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Air and Friction in the Celestial Region: Some medieval solutions to the difficulties of the Aristotelian theory concerning the production of celestial heat

This paper explores the medieval debates concerning problems with the Aristotelian theory of the production and transmission of solar heat as presented in De Caelo II, 7 and Meteorologica I, 3. In these passages, Aristotle states that celestial heat is generated by the friction set up in the air by the motion of celestial bodies. This statement is difficult to reconcile with Aristotle’s cosmology, which presupposes that the heavenly bodies are not surrounded by air, but by aether, and that the celestial spheres are perfectly smooth, and therefore cannot cause any friction. In their commentaries on De Caelo and on Meteorologica, the Latin commentators elaborated a model that solves these difficulties. In this attempt, they invoke a non-mechanical principle, namely celestial influence.