R. Yitzchak Yisraeli · c. 1310
Read the translation (PDF) · Editorial footnotes (PDF)
One of the great medieval Hebrew treatises on astronomy and the science of the Jewish calendar, composed in Toledo in the first half of the fourteenth century.
The work builds a complete system step by step, and the order is the argument: nothing is used before it has been established. It begins with geometry and trigonometry, rises through a model of the celestial spheres and the motions of Sun and Moon, and culminates in the rules of the Hebrew calendar together with a set of astronomical tables.
That progression makes Yesod Olam the fullest answer in the corpus to a question its other calendrical works raise but do not always pursue: not merely how the calendar is computed, but why those computations are the right ones — what model of the heavens they presuppose, and what geometry that model in turn requires. The manuscript opens with the author’s verse preface.
Toledo in this period was where Arabic, Latin and Hebrew learning met, and Yisraeli writes as an inheritor of all three. The book became a standard reference for later Hebrew writers on the calendar, and it is the natural companion to bar Ḥiyya’s Sefer HaIbbur two centuries earlier — where bar Ḥiyya established the discipline, Yisraeli gives it its most complete medieval statement.
This witness is the richly preserved 1484 copy in Bodleian MS. Huntington 299, chosen as a cleaner and higher-resolution alternative to the Vatican copy. The translation follows the manuscript from its first text leaf, and the volume preserves the codex’s front matter — binding, pastedown and flyleaves — before the text begins.