Isaac Israeli ben Joseph or Yitzhak ben Yosef (often known as Isaac Israeli the Younger) was a Spanish-Jewish astronomer/astrologer who flourished at Toledo in the first half of the fourteenth century. He was a pupil of Asher ben Yehiel, at whose request (in 1310) he wrote the astronomical work Yesod Olam, the best contribution on that subject to Hebrew literature.
It treats of geometry and trigonometry as introductory to the subject-matter; of the structure and position of the globe; of the number and movements of the celestial spheres; of the time differences in days and nights in the various parts of the earth; of the movements of sun and moon; of the solstices, the neomeniæ, the eclipses, and the leap-years; it contains as well astronomical tables (an ephemeris) and a perpetual calendar. It also deals (iv, § 17) with the chronological systems of other nations and religions, especially Christianity; and gives (iv, § 18) in chronological order the noted personages of the Biblical, Talmudic, and geonic periods, following the Sefer ha-Qabbalah of Abraham ibn Daud. This last was included by Zacuto in his Sefer ha-Yuḥasin.
The Yesod Olam was first published at Berlin, in 1777, by Jacob Shklower. A more complete edition, with a preface by David Cassel, was published by B. Goldberg and L. Rosenkranz (1848). Israeli's work was much studied in the Middle Ages. Isaac al-Hadib, Judah Bassan, and Elijah Mizrahi annotated it, and an anonymous author wrote a commentary to it (Neubauer, Cat. Bodl. Hebr. MSS. Nos. 2044, 746, 5). An abridgment was made in Arabic by the author's son Joseph Israeli ben Isaac, of which the Hebrew translation, Kitzur Yesod Olam is still extant (ib. No. 1319, 6).
Israeli was also the author of two other astronomical works, Sha'ar ha-Shamayim and Sha'ar ha-Milu'im, both extant in manuscript (ib. No. 2046).
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