pamphlet on the ancient origins of

geo-heliocentric hypotheses”, in Jardine et al., “Tycho v. Ursus” (ref. 13), 129–36.25. N. Jardine and A. Segonds, “The formal refutation of Ursus’s Demonstratio by Johannes Müller,

briefed by Tycho Brahe”, in Jardine et al., “Tycho v. Ursus” (ref. 13), 137–50.26. Jardine and Harloe, op. cit. (ref. 13), 151–65.27. Also fully translated into English by both Rosen and Jardine. See Rosen, Three Imperial

Mathematicians (ref. 5), 290–6, and Jardine, Birth of HPS (ref. 15), 67–71.28. Johannes Kepler gesammelte Werke (22 vols, Munich, 1938–2009), and Tychonis Brahe Dani opera

omnia (15 vols, Copenhagen, 1913–29).29. Though one is available in German, see ref. 4.30. M. Caspar, Kepler, transl. by C. Doris Hellman, 2nd edn (New York, 1993), and Thoren, The Lord

of Uraniborg (ref. 5).31. On the basic facts presented in this section, all authors who have seriously studied the Tycho–Ursus

affair agree: see Rosen, Three Imperial Mathematicians (ref. 5); Jardine, Birth of HPS (ref. 15); Gingerich and Westman, “The Wittich connection” (ref. 7); and O. Gingerich, The book nobody read: Chasing the revolutions of Nicolaus Copernicus (New York, 2004).

32. For more on Tycho’s struggle with Mars’s parallax, see Gingerich and Voelkel, op. cit. (ref. 14), and Gingerich and Westman, “The Wittich connection” (ref. 7), 69–76.

33. He had first broached the issue of Ursus’s plagiarism in letters of 4 November and 21 December 1588, addressed respectively to his former teacher Henry Van Den Brock (Brucaeus) and to his friend Heinrich Rantzau, the Governor of Holstein and Ursus’s former patron, through whom Tycho had first known about Ursus’s whereabouts in Kassel.

34. Letter from Christoph Rothmann to Tycho Brahe, 26 August 1586, in Rosen, Three Imperial Mathematicians (ref. 5), 224.

35. Ibid., 92.36. For an account of this, see Caspar, Kepler (ref. 30), 111–21.37. Letter from Johannes Kepler to Nicolaus Ursus, 15 November 1595.38. Letter from Tycho Brahe to Johannes Kepler, 11 April 1598.39. Letter from Johannes Kepler to Tycho Brahe, 19 February 1599.40. Letter from Tycho Brahe to Johannes Kepler, 9 December 1599.41. Tycho Brahe to Johannes Kepler, 28 August 1600, in Rosen, Three Imperial Mathematicians (ref.

44 Juan D. Serrano

5), 302.42. Ibid., 305.43. Ibid., 307.44. N. Jardine, A. Segonds and A. Mosley, “Introduction”, in Jardine et al., “Tycho v. Ursus” (ref. 13),

81–9, p. 85.45. Launert and Jardine, op. cit. (ref. 23), 93.46. Letter from Tycho Brahe to Georg Rollenhagen, 26 September 1600, in Rosen, Three Imperial

Mathematicians (ref. 5), 303–7. This explains why Ursus’s book is so rare. According to Gingerich, only around a dozen extant copies of the Tractatus have been located worldwide. See Gingerich, The book nobody read (ref. 31), 118.

47. Letter from Tycho Brahe to Georg Rollenhagen, 26 September 1600 (ref. 46).48. Jardine, Segonds and Mosley, op. cit. (ref. 44), 85.49. Letter from Tycho Brahe to Georg Rollenhagen, 26 September 1600 (ref. 46).50. Ibid.51. A full English translation of the Apologia is provided by Jardine in his Birth of HPS (ref. 15), 134–207.

I use the page numbers of this translation when I quote from the Apologia.52. Kepler, Apologia, 136.53. For the dating of the Apologia see E. Rosen, “Kepler’s defense of Tycho against Ursus”, Popular

astronomy, liv (1946), 405–8, and N. Jardine, “The circumstances of composition”, in Birth of HPS (ref. 5), 9–28.

54. Letter from Johannes Kepler to David Fabricius, 2 December 1602, in Rosen, Three Imperial Mathematicians (ref. 5), 318.

