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<title> ABSTRACT </title>Until Galileo's Dialogue on the Two Chief World Systems the Aristotelian and Ptolemaic cosmologies existed concurrently but incommensurably in ancient, Arabic, and Renaissance astronomy, precisely because the status of the observations themselves was in doubt. This paper reconstitutes an important long-standing observational tradition which both Ptolemaic and Aristotelian astronomers regularly deployed to critique their opponents. The Ptolemaic astronomers charged that Aristotelian homocentric spheres permitted no change in the distance between the earth and the planets, yet the phenomenon of planetary variations, especially the change in apparent magnitude of the planets throughout their motions, seemingly revealed such changes. Aristotelians, on the other hand, argued that while Ptolemaic eccentrics and epicycles might account for the phenomenon, it was at the cost of conceding real movements to circles not centered on the earth, and that even if real the predicted changes from Ptolemaic devices did not conform to observations. A fundamental tension remained between astronomical conceptual systems because the two traditions intersected at the point of observation: the 'true' position of a body necessitated witnessing the moment of maximum brightness or diameter. Incorporating a diverse range of practitioners, texts, and contexts reveals that both Ptolemaic discrete astronomical observations as well as Aristotelian common-experience observations contributed to Copernicus' cosmological formulation and were a core component of his lifelong devotion to the proposition for a moving earth.*

Affiliations: 1: Centre for the History of Science, Technology and Medicine. Imperial


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