In 2007, the 62nd General Assembly of the United Nations declared 2009 the International Year of Astronomy to mark the 400th anniversary of the first recorded astronomical observations with a telescope by Galileo Galilei and the publication of Johannes Kepler’s Astronomia nova (1609).
As could have been predicted from the start the year was very heavy on celebrations of Signor Galileo and his telescopic discoveries, which, by the way, were mostly made in 1610, with, in comparison, very little attention being paid to Kepler’s groundbreaking publication.
Of course, the invention of the telescope and those initial discoveries that Galileo rushed into print in his Sidereus Nuncius were a major earthquake in the development of science.
Science is based, first and foremost, on empirical observations made with the five senses and for the first time ever there was an instrument that radically increased the power of visual perception, enabling the user to see thing that had previously invisible to the naked-eye. That in itself was an absolute sensation and both news and availability of the instrument spread through Europe like a wild fire, although most of the initially available instruments were little more than toys.
However, it didn’t take long before people were able to improve the quality and magnification of their instruments and soon Thomas Harriot (c. 1560–1621), Simon Marius (1573–1625), and Galileo (1564–1642) were all making astronomical observations and discovering all sorts of wonderful new things in the heavens. Galileo, a brilliant polemicist rushed into print and won all the glory. One thing that tends to get forgotten or simply ridiculed is that Galileo’s initial, announcement as well as being acclaimed was also met with a great deal of, actually justified, scepticism. As we get reminded almost daily with the constant stream of retractions of scientific papers, great claims need great evidence and great evidence need independent confirmation. In the case of Galileo’s telescopic discoveries that independent confirmation took time but it did come from the most reliable of sources, the Jesuit astronomers of the Collegio Romano, and with it, Galileo was crowned, transmuting overnight from a middle aged, insignificant, university professor for mathematics to the greatest living astronomer. Telescopic astronomy had changed humanities perception of the heavens for the first time in about three millennia and that radically.
The change in humanities perception of the heavens that Kepler’s Astronomia nova wrought was actually just as cataclysmic, as that wrought by the telescope but initially very few people took very much notice. In fact, it is not an exaggeration to say that it was a flop. Galileo had pounced on Kepler’s Dissertatio cum Nuncio Sidereo (Conversation with the Starry Messenger) (1610), Kepler’s celebration of the Sidereus Nuncius, immediately publishing a pirate edition in Italy without asking and certainly without permission but he simply ignored the much more important Astronomia nova.
In fact, he never acknowledged it at all. Those who did read it were desperately unhappy with Kepler’s second law of planetary motion and would remain so for much of the seventeenth century. It was viewed as an unwieldy, clunky apology for a mathematical relation; how was one supposed to do anything with this ugly ducking of a law that was attempting to replace the cosmological axiom that planetary motion is uniform circular motion.
Another reason why Astronomia nova had little impact in comparison to Sidereus Nuncius is the style and presentation. Sidereus Nuncius is a comparatively small, short book–90 pages octavo in the English translation–written in Galileo’s bouncy polemic style. It’s more of a press report that a scientific one. Astronomia nova is a monster–600 pages in octavo–written in Kepler’s intense, complex scholarly style. Definitely not bedtime reading.
When talking about the Astronomia nova, people tend to simply reduce it to the first two of Kepler’s planetary laws, bundling them up with his third law as his total contribution to cosmology and astronomy but the book is very much more and I will attempt to explain what exactly Kepler had set out to do, what he had actually achieved and what its real significance for the history of astronomy was.
Astronomia nova was a consequence of Kepler going to Prague to work with Tycho Brahe (1561–1601). Although, he went because he desperately needed employment having been kicked out of Graz, as a consequence of the counter-reformation, he also had a personal scientific motive for going. In Graz he had written and published his Mysterium Cosmographicum promoting his ‘discovery’ that there were exactly six planets, Kepler was a Copernican, because the spaces between the spheres were filled out with the five regular polyhedra.
The book also contained his failed attempts to find linked ratios between the distances of the planets from the sun and some sort of laws governing the linear speeds of the planets and their distances from the Sun. Convinced that God had created a rational geometric cosmos, governed by mathematical laws Kepler was convinced that his failure to find those laws and the minor discrepancies in the fit of the polyhedra between the planetary spheres lay on inaccurate data. Tycho had the most accurate astronomical data ever measured and collated and Kepler wanted access to that data in order to perfect his model of God’s creation.
One thing that very much distinguished both the Mysterium Cosmographicum and the Astronomia nova from anything that had gone before was the status that Kepler attributed to his work and his endeavours. Since antiquity there had been a strict division between cosmology and astronomy. Cosmology was the realm of the philosophers and described with the celestial sphere truly was. Astronomy was the realm of the mathematician, who merely produced mathematical models to predict, as accurately as possible, the position of the celestial bodies at any moment in time. These models were not real, they just saved the phenomenon, to use the Platonic phrase. Both Copernicus and Tycho had taken the first steps to breaking down this barrier between cosmology and astronomy in that they at least hypothesised that the astronomical model they created did indeed describe reality, although it is difficult to imagine how a real version of Copernicus’ deferent and epicycle models would look or function. Kepler went a gigantic step further than both of them in that he was trying to develop, discover a real physical astronomical system and not just a mathematical model and certainly not one, such as the from Copernicus, that was virtually impossible to realise.
