Comets – from Rothmann to Rossetta

This is a guest blog post from Dr Adam Mosley of Swansea University, who was kind enough to write something for us about his current research project: a new assessment of the works of sixteenth-century German mathematician and astronomer Christoph Rothmann.


With the successful landing of Philae on the nucleus of Comet 67P, comets have certainly been in the news recently. Even the debate about the sartorial choices of the lead scientist – Dr Matt Taylor of the European Space Agency – has failed to overshadow just how astonishing an achievement it is, to intercept and land on an object hurtling through space at some 510 million kilometres from Earth. The whole ten-year Rosetta mission is testament to scientists’ powerful curiosity about the universe and drive to find out more about even those parts of it that are hardest to investigate.

Yet scholarly curiosity about comets has a very long history. My most recent publication, co-authored with colleagues from Cambridge University and the University of Barcelona, ostensibly focuses on a small part of that tale. As the title suggests, Christoph Rothmann’s Discourse on the Comet of 1585: An Edition and Translation with Accompanying Essays presents and interprets one sixteenth-century treatment of a comet which, like the Rosetta mission, was concerned with both the nature of comets themselves and what study of comets could tell us about the universe more generally.

Historians of science have long attributed to comets of the sixteenth and seventeenth centuries an important role in the transformation of ideas about the universe. Traditionally comets were considered to be meteorological phenomena – fiery conflagrations in the upper atmosphere. As astronomers increasingly became convinced that they were celestial, thanks to study of a sequence of comets seen in European skies from 1577 onwards, they developed new theories about what comets were and were forced to deal with the implications of those theories for understanding the constitution of the universe and the movement of bodies such as the planets within it.

Christoph Rothmann’s work on the comet of 1585 both fits within this history and challenges some elements of it. Employed as the astronomer of a German prince, Landgrave Wilhelm IV of Hessen-Kassel, Rothmann was also a correspondent of the Danish astronomer Tycho Brahe. It is Brahe who has typically been credited with demonstrating that comets were celestial objects and working out what that meant for understandings of the cosmos more generally. Yet Brahe was sent a copy of Rothmann’s work, or at least part of it, and discussed it with Rothmann and the Landgrave before he published his own views about comets. Thus, our book not only reprints Rothmann’s treatise, and translates it from Latin into English for the first time, it also considers its contribution to these important developments in astronomy and cosmology.

The book does more than just that, however. In one essay in the volume, I try to set the interest of Rothmann and Brahe in comets into the larger context of study of them in the Renaissance. A curious feature of the existing historiography is that historians of astronomy have typically not asked why astronomers were interested in comets before they were generally agreed to be celestial phenomena. In fact, the possibility that comets – or at least some of them – occupied celestial space had been entertained by some scholars in antiquity, and there was much discussion of these alternative views in the sixteenth century. In addition, however, most astronomers and natural philosophers who subscribed to the traditional view thought that comets, though not themselves celestial, had celestial causes. In other words, they were brought about by the action of the stars and the planets on the natural matter of the Earth, and so study of them fell into the domain of astrology. Comets were also widely considered to be portents of disasters, including plague and the death of princes as well as war and regime change, so rulers and their physicians also had good reason to be interested in them. So too did theologians, since, amidst the turmoil of the Reformation, they were thought to be amongst the signs in the sky by which God communicated his intentions to mankind. Much of what we used to assume was astronomical interest in comets was probably astrological, political, medical, or theological in origin.

It would be easy for us to sneer at the idea that comets, as natural phenomena, could be either portents of disaster or signs from God. But medieval and Renaissance scholars came up with many plausible physical explanations for the association of comets with terrible events – and in this period, there was no necessary contradiction between supposing that something was natural, and therefore operated according to the usual laws of the nature, and believing it to be divinely caused. In fact, as I show in my essay, sixteenth-century scholars possessed very good ‘evidence’ that comets were portents of disaster in the form of catalogues of comets that listed them alongside the wars, invasions, plagues, royal deaths and other terrible events that they had supposedly heralded. These catalogues were compiled from ancient historical and philosophical texts and medieval chronicles and annals. Thus, researching this topic led me to study some rather curious interpretations of events better known to some of my colleagues in the department, such as the Peloponnesian War and the Norman invasion of England.

Already in the sixteenth century some scholars were sceptical about the connection being made between the appearance of a comet in the sky and the events it supposedly portended. This scepticism led them to advance some sophisticated arguments about how the historical record had been produced and what constituted a properly historical inference about the causal connection between different past events. Thus, having started to study Renaissance scholarship on comets because of what it could tell us about changing ideas about the universe, I found myself learning a lot about sixteenth-century study of my own discipline, history. That’s one of the things I like best about being a historian: following a trail of evidence can lead you to some very surprising destinations.

Written by Adam Mosley