R. Avraham ibn Ezra · c. 1148
Read the translation (PDF) · Editorial footnotes (PDF)
The earliest surviving Hebrew treatise on the astrolabe, composed in the mid-twelfth century. It explains how to build the instrument and how to use it to tell time, determine latitude, locate the stars, and cast astrological judgments.
The work runs to some thirty-four gates, and reads as a graded course in what one instrument can be made to do. The first names the parts of the astrolabe and the markings upon it. The next gates take the altitude of the sun by day and the stars by night, find the sun’s true place from the rete, and locate the point opposite the sun.
Then comes a long, patient sequence on time. Ibn Ezra distinguishes seasonal (“crooked”) hours from equal hours and works out how to obtain each from the instrument by day and by night, how many of each the day or night contains, and how to convert between the two systems in either direction — the practical problem of a world where two incompatible reckonings of the hour were both in use.
Later gates handle declinations, risings and settings, the equinoctial point, the day-arc and night-arc, and the breadth of a city — that is, its latitude. A catalogue of the great fixed stars supplies the coordinates and Arabic-derived names needed to set the instrument, followed by gates on where a star will rise and set. The book ends on a practical note that has nothing to do with the heavens: measuring the height of a tower or tree, and the depth of a thing below you.
The astrolabe was the period’s general-purpose scientific instrument, and this is the first Hebrew account of it — ibn Ezra writing for readers who had no such text in their own language. Remarkably, the treatise is no longer the only witness: an astrolabe bearing Hebrew markings that follow its instructions came to light in Verona in 2024, the very city where the work was composed.
The printed volume opens with the author’s dedicatory poem and introduction, and closes with a table of errata.