Various people and organisations tweet historical scientific facts or events of the day, one of these is the Mathematical Association of America under the Twitter handle @maanow. Today they tweeted the following:
Tycho Brahe first observed a supernova in the constellation Cassiopeia. It provided important evidence to support the Copernican hypothesis.
Put quite simply the second statement is pure bullshit. Once again we have people confusing cosmology with mathematical astronomy. Aristotelian cosmology divided the cosmos into two spheres. The sublunar sphere, i.e. everything below the moon’s orbit around the earth was mutable, that is subject to change. The superlunar sphere, i.e. everything above the moon’s orbit, was immutable, that is unchanging.
In 1572 a stellar nova became visible from the earth. Cornelius Gemma, the son of Gemma Frisius, made the first recorded observation of it on 9 November. Tycho Brahe first saw it on 11 November. Cornelius, Tycho, and others all observed the nova and determined it to be superlunary, thereby signalling a change in the superlunar sphere contradicting Aristotelian cosmology. However this says absolutely nothing about the astronomical model of the cosmos.
Aristotle’s was not the only geocentric cosmology. Stoic cosmology, which was dominant in the later part of antiquity, rejected Aristotle’s two-sphere model for a cosmos that was homogenous and filled with pneuma. The Stoics who regarded comets as being superlunary also accepted change in the heavens, whilst propagating a geocentric astronomy. Stoic cosmology was experiencing a renaissance in the 16th century even before Tycho began his astronomical observations so the discovery that the nova was superlunary had no implication pro or contra for a heliocentric astronomical model.