The emerging new astronomy of the 17th century: Comets were not that important
Here I will be arguing that comets were not instrumental in the emergence of modern astronomy in the 17th century. This view, most notably propounded by Kuhn and Hellman , where observations of comets were of paramount importance in ushering in a post-Newtonian modern astronomy by the end of the 17th century. Heidarzadeh posits that the history of comets falls into four periods, the first two being most relevant to this discussion: from Aristotle to Brahe comets were assumed to be meteorological phenomena; from Brahe to Newton, comets were admitted as celestial bodies but with unknown trajectories; from Newton to Laplace, they were treated as members of the solar system; and the post-Laplacian period, in which the mass and density of comets was calculated to be much less than the planets.
The focus here is particularly on observations of the 1577 comet by Tycho Brahe and Michael Mästlin and the 1607 comet by Johannes Kepler and Edmond Halley. From their observations and subsequent explanations it is obvious that there was more than science involved in their arguments: their actions cannot be removed from the political, social and cultural context of their time. In agreement with Nouhuys and Schechner Genuth I find that in the contemporary 16th and 17th century view there was no distinction between magical and scientific ideas, and to assess the science without appreciating this leads to a misleading understanding of the role played by comets in this period. In Kepler’s extensive writing we also find concurrence; he found comets to be unimportant in his arguments for a new cosmology. In addition, the shared discourse of comet lore, into the 18th century, was for comets still seen as agents of upheaval, renewal and divine justice.
A tour of the universe in 1577
By modern astronomy, I mean a world-view where the solar system is heliocentric, with planets and comets for example, having elliptical orbits, described by Kepler’s laws and Newtonian mechanics. In the late 16th – early 17th centuries though, there were a variety of opinions advanced regarding the universe: the shaping of ‘scientific’ ideas is marked by the adaptability of the Renaissance Aristotelians, the role played by humanism, as well as astrology and divinatory beliefs.
Aristotle (384-322 BC) presented a long lasting model of the universe. The key idea, for our purposes, was it comprised a terrestrial and celestial tier. The lower terrestrial tier, from the moon down to the centre of the earth, was composed of four elements: earth, water, air and fire. The celestial tier, containing the planets (which included the moon and sun) and the stars, was made of a fifth, non-material, element; aither. The upper tier was unchanging. The stars were fixed to an outer, perfect, sphere revolving around the centre of the universe, the earth. Likewise the planets were fixed to interlocking spheres that transported motion from planet to planet. A key idea here, and somewhat challenging to our modern sense of material and immaterial, is that these non-material spheres were hard and impenetrable, ‘adamantine’, solid enough to keep the planets fixed in their motions and ensure that the spheres did not overlap. The terrestrial tier was the home of all change and corruption. Here was where Aristotle placed comets – they were sub-lunar phenomena composed of hot dry exhalations. They are referred to in his text on “Meteorology”, not “On the Heavens”.
This, highly philosophical model of Aristotle, was complemented by the mathematical model of Claudius Ptolemy (c.100-c.178 AD). Here Ptolemy successfully incorporated 800 years of observational data into a geometric model based on Aristotle’s. By incorporating epicycles and equants Ptolemy successfully predict planetary motions. Ptolemy accepted Aristotle’s aither, but not his interlocking spheres that transmitted motion from the stars to each successive planet, finishing before the terrestrial realm. Along with its predictive accuracy another of its successes is its simplicity from the point of view of the earth-stationed observer .
The challenge to the geocentric model of Aristotle and Ptolemy by Nicolaus Copernicus (1473-1543) in his 1543 work De revolutionibus orbium coelestium is well documented. The fundamental objection that Copernicus had to geocentrism was directed against both Aristotle and Ptolemy. He found it strange that motion was imparted to the ‘fixed’ stars to make the planets move. Copernicus argued it is better to have the Earth in motion and to reduce the complexity of the epicycles, and remove the equants needed to describe the geocentric system. This argument was one contemporary to Aristotle – it was reworking a Stoic criticism. However comets did not appear in Copernicus’ universe. However they did make an appearance in 1577.
Brahe, Mästlin and the comet of 1577
On the afternoon of November 13, 1577 Tycho Brahe (1546-1601), Danish royal consultant on astronomical and astrological matters, noticed a bright star. When a long ruddy tail, stretching in the opposite direction from the sunset, grew visible Brahe realised it was a comet – at age 31, it was the first he had seen. For the next two and a half months he observed and recorded its position against the fixed stars. From these parallax measurements, Brahe concluded that the comet was about one third of the way from the earth to the stars. This was the first major comet post Galileo’s telescopic discovery of the moons of Jupiter. The parallax measurements, like Galileo’s moons, challenged the logicality of immutable heavens, both Aristotle’s celestial sphere and the existence of crystalline spheres supporting the planets, rather than the geocentric model. Brahe had been skeptical of Aristotle’s distinction between the celestial and terrestrial regions, a subject he had delivered a lecture series on in 1574-5. In addition he had witnessed a nova, the ‘new star’ of 1572, which revealed the heavens to be mutable.
In 1578 Brahe wrote a brief manuscript on the 1577 comet in German. The 1577 comet had also stimulated him to develop a new model of the planetary system with a stationary Earth still as the centre of the universe, around which the sun and moon revolved, with the planets revolving around the sun. This model was developed by 1583 and published by Brahe in 1588 in De mundi aetherei recentioribus phaenomenis liber secundus. While the rejection of the crystalline spheres and the imperfection of the celestial region led Brahe to examine the Copernican model, they were not sufficient to overthrow the ancient models.
