The Astronomical Tables of Isaac ben Joseph Israeli of Toledo

Abstract

Isaac Israeli (early fourteenth century) is best known for Yesod ʿolam (Berlin, 1846–1848), a treatise on astronomy and chronology. Here our focus is on his astronomical tables, mainly those in his unpublished Saʿar ha-samayim , which have not been studied previously. We present an analysis of some of these tables, preserved in various manuscripts, and find that they are largely dependent on similar sets of tables available at the time in Spain. Of special interest is that in the headings of a few of his tables he introduces an unusual term for table, ʿarugah , whereas in others we find the usual luaḥ . Moreover, he uses ḥalaqim or parts (that is, 1/1080 of an hour) in astronomical and geographical contexts having nothing to do with the treatment of the Hebrew calendar where they commonly occur. In Yesod ʿolam Isaac mentions Isaac ben Sid, an astronomer who worked under the patronage of Alfonso X of Castile (d. 1284) and one of the authors of the Castilian Alfonsine Tables (of which only the canons or instructions survive). Unfortunately, we have not found any trace of those tables in Isaac's tables. On the other hand, we have found evidence that, for the compilation of some of his tables, Isaac used the Toledan Tables (arranged for the Hijra calendar).