Levi ben Gerson on the Principles of Cosmology

Levi ben Gerson (Gersonides: 1288–1344) included discussions of cosmology in his magnum opus on astronomy (Wars of the Lord, Book V, Part 1). In particular, he addressed issues concerning the arrangement of the planets and their orbs in space. In chapter 29 of this work, Levi argues that the lowest of the set of orbs for each planet moves with the daily rotation and that the planet itself lies on the highest of its set of orbs. He claims that observations of certain phenomena visible in the sky provide evidence for his view and quotes a passage from Book II of Ptolemy’s Planetary Hypotheses in support of the principle that the surrounding orb is moved by the orb it encloses. Although Levi took Ptolemy’s work as his point of departure, he developed his own theories, which display considerable innovation. An edition of chapter 29 (based on three Hebrew manuscripts), with an English translation, is presented here.