Politics and astrology were inextricably linked with the 1577 astronomical observations for Brahe. Within five weeks of the comet sighting a pamphlet had been produced by Jørgen Dybvad, a professor at the University of Copenhagen. He had first sighted the comet on November 11, 1577, whilst in the company of King Frederick of Denmark. His pamphlet, written for a public audience in the vernacular German and dedicated to the King, can be seen more as a political document than a scientific one. In it he delivered an apocalyptic message based on the “terrible great comet” including “2000 years of astrological and historical evidence” to bolster his assertions. Brahe’s pamphlet response was then as much to assert his own political credibility and influence with the King, as to develop his scientific endeavours. In his pamphlet on the comet Brahe states:
It is regrettable that this comet, no less than former ones, brings and arouses the same evil effects and misfortunes here on earth, so much the more so because this comet has grown so very much greater than others and has a saturnine, evil appearance, which was revealed by its pallid appearance and unclearly shining color like the star Saturn.
Brahe then went onto presage, for example, calamities for Europe west of Denmark, as the comet had “first let itself be seen with the setting of the sun.” The arena was more about astrology and royal influence than modern scientific debate.
Michael Mästlin (1550-1631), astronomer at the University of Tübingen and teacher of Johannes Kepler (1571-1630), also observed the 1577 comet. Voelkel claims “Mästlin was the only convinced Copernican teaching at a university in Europe when Kepler was a student.” Mästlin was another who concluded the comet being located beyond the moon. Mästlin’s most important contribution was subtle; he attempted to compute the comet’s orbit. With only limited orbital information available he was genuinely innovative in trying to find an orbit for this transitory phenomenon: assigning a circular heliocentric orbit, between that of Earth and Venus, for the comet. Mästlin used, in his published demonstratio, the Copernican model as a calculation tool, without directly supporting the model. Mästlin’s orbital results were overshadowed by Brahe’s study; however he greatly influenced his student, Kepler.
In addition to these astronomical innovations, the observations also changed the nature of augury. However, new models and instruments did not change comets remaining as portends, this despite the growing interest in physical models of comets. Mästlin, along with others including Brahe, proposed that the comet tales were produced as an optical effect caused by sunbeams shining through translucent spheres. In addition to Brahe’s politically motivated divinations others such as John Bainbridge, future Savilian professor of astronomy at Oxford, were still seeing evidence of divine providence in the comet of 1618, this comet lore starting to decline in learned circles by the end of the 17th century.
Kepler, Newton and Halley and the comet of 1607
Kepler first acknowledges his faith in the Copernican model arose from the studies of the 1577 comet by Mästlin. Kepler’s 1593 student disputation argued for the new order of the inferior planets and the dispensability of Aristotle’s adamantine spheres. This can be seen as a preference rather than necessity, as Kepler was also the first to demonstrate the geometrical equivalence of the Ptolemaic and Copernican models. Most importantly for my argument by 1604 Kepler had downplayed the influence of the observations of the 1577 comet and had decided that all comets had rectilinear paths. This is despite publishing the idea of elliptical orbits for the planets in his 1605 Astronomia Nova. By the 1621 second edition of his Cosmographic Mystery Kepler had decided that the comet had entirely outlived its original usefulness as an argument in support of his new cosmology.
In 1619 Kepler published his book De Cometis Libelli Tres which included a diagram of the 1607 comet, clearly identifying the comets path as rectilinear. In 1695 Edmond Halley used this and observation data from John Flamsteed (1646-1719), the first Astronomer Royal, to compute the path for the comet of 1680-81, and conclude that it was the same as the comets of 1607 and 1531. Working with Isaac Newton they found that the comets travelled in closed elliptical orbits, following Kepler’s descriptions and the soon to be published universal laws of Newton’s Principia. Furthermore Halley predicted the comet to reappear in 1758.
While this appears straight forward from the perspective of ‘progress’ in physics it was neither necessary nor sufficient for the acceptance of the Copernican model. Ariew has argued that the Aristotelian theory of comets, suitably modified, survived in text books and university theses well into the second half of the 17th century. For example by 1657 Aristotelians accepted comets as celestial objects, as stars not a fire, and therefore not sublunary; therefore not challenging the idea of a firmament, some adopting a view along the lines of a Tychonic or semi-Tychonic system on account of comets. Kepler, as argued, saw the comet as extraneous to his model, and both Newton and Halley “continued to see comets as harbingers of cataclysmic events and world reform.” Newton noted that the path of comets made them perfect to distribute “vital material” throughout the heavens as well collide with the sun or alter the solar system, Halley thought it possible that a comet caused the biblical flood.
I have focused particularly on observations of the 1577 and 1607 comets because their influence is most cited as being instrumental in the emergence of modern astronomy in the 17th century. What I have argued is that there was as much political and social reasoning behind their observation and explanation as there was science. Kepler, who put Copernicus’ model together with Brahe’s observations to create his “new astronomy’, did not see the place for comets in this model; consigning them to traveling linear paths rather than the elliptical orbits of the planets. Finally I have shown , into the 18th century, the shared discourse of comet lore was for them to be, in addition to being supra-lunary objects, agents of upheaval, renewal and divine justice.
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This is an expanded version of an essay submitted in March 2015, by the author, as an partial assessment task for HPSC20015 “Astronomy in World History” in the Post Graduate Diploma Arts (History and Philosophy of Science) program at the University of Melbourne.