The Keplerian Revolution

Edward Worth collected a host of works by Johannes Kepler, 1571-1630, a German mathematician and astronomer who is perhaps best known for providing a physical structure for the Copernican system. From his earliest work, the Mysterium cosmographicum (1596), to his Tabulae Rudolphinae (1627), a number of key themes emerge: his physical understanding of the Copernican system; his insistence on the motive power of the Sun; his unabashed advocacy of his own work as something innovatory. The title of his work of 1609, the Astronomia Nova, sums up Kepler’s approach: his astronomy was new and that, in Kepler’s view, was its chief recommendation. He regarded himself as at the frontiers of knowledge and was not afraid to launch onto sometimes perplexing exploratory trails. Some would lead him to write the Harmonices Mundi (1619), a text which he viewed as the culmination of his life’s work but which today is generally ignored in favour of the more concrete of his achievements: his three planetary laws.

Kepler’s Optics

The earliest publication by Kepler purchased by Worth was the former’s fascinating work on optics, the Ad Vitellionem paralipomena, quibus astronomiæ pars optica traditur (Frankfurt, 1604).

Johannes Kepler, Ad Vitellionem paralipomena, quibus astronomiæ pars optica traditur 
(Frankfurt, 1604), title page.

 

Witelo’s Perspectiva was a source book for works on optics by the leading ancient and medieval writers on the subject: Euclid, al-Kindi, Alhazen and Roger Bacon, to name only the most famous. The last two authors were undeniably the guiding forces behind Witelo’s perspectivist understanding which proved popular, judging by the fact that 19 manuscripts copies of it survive, not to mention the fact that it was printed three times in the sixteenth century (1535, 1551 and 1572). However, these treatments differed wildly from that of Kepler whose treatise of 1604 was a refutation of the thirteenth-century Witelo and via him, all medieval conceptions of vision.

Although grounded in medieval optical tradition himself, Kepler was aware of its limitations. He pointed specifically to two different areas: the medieval perspectivist understanding of the anatomy of the eye and, secondly, the issue of refraction (the last being of particular interest to him as an astronomer). According to perspectivists such as Witelo, the crystalline humour ‘felt’ light falling on it which it transferred to the optic nerve via the aranea and retina. It was enabled to do this by its flat nature. Kepler, however, pointed out that this was based not only on a flawed understanding of the role of the crystalline humour but also of its anatomy. Citing the anatomical works of Felix Platter’s De corporis humani structura et usu, he demonstrated that the crystalline humour was not flat (as Witelo argued) but rounded.

The anatomy of the eye: 
Kepler, Ad Vitellionem paralipomena, (Frankfurt, 1604), plate.

He also challenged Witelo’s theory of refraction for like many other perspectivists Witelo had simply ignored rays which were not perpendicular. Kepler realised that this position was untenable and that refractions could not be ignored. While both he and medieval perspectivists were roughly agreed on how light reached the eye it was what happened after the crystalline humour which was the field of contention. As Lindberg (1976) states, Kepler argued that ‘all the rays coming from one visible point finally converse at another point’ and focused on the role of the retina ‘as the seat of all visual power within the eye’. He thus replaced the medieval theory of touch with a new understanding of the retinal image by providing the first explanation of the formation of an inverted image on the retina of the eye.

 

Caption: Kepler, Ad Vitellionem paralipomena, (Frankfurt, 1604), p.

He continued on his researches in the field of optics in another seminal work, his Dioptrice (1611). This book, likewise collected by Worth, continued on his ground-breaking insights into vision but specifically concentrated on lenses of all kinds, a subject which had not received much coverage in his Ad Vitellionem. However, the invention of the telescope in the intervening years ensured that Kepler turned his thoughts to the type of lenses which should be used. In fact his Keplerian telescope, with its two convex lenses gained greater popularity than Galileo’s because it allowed greater magnification.

Kepler’s Nova and the Date of Christ

But Worth also collected another section of Kepler’s oeuvre. His third text was Kepler’s De stella nova in pede Serpentarii (Prague, 1606), a treatise on the supernova first seen in October 1604 which contains such a wealth of detail on the phenomenon that subsequent astronomers simply refer to it as ‘Kepler’s nova’. As the title of his treatise suggests, it was sighted in Ophiuchus’s foot – as is visible in this foldout plate from Worth’s copy.