Arriving in Prague, Kepler had hoped to gain access to all of Tycho’s data but Tycho was afraid of being plagiarised by other astronomers, as he believed Nicolaus Reimers Baer (1551–1600) had done, and restricted access to his data.
Tycho first set Kepler to the task of writing A Defence of Tycho against Ursus in their plagiarism dispute as a punishment for written an obsequious letter to Reimers Ursus when he sent him a copy of Mysterium Cosmographicum and which Ursus then published in one of his own books attacking Tycho. Kepler never finished this work but Tycho then gave him the task of determining the correct orbit of Mars based on his collected observational data and the rest is, as they say, history.
Kepler’s war with Mars, as he described it, began in 1601 but the Astronomia nova, of which his description of the battle forms the central part, was first published in 1609. Popular myth says that Kepler took nine years to defeat Mars but he, himself, says it only took six. Even this is misleading. During those six years he fought a protracted battle with Frans Gansneb genaamd Tengnagel van de Camp (1576–1622), who had married Tycho’s daughter Elizabeth, over Tycho’s data which had been inherited by Tycho’s children and not Kepler. Kepler had wisely taken possession of the data on Tycho’s death and was not about to surrender them. Tengnagel blocked Kepler from publishing anything based on the data, but after years of negotiations it was agreed that Kepler could publish but Tengnagel would write the preface, which he duly did for the Astronomia nova.
In those years Kepler also wrote and published his Astronomia Pars Optica (1604) another monumental volume with four hundred and sixty pages.
He also observed and wrote up and published his observation of the supernova from 1604, De Stella Nova in Pede Serpentarii (On the New Star in the Foot of the Serpent Handler) 1606, a mere two hundred and fifty pages.
He also conducted extensive scientific correspondence with both Thomas Harriot (c. 1560–1621) and David Fabricius (1564–1617) the latter on the progress of his war with Mars, using Fabricius as a sounding board for his final text.
Kepler’s battle to try and determine the orbit of Mars is, of course, the central theme of the book and totally unusual for a scientific publication, he gives a very detailed account of all the errors he made along the way and all of the false paths that he took before he finally arrived at the correct solution. There is, however, method in Kepler’s madness. The whole exercise is a rhetorical strategy aimed at convincing the reader that his conclusions are inevitable and factually true and that no other conclusion could be possible. His conclusions are, of course, well know and encapsulated in his first two laws of planetary motion.
- The orbit of a planet is an ellipse with the Sun at one of the two foci.
- A line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time
Interestingly, although Kepler discovered the second law first and it played a central role in the discovery of the first law, there is no clear statement of the second law in the Astronomia nova. The first time that Kepler stated it in the form we now know was in the Epitome astronomiae copernicanae published in 1618-1622
The laws alone would guaranty the book the status of one of the greatest publications in the history of astronomy but there was more. Kepler devotes a lot of time and effort to explaining and proving that there must be some sort of force emanating from the Sun that drives the planets around their orbits, because as he states quite clearly several times in the book, the first time in the introduction, “… that there are no solid orbs, Brahe has demonstrated most firmly.” The orbs were that which carried the planets around their orbits in the previously accepted Aristotelian cosmology.
In Kepler’s astronomy the real Sun is at the centre of all the orbits of the planets and he argues that either the Sun at one end of the line between it and the planet, or the planet itself, or both must be the cause of the movement of the planet around the Sun. He brings philosophical arguments why it can’t be the planet and must be the Sun and also why the Sun as a result must rotate about its axis. The proof that the Sun rotates about its axis was first brought by Galileo in his dispute with Christoph Scheiner (1573–1650) about the nature of sunspots. Kepler argues that the power that moves the planets resides in the Sun and is an immaterial species similar to light, whilst arguing that it can’t actually be light itself. He argues by analogy that William Gilbert (1644–1603) has demonstrated that the Earth is a magnet in his De magnete (1600), so the Sun must also be a magnet and the power driving the planets around their orbits is magnetism. He argues that it is not magnetic attraction but a force that goes out from the Sun to the planet pushing it along is orbital path. Kepler is the first to use the Latin ‘vi’ for force rather than ‘anima’ for life force as that which moves the planets showing personal progress in his thoughts as he was still using anima in his Mysterium Cosmographicum.
Interestingly, Kepler makes one fundament perceptual error in his arguments that is somewhat strange. He argues that the force he is describing is only two dimensional existing only in the plane of the ecliptic and therefore diminishes in simple inverse ratio to the distance covered. We know that the universal force of gravity diminishes in inverse ratio to the square of the distance, as does light in the inverse square law of the spreading of light from a point source, which Kepler himself first formulated. This is because the spread is actually three not two dimensional. This perceptual error in Kepler’s though process is strange because from the beginning of his Mysterium Cosmographicum he insisted on treating his cosmology and astronomy as three dimensional as opposed to the traditional presentation of the planetary orbits as two dimensional.
