R. Levi ben Gershom · c. 1328
Read the translation (PDF) · Editorial footnotes (PDF)
A complete medieval system of mathematical astronomy, formally Book V, Part 1 of the Wars of the Lord. Across 136 chapters it sets out solar and lunar theory, eclipses, the motions of the planets, and the fixed stars, grounded throughout in its author’s own instruments and observations.
The manuscript opens with a preface and a chapter list, and the work proper declares its own scale: divided into one hundred and thirty-six chapters, as promised. The early chapters build the mathematical ground before any astronomy is attempted; the middle of the work turns to the models, and the folios carry not only prose but the apparatus of a practising astronomer — double-entry tables with cycle-argument columns, argument columns running along the leaf edges, footer captions beneath the grids, and geometrical diagrams set into the text.
The codex also preserves the traces of use. Marginalia appear in a second hand in the inner margins, and notes run down the outer margins of the early folios — evidence that this was a text people worked through rather than merely owned.
Gersonides did not take Ptolemy on authority. He built his own instruments — the cross-staff known later as the Jacob’s staff among them — made his own observations, and where the measurements disagreed with the received models he changed the models. That is an unusual stance for the fourteenth century, and it makes Sefer HaTechunah one of the most original works of medieval astronomy in any language.
It is also the least accessible. The work was never printed and is almost entirely untranslated, surviving only in manuscript — which is why most accounts of medieval astronomy discuss Gersonides briefly and at second hand.
The translation works from a Sephardic-hand copy made in 1397 (Paris, Bibliothèque nationale de France, Hébreu 724, ff. 1r–257v), among the earliest witnesses to the text.