Johannes Kepler, De stella nova in pede Serpentarii (Prague, 1606), foldout plate.

Kepler’s work on the nova was not solely an astronomical investigation of its various sightings: he was also concerned with its implications beyond the astronomical field. Writing to J. G. Herwart von Hohenburg in 1605 he outlined that his main aim was to ‘show that the celestial machine is not so much a divine organism but rather a clock-work’ (Gingerich, 2003). He had, however, no intention, indeed no conception, of a celestial physics which was sufficient unto itself. If his universe behaved like clockwork it was because God was the clockmaker. This aspect of his work is particularly apparent in the works chosen by Edward Worth. Worth didn’t collect the Mysterium cosmographicum (1596), or Astronomia Nova (1609) or Kepler’s Epitome astronomiae Copernicanae (1617-1621), with their intensive discussions of the physical nature of the cosmos, and their analysis of his three laws. Instead, the running theme in the Keplerian works in the Worth Library was their historical-theological application. This is particularly apparent in Kepler’s De stella nova in pede Serpentarii (Prague, 1606) which, as the title page makes clear, was not only concerned with the 1604 nova but also linked his discussion of its astronomical significance with its historical relevance: was the Star of Bethlehem a similar nova and if so could the date of Christ’s birth be dated on astronomical grounds?

 

Johannes Kepler, De stella nova in pede Serpentarii (Prague, 1606), title page.

The significance of the 1604 nova for Kepler lay not only in the possibility of linking his own data and that of Brahe for the 1572 nova but also because it was linked in his mind to the conjunction of Jupiter, Saturn and Mars which took place in early 1603 and in the first months of 1604. The conjunction of Saturn and Jupiter, taking place as it does every twenty years, was not unusual but as astrologers pointed out, this time the conjunction was taking place in the fiery signs of Sagittarius, Leo, and Aries. While astrologers concentrated on what the nova and conjunction might mean for the future, Kepler turned his gaze backward to see if it could shed light on the past. His argument, put forward in the appendix of De Stella Nova in pede Serpentarii was that similar celestial events could help date the birth of Christ – which he confidently asserted was in the year 5BC. He outlined this thesis in more detail in another book, again collected by Worth, Kepler’s De vero Anno, quo aeternus Dei Filius humanam naturam in Utero benedictae Virginis Mariae assumpsit (Frankfurt, 1614).

The Harmony of the World

‘At last… I brought it into the light, and beyond what I had ever been able to hope, I laid hold of Truth itself: I found among the motions of the heavens the whole nature of Harmony, as large as that is, with all its parts as explained in book 3. It was not in the same way which I had expected – this is not the smallest part of my rejoicing – but in another way, very different and yet at the same time very excellent and perfect.’

Bruce Stevenson, The Music of the Heavens. Kepler’s Harmonic Astronomy. Princeton: Princeton University Press, p. 128.

The mystical strain within Kepler’s work came to full flowering in his Harmonices Mundi (Linz, 1619). This was a wide ranging work which covered a host of topics: geometry, music, astronomy and astrology, and in which Kepler expounded his third planetary law, also known as his ‘harmonic’ law, where he set out the relationship between the size and period of a planet’s orbit. Perhaps the book’s rather diffuse style accounts for the fact that it was, according to Russell (1964), the least read of Kepler’s works – a fact that would have severely disappointed the author who regarded it as the culmination of theories initially explored in his first work, the Mysterium Cosmographicum (1596). His underlying theme was universal harmony – the theory that the universe was built on harmonic laws. This illustration of the harmonies of the various planets illustrates one of his more basic points.

Johannes Kepler, Harmonices Mundi (Linz, 1619), p. 207.

For his theory of harmony to work properly Kepler had to turn his attention to the complex subject of relative planetary distances – and by doing so invented his third planetary law.

The Rudolphine Tables and Kepler’s Reputation

The last of Worth’s collection of Kepler’s astronomically minded works was Jean Baptist Morin’s edition of Kepler’s Tabulae Rudolphinae (Paris, 1657).

