Claudius Ptolemy? Pseudo-Ptolemy? The Main Source of Moses Almosnino's Treatise on Astrolabe

In the late 1960s, the author discovered an unknown Judeo-Spanish treatise on the astrolabe among the Hebrew manuscripts in the Russian National Library. He identified it as the Treatise on the Astrolabe (1560) by Moses Almosnino of Salonica. In the treatise, Almosnino alleged more than once that Ptolemy's Canons was his source for both the contents and the form of it. The researcher established that Almosnino's insistence on Ptolemy as his source is questionable. However, the search for the true source took quite a long time. Finally, the genuine source is identified as the Kitab al-amal bi-asturlab (The Book of the use of the astrolabe) by the Andaluzian Arab astronomer Abu-l-Qasim Ahmad Ibn al-Saffar (d. 1035). This article also explains why Almosnino ascribed his source to Ptolemy.