The last couple of days have seen two astronomical anniversaries associated with the great Danish observational astronomer Tycho Brahe. Tycho first observed the super nova of 1572 from Herrevad Abbey in Southern Sweden on 11th November and five years later he first observed the great comet of 1577 from his observatory of Uraniborg on the island of Hven on 13th November. These two events have entered the folklore of the history of astronomy and their supposed impact has defined Tycho’s position in the pantheon of the astronomers who ushered in the new astronomy. Unfortunately most of what is said about them is false or at best distorted.
In both cases it is claimed that through his observations of these events Tycho singlehandedly demolished the previously held belief in the incorruptibility of the heavens and with it geocentricity and thereby paved the way for the entrance of heliocentricity. Although there is a grain of truth hidden somewhere in this claim it is an oversimplified and largely incorrect version of what actually took place in the sixteenth century.
The oversimplified version ignores the fact that the standard mediaeval view of the heavens was an uneasy mix of Aristotelian cosmology and Ptolemaic geocentric astronomy. I say uneasy because the two theories were not necessarily conform and in the view of some contemporary experts even contradictory. The belief that the heavens were incorruptible was part of the Aristotelian cosmology and it was perfectly possible to hold and defend a geocentric view of astronomy without believing in it; in fact in the later part of antiquity a leading philosophical school, the Stoics, did just that. Observational evidence refuting the incorruptibility of the heavens by no means led to a heliocentric astronomy.
The story becomes even more complex when you realise that Tycho was by no means the first important astronomer in the sixteenth century to question Aristotelian cosmology on just this point. In the 1530s there had been a series of major comets in Europe, observation of which triggered a widespread debate on the nature of comets and whether they are sub- or supra-lunar. Independently of each other Fracastoro and Cardano in Italy as well as Gemma Frisius and Jean Pina in Northern Europe concluded that contrary to Aristotle’s teachings comets were corruptible supra-lunar phenomena simultaneously rejecting Aristotle’s dictum on the incorruptibility of the heavens and his strict division between the sub-lunar and supra-lunar spheres. This position was conform with the cosmological teachings of the Stoics, which were not coincidentally undergoing a renaissance in Europe at this time. Forty years before Tycho made his much celebrated observations leading European astronomers had driven a very large nail into the coffin of Aristotelian cosmology without even considering abandoning Ptolemaic geocentric astronomy.
In the case of the super nova of 1572 it was not Tycho’s influence that led to the general acceptance that this was a supra-lunar change in the heavens but that of Christoph Clavius a dyed in the wool defender of the Ptolemaic geocentric astronomy. Again Clavius, a Jesuit mathematician and the leading astronomical authority in Europe in the last quarter of the sixteenth century, was prepared to abandon a philosophical point of Aristotelian cosmology but did not consider this is anyway grounds to abandon the geocentric astronomy.
Of course in the long run these sixteenth century observations did play a role in the gradual disintegration of the mediaeval concept of the cosmos and the acceptance of the first stage of the one we hold today but Tycho’s role in the story was by no means so central or innovative as it is usually painted.