R. Isaac ibn al-Aḥdab · late 14th c. (composition; this is a later manuscript copy)
Read the translation (PDF) · Editorial footnotes (PDF)
Isaac ibn al-Aḥdab’s Keli Ḥemda (“The Precious Instrument”) — a description of the construction and use of an equatorium, an instrument for computing planetary positions geometrically.
The treatise divides cleanly in two, and the division is the point. The first part concerns the craft of making the instrument, proceeding by gates through the work of construction. The second is a separate treatise on its use, opening with definitions of the instrument’s parts and continuing through seven gates of operation. A standalone ruled astronomical table follows at the end of the witness, its argument column headed mispar kelali.
An equatorium answers a specific difficulty. Computing a planet’s position from tables means a long chain of arithmetic — mean motions, equations, corrections — and every step invites error. An equatorium replaces that chain with a physical model: set the instrument for the date, and read the position off directly. It trades arithmetical labour for geometrical construction, which is why the treatise has to teach both making and using.
This is one of a set of medieval Hebrew instrument treatises preserved together — alongside the quadrant Roba’ Yisrael, the astrolabe Keli ha-Neḥoshet, and ibn Verga’s horizon instrument, all in the same codex. Read together they show a tradition thinking hard about how to get astronomical results without endless computation, each instrument attacking a different part of the problem. Ibn al-Aḥdab, who moved from Castile to Sicily, is also the author of the widely copied tables Orah Selulah — a reminder that the table-maker and the instrument-maker were often the same person, pursuing the same end by two routes.
The translated witness is BnF Hébreu 1031, folios 208r–215v — a richly rubricated Hebrew astronomy anthology that also contains a Hebrew De Sphaera and works of ibn Ezra.