55. Johannes Kepler gesammelte Werke, xx/1 (Manuscripta astronomica I).56. Letter from Kepler to Ursus, 15 November 1595, in Jardine, Birth of HPS (ref. 15), 53–4. Rosen,

Three Imperial Mathematicians (ref. 5), 86, gives a slightly different translation (“I love your hypotheses”).

57. Caspar, Kepler (ref. 30), 119.58. Rosen, op. cit. (ref. 53), 408.59. Rosen, Three Imperial Mathematicians (ref. 5), 15.60. Jardine, Birth of HPS (ref. 15), 2.61. Ibid., 5.62. R. Martens, Kepler’s philosophy and the new astronomy (Princeton, 2000), 57.63. J. Voelkel, The composition of Kepler’s Astronomia Nova (Princeton, 2001), 125.64. Westman, “The astronomer’s role” (ref. 12), 126.65. See Jardine, “The scope and form of the Apologia”, in Birth of HPS (ref. 15), 72–9.66. Schofield, Tychonic and semi-Tychonic world systems (ref. 3), 132.67. Kepler, Apologia, 135–6.68. Letter from Kepler to Herwart von Hohenburg, 30 May 1599, in Jardine, Birth of HPS (ref. 15),

61. By “his confession” Kepler refers to Ursus’s quip in the Tractatus: “Let it be a theft, but a philosophical one. Learn to safeguard your things hereafter.”

69. Kepler, Apologia, 190.70. Ibid., 206.71. Letter from Kepler to Herwart von Hohenburg, 30 May 1599, in Jardine, Birth of HPS (ref. 15), 64–5.72. Jardine and Harloe, op. cit. (ref. 13), 162.73. Such as, for instance, in his above-quoted letter to Herwart von Hohenburg. See Jardine, Birth of

HPS (ref. 15), 59–60.74. Letter from Kepler to Ursus, 15 November 1595. Fully translated in Jardine, Birth of HPS (ref.

15), 53–4.75. Kepler, Apologia, 139.

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76. Ibid., 144.77. Ibid., 153.78. Ibid., 156.79. Ibid., 185–6.80. Ibid., 144–5.81. J. Kepler, New astronomy, transl. by W. Donahue (Cambridge, 1992), 28.82. Kepler, Apologia, 191–2.83. Jardine and Harloe, op. cit. (ref. 13), 163.84. Ibid.85. Kepler, Apologia, 206.86. Rosen, Three Imperial Mathematicians (ref. 5), 333. Rosen’s suggestion is based upon Kepler’s own

words in the introduction to the Apologia: “And in itself the undertaking was an honourable one: to root out the erroneous opinions from the minds of students, and to obliterate the calumnies against the mathematicians impressed on the minds of patrons” (Kepler, Apologia, 136).

87. A member of the Council of the Kingdom of Denmark.88. According to the account given in the letter from Tycho Brahe to Georg Rollenhagen, 26 September

1600. See Rosen, Three Imperial Mathematicians (ref. 5), 305.89. N. Jardine and K. Tybjerg, “The lost copy of Ursus’s De astronomicis hypothesibus marked for use

in his trial”, in “Tycho v. Ursus” (ref. 13), 97–106.90. Launert, op. cit. (ref. 22), 89–92.91. Translated in Rosen, Three Imperial Mathematicians (ref. 5), 289–96, and Jardine, Birth of HPS

(ref. 15), 67–71. Kepler himself dates it around a year before the Apologia.92. Launert, Jardine and Segonds, op. cit. (ref. 24), 129–36.93. Uraniborg opened in 1576, but serious and systematic observing began later. Stjerneborg, the

underground observatory, opened in 1584.94. Whereas in the Ursine system, Mars and the Sun are equidistant from the Earth at opposition.95. Letters, from Tycho Brahe to the Landgrave Wilhelm IV, 18 January 1587; Tycho Brahe to Christoph

Rothmann, 21 February 1589; and Tycho Brahe to Thaddeus Hayek (Hagecius), 1 November 1589.96. Letter from Tycho Brahe to Henry Van Den Brock (Brucaeus), spring of 1584 (undated).97. Some of these diagrams, described in detail by Gingerich and Westman in “The Wittich connection”

(ref. 7), are bound at the end of Wittich’s personal copy of the first edition of Copernicus’s De revolutionibus, now at the Biblioteca Apostolica Vaticana (this copy had been first mistakenly identified as Tycho’s copy). Upon learning of Wittich’s death, Tycho acquired some items from Wittich’s personal library, including this volume.

