“…like birds in the air and fish in the sea.”

In popular accounts of the transition from geocentric to heliocentric cosmology and astronomy it is often stated that in Aristotle’s geocentric cosmology the orbs of the planets were solid crystalline spheres, the existence of which were disproved when Tycho Brahe demonstrated that comets were not sublunar meteorological phenomena, as claimed by Aristotle, but supralunar astronomical objects. ‘Tycho’s comet of 1577 shattered Aristotle’s crystalline spheres’ is a common trope in such writings, but how true is it?

Aristotle’s cosmology divides the cosmos into the sublunar and supralunar spheres, i.e. below the moon and above the moon. The sublunar sphere, i.e. the earth, consists of the four elements earth, water, air and fire. The supralunar sphere consists of aether, the fifth element or quintessence. For his astronomy he adopted the homocentric planetary spheres of Eudoxus with the earth at their centre. As everything in the supralunar sphere consists of aether so do the planetary orbs. However Aristotle doesn’t actually say what aether is or what its qualities or characteristics are, it just is.

In his Mathēmatikē Syntaxis, Ptolemaeus adopted Aristotle’s cosmology putting it together with the deferent/epicycle model of the planetary orbits developed by Apollonius. He also offers no details as to the nature of aether. The pattern repeats itself with the astronomers of the Islamic Empire, who largely adopted the cosmology of Aristotle and the astronomy of Ptolemaeus without offering an explanation of the nature of aether.

It is first in the High Middle ages that the European, Christian, Aristotelian scholars first begin to ask about the nature of the aether and its properties; they were at least as motivated by the Bible as by the works of Aristotle and Ptolemaeus. The relevant Bible text is from the account of creation in Genesis:

6 And God said, Let there be a firmament in the midst of the waters, and let it divide the waters from the waters.

7 And God made the firmament and, and divided the waters which were under the firmament from the waters which were above the firmament: and it was so.

8 And God called the firmament Heaven.

Combining Aristotle’s aether and the waters above the firmament those European, Christian, Aristotelian scholars in the thirteenth century said that the planetary orbs must be fluid. However they were worried about the waters above the firmament would rain down on the earth so they thought of the orbs as having firm boundaries, like chocolate with soft centres or balloons full of water (my analogies not those of the medieval scholars). The orbs are described as solid (Latin: solidum), but this originally means that they are three-dimensional structures rather than flat disks and does not mean that they are hard.

During the fourteenth and fifteenth centuries the opinions changed and there developed a view that the orbs were not fluid but hard, however we are still far away from the crystalline spheres smashed by Tycho’s comet. In fact it is not actually known when they first appeared in the debate, although Tycho is convinced that they are propagated by his opponents. They don’t play any role in Copernicus’ astronomy so it is thought that they come into the debate somewhere between Copernicus and Tycho.

The story goes that following Tycho’s proof that the comet of 1577 was definitely supralunar the debate reverts to the possibility of fluid rather than hard planetary orbs but there are a couple of problems with this story line. Firstly, that comets were supralunar was being discussed well before Tycho’s 1577 measurement of cometary parallax. Already in the early fifteenth century Paolo dal Pozzo Toscanelli wrote a thesis in which he treated comets as astronomical phenomena and not meteorological ones. He didn’t publish his work but he did have contact with Georg Peuerbach and it can’t be just coincidence that Peuerbach and his pupil Regiomontanus also considered comets to be astronomical. Regiomontanus even wrote a paper on the problems of measuring the parallax of a moving comet, a paper that was discussed in correspondence between Tycho and John Dee. In the 1530’s there was a lively discussion on the supralunar nature of comets in which various notable European astronomers, including Copernicus, took part. When the comet of 1577 appeared it was observed very carefully and its parallax was measured by astronomers all over Europe exactly because of the earlier discussions. Although the results of those attempted measurements were hotly disputed by the various fractions, Tycho was by no means the only astronomer to determine that the comet was supralunar. The determination made by Kepler’s teacher, Michael Maestlin, probably had more impact on the debate than that of Tycho. The biggest impact, however, was made by Christoph Clavius, the leading Jesuit astronomer, who although by definition an Aristotelian scholar accepted that the comet was definitely supralunar.

If Clavius accepted that the comet was supralunar, and he did, how does this square with the fact that as a Jesuit he was required to follow a fairly strict Thomist, Aristotelian philosophy of nature. In fact this was less of a problem than one might imagine. Roberto Bellarmino, who would go on to become the most important Jesuit authority of the age, had already rejected the crystalline spheres before the appearance of the 1577 comet. In his astronomy lectures at the University of Leuven between 1570 and 1574 he taught that the whole deferent/epicycle model was just an abstract construct, which didn’t exist in reality and that the planets moved freely through a fluid medium “like birds in the air and fish in the sea.”

Tycho’s comet smashing Aristotle’s crystalline spheres is a nice story but a closer examination of the historical facts shows it to be just that a nice story but not really a true one.