The emergence of modern astronomy – a complex mosaic: Part XIX

Tycho and Kepler was one of the most important partnerships in the history of Early Modern science and a good counter example to those who mistakenly believe in the lone genius myth. Tycho the observational astronomer with an obsession for accuracy, who produced an unequalled volume of raw data


Tycho Brahe Source: Wikimedia Commons

and Kepler the theoretical astronomer with an equal obsession for accuracy, who would come to turn that data into a completely new heliocentric model of the cosmos.


Johannes Kepler Source: Wikimedia Commons

Superficially a marriage made in heaven but when the two men first met in Prague their future cooperation almost ended before it even started. Arriving in February 1600 at Tycho’s new home in Benátky Castle Kepler was initially welcomed as a guest and observer.


Benátky Castle Source: Wikimedia Commons

He was impressed by Tycho’s setup and his data collection to which he hoped to gain access to fine tune his Platonic solids model of the cosmos. Tycho was impressed by the young theoretician but, suspicious of possible plagiarism, was not prepared to make his data freely available. Kepler was of course desperate for paid employment and after two months entered negotiations with Tycho over some sort of fixed employment. The young, working class, German mathematician took umbrage at the arrogant attitude of the Danish aristocrat and in a strop broke off the talks, departing for Prague. Interestingly Tycho, needing fresh workers, proved himself surprisingly conciliatory and through the diplomatic efforts of the Bohemian physician Jan Jesenius (1566–1621) Kepler was persuaded to return to Benátky, where conditions of employment for him were eventually arranged.


Jan Jesenius

Kepler, who had left his family in Graz, now returned there and after a failed attempt to obtain employment from Archduke Ferdinand in August 1600 he finally set off for Prague with household and family. Initially he was employed directly by Tycho his first task being to write an account of the plagiarism dispute between Tycho and Ursus, probably as a punishment for having supplied Ursus with munitions in that dispute, and naturally he was expected to find in Tycho’s favour. This work, which turned out to be very impressive, was never published in Tycho’s or Kepler’s lifetimes and much more significant was the first real astronomical work that Tycho assigned to him.

When Kepler had first arrived in Benátky, Christen Longomontanus (1562–1647), who had returned to the fold having initially left Tycho’s circus when he quit Denmark, was working on the raw data for the orbit of Mars.


Christen Longomontanus

Tycho now removed him from this work, assigning him instead to the Moon’s orbit, and gave the task instead to Kepler under Longomontanus’ supervision. This turned out to be one of the most fateful decisions in the entire history of astronomy. Kepler initially thought that he could knock off this task in a couple of weeks but in fact it took him six years to complete but as well as the mathematical difficulties involved there were extenuating circumstances. Before turning to the results of what Kepler called his war with Mars, a play upon the fact that Mars was the Roman god of war, we need to take a look at some of those circumstances.

Tycho’s financial resources were stretched so he negotiated with Rudolf, the Emperor, for Kepler to be appointed directly to the court in order to produce a new set of planetary tables, using Tycho’s data, to replace the existing Prutenic Tables of Erasmus Reinhold and to be named, of course, after the Emperor. What seemed like a step up for Kepler proved to be very problematic, as Rudolf, always strapped for cash, was very bad at paying his staff.


Rudolf II Portrait by Martino Rota Source: Wikimedia Commons

The next major event was the death of Tycho on 24 October1601 barely a year after Kepler had started working with him. For Kepler this was both good and bad. Already appointed to produce planetary tables for Rudolph, he now inherited Tycho’s title as Imperial Mathematicus, as the obvious candidate Longomontanus had left Benátky and Tycho’s service the previous year. This was truly a major step up but with the same caveat that Rudolf was extremely bad at paying salaries. Kepler was, of course, now in physical possession of all of Tycho’s data but unfortunately not in legal possession. Although he had been Imperial Mathematicus, Tycho’s data did not belong to Rudolf but was his own private property and was on his death inherited by his family. Kepler was faced with the problem of negotiating with Tycho’s son in law Frans Gansneb genaamd Tengnagel van de Camp over the use of the data. Frans Tengnagel initially claimed that he would work with the data but he was a diplomat and not an astronomer or mathematician and in the end a compromise was reached in that Kepler could use the data but that Fans Tengnagel would be named as co-author in any resulting publications. In fact, in the end Frans Tengnagel’s only contribution was a preface to the Astronomia nova.


Frans Gansneb genaamd Tengnagel van de Camp

Even though he now possessed the desired data Kepler did not sit down solely to finish calculating the orbit of Mars. In 1604 he published his Astronomia Pars Optica, the most important work in optics since the Middle Ages, which laid the foundations of the modern science of optics.


Astronomiae Pars Optica

It included the first explanations of how lenses work to correct short and long sight and above all the first-ever correct explanation of how the image is formed in the eye. This work would, as will see, proved extremely important following the invention of the telescope at the end of the decade. Also in 1604 a supernova appeared in the skies and Kepler systematically observed it, confirmed it was definitively supralunar (i.e. above the moon’s orbit) and wrote up and published his findings, De Stella nova in pede Serpentarii, in Prague in 1606. This of course confirmed what had already been demonstrated in the 1570s that the heavens were not incorruptible, driving another nail into the coffin of Aristotelian cosmology.