98. For a useful chronology of Wittich’s wanderings, see Gingerich and Westman, “The Wittich connection” (ref. 7), 67–69. Tycho carefully recorded the dates of people’s visits to Uraniborg in his meteorological diaries.

99. Ibid.100. Even Tycho admits this in his letter to Kepler of 9 December 1599: “He [Hansen] arrived a little

after Ursus had been with me, and therefore he found out from my other students how deceitfully Ursus had behaved.” See Rosen, Three Imperial Mathematicians (ref. 5), 242.

101. Ibid., 251.102. Interestingly, in his letter to Anders Vedel (Velleius) of 18 September 1599, Tycho relates that Lange

had been “compelled” to testify before a notary against his former employee Ursus, “even though he did this reluctantly”. See Rosen, Three Imperial Mathematicians (ref. 5), 254. We do not have Lange’s testimony, but we can surmise Tycho somehow pressed him to testify on his behalf.

103. See Schofield, “The Tychonic and semi-Tychonic world systems” (ref. 3), 43.104. For more on the Tycho–Liddell affair, see Schofield, Tychonic and semi-Tychonic world systems

(ref. 3), 145–60, and Thoren, The Lord of Uraniborg (ref. 5), 455–8.105. From Ursus’s Tractatus, translated by Jardine, Birth of HPS (ref. 15), 35–6. Tycho’s wife, Kirsten

46 Juan D. Serrano

Jørgensdatter, was a commoner to whom he was informally engaged, as given his social status they could not legally marry. Despite his protests about Ursus’s calumnies, Tycho repaid him in his own coin, at least in his private correspondence: in a letter of 18 September 1599 to his former teacher Anders Vedel (Velleius), he refers to Ursus’s wife as “a notorious adulteress” (Rosen, Three Imperial Mathematicians (ref. 5), 254) and in his letter to Rollenhagen of 26 September 1600, he reports that Ursus’s death may have been caused by an attack in the French camp (syphilis), “to which he was accustomed” (Rosen, Three Imperial Mathematicians (ref. 5), 303).

106. Jardine and Tybjerg, op. cit. (ref. 89), 97–106, esp. pp. 100–2.107. Letter from Nicolaus Ursus to Emperor Rudolph II, 1 June 1597. Transl. by Launert, op. cit. (ref.

22), 90–1.108. For this reason, I find Schofield’s assertion that “Ursus alleged actual priority of discovery over

Tycho” (Tychonic and semi-Tychonic world systems (ref. 10), 166) questionable.109. See Jardine, Birth of HPS (ref. 15), 41–2.110. Ibid., 48.111. A version of the geo-heliocentric system in which Mercury and Venus circle the Sun, which in

turn circles the Earth, but the superior planets (Mars, Jupiter and Saturn) circle the Earth. In the Tychonic system, all planets circle the Sun, which revolves around the Earth (unlike Copernicus, Tycho did not regard the Earth as a planet).

112. However, the part of Kepler’s refutation of Ursus’s claims regarding Copernicus is missing from the Apologia, which Kepler left unfinished.

113. Transl. by Rosen, Three Imperial Mathematicians (ref. 5), 325.114. Transl. by Jardine, Birth of HPS (ref. 15), 42.115. Transl. by Rosen, Three Imperial Mathematicians (ref. 5), 325. Kepler also pointed out this

contradiction in the Apologia, 148.116. See Schofield, Tychonic and semi-Tychonic world systems (ref. 3), 118–19.117. Jardine, Launert, Gingerich and Westman all agree on this translation. Rosen offers a slightly

different translation (“intellectual” instead of “philosophical”) in Three Imperial Mathematicians (ref. 5), 195.

118. Both Kepler and Rosen take Ursus’s remark at face value, namely as a plain admission of theft, but Jardine contends that it was clearly intended as a rhetorical strategy. Gingerich and Westman endorse the latter interpretation and further suggest that Wittich may have been the source of Tycho (and perhaps Ursus).

119. The complete title in English translation is “Demonstration: that the Apollonian (that is, of Apollonius of Perga) hypotheses of the celestial motions are explicitly described in the surviving works of Martianus Capella, a most ancient author, and of Nicolaus Copernicus”. See Launert, Jardine and Segonds, op. cit. (ref. 24), 129–36.

120. Ibid., 134.121. Ibid.