Superlunar mutability does not imply heliocentricity!

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.


Filed under History of Astronomy

4 responses to “Superlunar mutability does not imply heliocentricity!

  1. There’s a sort of valid point here, in that Tycho’s observation showed that the ancients were wrong in one respect. So one might then ask what other ways they were wrong. Copernicus himself went back and forth about how much to trust the ancients (in terms of both models and in terms of observations), and to some extent Kepler succeeded because he paid much more attention to Tycho’s careful observations and less on ancient observations.

  2. Tycho showed that the heavens were mutable. The Aristotelian two-sphere cosmology presumed as its foundation that the only change that occurred did so in the sublunary realm, where generation and decay occurred. If the heavens were mutable, then the justification for Aristotle’s cosmology was gone, simple as that.

    The Ptolemaic system was not a cosmology, however, although it was more accurate than the Copernican at the time of Tycho. So if Aristotle’s realm was false due to Tycho’s stars, the alternatives left on the board were Ptolemy’s computational system, or Copernicus’ hybrid cosmological astronomical system. Indirectly, then, the failure of Aristotle led to the acceptability of Copernicus.

    So I think you over state things here…

  3. Federico Quallbrunn

    Actually, as is documented in Arthur Koestler’s “The Sleepwalkers”, Tycho Brahe himself favoured a geocentric system as astronomical model, even after the observation of a stellar nova.

  4. I guess Koyré started this particular hare, or perhaps it goes back even earlier. It’s tempting to take a foreshorted perspective, and view the transition from Aristotle+Ptolemy to Copernicus+Kepler+….+Newton as one big gestalt switch. (Traditionally one replaces the three dots with Galileo.) On the other hand, logically speaking, we have three separate questions: the geometrical model of planetary motion, the status of the earths physical motion, and the nature of sublunar vs. celestial matter.

    Arguing for the fine-grained viewpoint, it’s a curious gestalt-switch that takes about 200 years to complete.

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