The Theory of Access and Recess in Levi ben Gerson’s Astronomy, and its Sources

Despite Gersonides’ long relationship with a Christian circle for which he redacted Provençal and Latin versions of his works, no evidence suggests that scientific information came to Gersonides from this circle. An instance of this lack of transfer is provided by the theory of access and recess (trepidation) of the fixed stars, widely diffused in the Latin West but never mentioned by Gersonides, neither in its original form associated to the Toledan Tables and pseudo-Thābit’s De motu octavae spherae nor in the form it was given in the Latin Alfonsine Tables circa 1320. The analysis of the astronomical part of the Wars of the Lord shows that Gersonides depended on a small number of Hebrew sources: he was unacquainted with the Hebrew version of Azarquiel’s Treatise on the fixed stars and the content of chapter 52 of al-Battānī’s Zīj, and he knew the Hebrew version of al-Biṭrūjī’s Kitāb fī l-hay’a in a shortened version that omitted trepidation. Judging from the relevant passages in the Astronomy, the only information on the history of the theory of access and recess that reached Gersonides was a brief description of a homocentric model of trepidation, earlier than those of Azarquiel and pseudo-Thābit, which he found in the works of Abraham Bar Ḥiyya and Averroes.