Gersonides' Astrology and Abraham Ibn Ezra

This paper investigates Gersonides' use of the astrology prevalent in his day, namely, horoscopic astrology. It shows that for Gersonides, Abraham Ibn Ezra was a central source and point of reference on which he relied wherever he drew on astrology. The paper is divided into three parts. The first examines the extent of Gersonides' acquaintance with astrological literature, as reflected in his private library and writings. The second part looks at several instances in his biblical commentaries, plus one from a philosophical work, in which Gersonides draws on astrology. The third part considers the extent to which Gersonides followed or went beyond Abraham Ibn Ezra and compares their respective attitudes towards key astrological issues. An appendix contains a critical edition and English translation of a section of an unpublished commentary on a selection of astrological passages from Ibn Ezra's works, which has been tentatively attributed to Gersonides, and discusses its authorship. Finally, a postscript offers a preliminary inspection of Ḥug šamayim (Circuit of heavens), the name Gersonides gave to an armillary sphere of his own design and also the name of a treatise, composed in 1325, in which he described this astronomical instrument and its uses. Ḥug šamayim is examined from the narrow angle of its bearing on Gersonides' astrology and Gersonides' reliance on Ibn Ezra's work.