A Comparative Analysis between the System of the Nine Spheres in Sacrobosco and the Aristotelian Cosmological Model

Abstract

This article offers a comparative analysis of John of Sacrobosco’s “Tractatus de Sphaera” and Aristotelian cosmology, with the aim of understanding the processes of transmission, systematization, and adaptation of astronomical knowledge within the medieval university context. Initially, the study examines the place of the “Tractatus” in the teaching of the liberal arts and its function as an introductory manual of astronomy, highlighting the didactic character of the definition of “sphere,” grounded in Euclid and Theodosius, and its articulation between geometry and natural philosophy. The paper then presents the principles of Aristotelian cosmology, emphasizing the distinction between the sublunary and supralunary worlds, the theory of the elements, the notion of ether, and the grounding of circular motion as proper to the celestial bodies. Finally, it discusses the correspondences and differences between the complex system of multiple spheres developed by Aristotle and the simplified version adopted by Sacrobosco, organized into nine spheres. It concludes that this simplification derives from Macrobius’s influence on Sacrobosco’s text.