Stella nova in pede Serpentarii title page

During this period Kepler also finished his determination of the orbit of Mars, in the course of which he changed the course of astronomy forever. Published in 1609 the Astronomia Nova ΑΙΤΙΟΛΟΓΗΤΟΣ seu physica coelestis, tradita commentariis de motibus stellae Martis ex observationibus G.V. Tychonis Brahe, to give it its full title, is without any doubt whatsoever one of the most important books in the whole history of astronomy, although it was not recognised as such until long after it appeared.


Astronomia Nova title page Source: Wikimedia Commons

Unique amongst major scientific publications the book appears to outline in great detail the path that the author took in his determination of the Planet’s orbit, including all the false paths he followed, the errors that he made and even his calculating mistakes. Normally scientists leave all of this out of their published work presenting only the successful conclusions of their battles with the evidence and data. So why did Kepler include all of his six years of strife in his finished product. The answer lies in the statement above, appears to outline. In fact the account presented is to some extent, to use an actual term, fake news. Kepler is deliberately misleading his readers but why?

Kepler was a convinced Copernicaner in a period where the majority of astronomers were either against heliocentricity, mostly with good scientific reasons, or at best sitting on the fence. Kepler was truly revolutionary in another sense, he believed firmly in a physical cause for the structure of the cosmos and the movement of the planets. This was something that he had already propagated in his Mysterium Cosmographicum and for which he had been strongly criticised by his teacher Mästlin. The vast majority of astronomers still believed they were creating mathematical models to save the phenomena, irrespective of the actually physical truth of those models. The true nature of the cosmos was a question to be answered by philosophers and not astronomers. Kepler structured the rhetoric of the Astronomia Nova to make it appear that his conclusions were inevitable; he had apparently no other choice, the evidence led him inescapably to a heliocentric system with a real physical cause. Of course, he couldn’t really prove this but he did his best to con his readers into thinking he could.

Kepler tested and refined his arguments in one of the most fascinating correspondences in the history of astronomy, which took place with the Frisian amateur astronomer David Fabricius (1564–1617) over a total of eight years; a correspondence that also makes a mockery of the lone genius myth. Fabricius was a Protestant pastor and a passionate amateur astronomer. He first emerged on the European astronomical scene when he took up contact with Jost Bürgi (1552–1632) in Kassel in 1592 to request his advice on constructing astronomical instruments. In 1596 having, as the first astronomer to do so, observed the variable star Mira he wrote a letter to Tycho Brahe in Hven describing his discovery. This was the start of an extensive correspondence between the two that lasted until Tycho’s death in 1601. In that year he visited Tycho in Prague, where he met Simon Marius (1573–1625) with whom he would also correspond but not Kepler who was in Austria on family business. The letters that Kepler and Fabricius exchanged were more in the nature of academic papers often running to forty or fifty pages.


Monument to David Fabricius & his son Johannes in the church yard in Osteel his parish Source: Wikimedia Commons

Fabricius was neither a geocentrist nor a heliocenrist but a solid supporter of Tycho’s geo-heliocentric compromise and so was an ideal sparing partner for Kepler. Kepler would outline his latest heliocentric theories and Fabricius would do his best to demolish them and his best was very good indeed. This meant that over the years of their correspondence Kepler could really develop and refine the complex arguments that he would then finally present in the Astronomia nova. Probably frustrated by his failure to convert Fabricius to his way of thinking, Kepler rather abruptly broke off the correspondence in 1609. Fabricius, who Kepler acknowledge as the best observational astronomer in Europe following Tycho’s demise, died tragically in 1617, beaten to death with a spade by a local farmer, who thought Fabricius had accused him of being a thief in a sermon.

Despite Kepler’s best efforts the Astronomia nova was largely a flop when first published. Those who read it largely rejected his argument for heliocentricity. The book however contains two of the most important discoveries in the history of astronomy, Kepler’s first two laws of planetary motion:

1) That planets orbit the Sun on elliptical paths with the Sun situated at one focus of the ellipse

2) That a line connecting the planet to the Sun sweeps out equal areas in equal periods of time.

Kepler actually developed the second law first using it as his primary tool to determine the actually orbit of Mars. The formulation of this law went through an evolution, which he elucidates in the book, before it reached its final form. The first law was in fact the capstone of his entire endeavour. He had known for sometime that the orbit was oval and had even at one point considered an elliptical form but then rejected it. When he finally proved that the orbit was actually an ellipse he knew that his battle was over and he had won.

Although, at the time, Kepler had temporarily lost the public battle in the larger war for the recognition that the cosmos, as it was then known, was indeed heliocentric the publication of the Astronomia nova represents one of the most important steps towards the final victory in that war. This would not remain Kepler’s only contribution to that war but before we look at his further efforts we need to turn to what was possibly the most important event in the history of Early Modern astronomy, the invention of the telescope.

[Attentive readers of this blog might have noticed that I have ‘plagiarised’ my own post on the Astronomia nova from December last year. I simply couldn’t be bothered to find new ways of expressing things that I had already expressed to my own satisfaction.]
















Filed under History of Astronomy, History of Optics, History of science

3 responses to “The emergence of modern astronomy – a complex mosaic: Part XIX

  1. Pingback: The emergence of modern astronomy – a complex mosaic: Part XIX — The Renaissance Mathematicus | O LADO ESCURO DA LUA

  2. Check the caption under Longomontanus… a truly impressive camera.

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