 

Morin’s 1657 edition of Kepler’s Tabulae Rudolphinae (Paris, 1657), title page.

The text itself, first published at Ulm in 1627, was the culmination of years of labour and can be viewed as the high point of Kepler’s career for in this text he accomplished his cherished aim: the establishment of the Copernican system on a firm observational footing. Worth’s copy unfortunately lacks the superb frontispiece in which Urania’s temple is constructed from pillars symbolising the various cosmographical systems, overseen above by the imperial eagle himself, a symbol of that Maecenas of early modern astronomy, Emperor Rudolf II. The echoes of Tycho Brahe’s Urania were very loud indeed – and not just because Kepler’s Temple of Urania was modelled on one in the Island of Hven. What Kepler provided with his Rudolphine Tables was his continuation of Brahe’s observational masterpieces: a new, updated version which took into account the new utility of logarithms. Having seen off initial competition from the simpler but error laden astronomical tables of Philip Landsbergen, it became a standard text for would be astronomers and did much to make Kepler’s reputation in the seventeenth century by providing accurate tables which were ultimately based on the first two of his laws.

Given the fascination of his three Planetary Laws today it is easy to ignore the fact that they took some time to gain acceptance. His first law, which stipulated that orbits around the Sun were elliptical in shape with the Sun at one focus, was the first to gain widespread recognition though, as Russell relates (1964), it was not until the 1640s that it came to be generally accepted by astronomers – at least in France. That it was accepted at this time owed more to the 1627 Rudolphine Tables than his monographs such as the Astronomia Nova in which it had first been promulgated. The Tabulae Rudolphinae thus served to publicise at least the first two planetary laws far more successfully than those works in which they had been initially expounded. The reason for this was not hard to see: Kepler’s works were challenging to even the most adept astronomer as many who struggled with his second area law admitted. He was presenting at times a completely revolutionary understanding of the cosmos – whether it was his idea that celestial physics could ultimately be understood as a combination of various polygonal shapes or his use of the true, rather than the mean Sun, by which to determine conjunctions and oppositions of planets. Kepler was not easy simply because he was not content to sit ‘on the shoulders of Giants’ but rather preferred to track his own way through the complexities of the cosmos. What he presented to his fellow astronomers was indeed a ‘New Astronomy’, an astronomy which revolutionised understanding of the cosmos.

Other Works by Kepler in the Worth Library

Kepler, Johannes, De nive sexangula. (Frankfurt, 1611). 4o.

Kepler, Johannes, Chilias logarithmorum (Marburg, 1624). 4o.

Kepler, Johannes, Supplementum Chiliadis Logarithmorum continens (Marburg, 1625). 4o.

Selected Reading

Dupré, S. (2006). ‘Visualisation in Renaissance Optics: The Function of Geometrical Diagrams and Pictures in the Transmission of Practical Knowledge’ in Kusukawa, S. and Maclean, I. (eds). Transmitting Knowledge. Words, Images, and Instruments in Early Modern Europe. Oxford: Oxford University Press.

Gingerich, O. (2003). ‘Johannes Kepler’, in R. Taton and C. Wilson (eds). Planetary Astronomy from the Renaissance to the rise of astrophysics. Part A: Tycho Brahe to Newton. Cambridge: Cambridge University Press. The General History of Astronomy Series.

Lindberg, D. C. (1976). Theories of Vision from Al-Kindi to Kepler. Chicago: University of Chicago Press.

Ronchi, V. (1991). Optics. The Science of Vision. New York: Dover publications.

Russell, J. L. (1964). ‘Kepler’s Laws of Planetary Motion: 1609-1666’, The British Journal for the History of Science 2, no. 1, 1-24.

Stephenson, B. (1987). Kepler’s Physical Astronomy. Princeton: Princeton University Press.

Stephenson, B. (1994). The Music of the Heavens. Kepler’s Harmonic Astronomy. Princeton: Princeton University Press.

Wilson, C. (1989). Astronomy from Kepler to Newton. Historical Studies. London: Ashgate.