Of special interest in Astronomia Nova is the introductory material. In the English translation by William Donahue, it starts with a twenty-one page introduction, which ends with a two page Synopsis of the Entire Work. Basically, a detailed list of contents laid out in tabula form.
This is followed by a twenty-five page Summaries of the Individual Chapters. All of this means that you only have to read the introductory material to have a clear understanding as to where Kepler is going to take you on his intellectual exploration. Of particular interest in the introduction, itself, as this is the most widely read of all of Kepler’s writings and the only one translated into English in the seventeenth century. This is because it was added as an appendix to the printed editions of Galileo’s Dialogo.
I will close by taking a brief look at the introduction. He starts of by complaining about the difficulties of writing mathematical books. On the one side to be truly mathematical it must maintain a formal structure–proposition, construction, demonstration, and conclusion–but on the other side a formal mathematical structure makes it basically unreadable. This problem is why he is writing “a kind of elucidating introduction.” He also makes clear that he is breaking down the tradition division between celestial physics and cosmology, between the territory of the philosopher and that of the mathematician–“For since I have mingled celestial physics with astronomy in this work, no one should be surprised at a certain amount of conjecture.”
Perhaps the most significant aspect of the introduction is the amount of space he allots to debunking “objections to the earth’s motion.” Two things in particular stick out. The first is his redefinition of the Aristotelian concept of gravity. He rejects the idea that gravity is heavy object seeking “out a mathematical point (in this instance the centre of the world)” and replaces it with a definition surprisingly close to that of Newton et al.
The true theory of gravity rests upon the following axioms. Every corporeal substance, to the extent it is corporeal, has been made as to be suited to rest in every place in which it is put by itself, outside the sphere of influence of a kindred body. Gravity is a mutual corporeal disposition among kindred bodies to unite or join together; thus, the earth attracts a stone much more than the stone attracts the earth. (the magnetic facility is another example of this sort.) Heavy bodies (most of all if we establish the earth in the centre of the world) are not drawn towards the centre of the world because it is the centre of the world, but because it is the centre of a kindred spherical body, namely, the earth.
He then goes on to explain that if the moon and the earth were not held back in their own circuits the earth would ascend to the moon and the moon would descend to the earth each in the ration of their apparent angular size.
The second prominent aspect of Kepler’s debunking is his take on the interpretation of Holy Scripture. Over almost five pages he explicates, analyses and disposes of all the Bible passages that either directly say or even imply that the earth doesn’t move. His central argument is as simple, as it is direct, as it is correct. What the people were saying is what they had perceived in a given situation not what actually happened. It is the opposite of seeing is believing, it is seeing is deceiving. Kepler’s polemic here is a tour de force, which he originally wrote for his first work, the Mysterium Cosmographicum and which he was required to remove by the theologians of the University of Tübingen, as he was then still under their authority. Now he is the highly respected Imperial Mathematicus, a leading figure in the world of scholarship and is free to decide what he can or cannot include in his publications. Much is made in the literature of Galileo’s dismissal of the Biblical objections to heliocentricity in his Letter to Castelli and the in his Letter to the Grand Duchess Christina, almost nobody mentions Kepler’s equally powerful dismissal in the introduction to Astronomia Nova, published six years earlier.
Following his Biblical exegesis Kepler offers two pieces of advice. He asks the astronomers not to forget to pay due reverence to God’s creation, reminding the reader of his own deep religious beliefs and then a wonderful Advice for idiots:
But whoever is too stupid to understand astronomical science, or too weak to believe Copernicus without affecting his faith, I would advise him that, having dismissed astronomical studies and having damned whatever philosophical opinions he pleases, he mind his own business an betake himself home to scratch in his own dirt patch, abandoning this wandering about the world.
Kepler adds a comment for the pious:
So much for the authority of holy scripture. As for the opinions of the pious on these matters of nature, I have just one thing to say: while in theology it is authority that carries the most weight, in philosophy it is reason. Therefore, Lactantius is pious, who denied that the earth is round. Augustine is pious, who, thought admitting the roundness, denied the antipodes, and the Inquisition nowadays is pious, which, though allowing the earth’s smallness, denies its motion. To me, however, the truth is more pious still, and (with all due respect for the Doctors of the Church) I prove philosophically not only that the earth is round, not only that it is inhabited all the way around at the antipodes, not only that it is contemptibly small, but also that is carried along among the stars.
In Astronomia nova Kepler presents a real physical description of a heliocentric cosmos, not some hypothetical mathematical model, in which the planets orbit the sun on elliptical orbits with all of those orbits centred on the real sun, driving around those orbits by a force emanating from a rotating sun. He also makes it very clear that he is prepared to defend his description against all comers whether astronomical, philosophical or theological. It is not an exaggeration to describe the publication of Astronomia nova as the birth of modern celestial physics. He got a lot basically right but also a lot simply wrong. However, the child would grow during the seventeenth century, it would evolve, change, mature, but its roots would remain forever in Kepler’s book.