Al-Hasan ibn Musa ibn Shakir

Al-Hasan ibn Musa ibn Shakir was the youngest of the Banu Musa, the three sons of the astronomer Musa ibn Shakir who were educated at Baghdad's House of Wisdom under the caliph al-Ma'mun and made it a centre of translation and original science. ^{1 2} The Arabic article calls him the most learned of the brothers in mathematics and astronomy. ² No birth or death year is attested for him in the scholarship the English article rests on: that article dates him only to the ninth century and states that of the three brothers only Muhammad's death year, 873, is known. ¹ The Arabic article prints the years 810–873 for him — its narrative repeats the 873 death year — and his Wikidata item carries the same two years; all are unsourced and of one modern tradition, and they are preserved here as a tradition, not built into this record as dates. ^{2 3}

Al-Hasan's one attributed work is the Kitab al-shakl al-mudawwar al-mustatil, the Book of the Elongated Circular Figure, and it is lost. ^{1 4} What it contained is known from the tradition: a procedure for drawing an ellipse using two pins and a length of string, the technique still taught today as the gardener's construction of the ellipse. ¹ It was a characteristically Banu Musa piece of work — a Greek geometrical topic, the conic sections on which his brother Muhammad also wrote, turned into a construction that could be carried out with string and pins. ¹

Like his brothers, al-Hasan was trained by the court astrologer Yahya ibn Abi Mansur and shared in the enterprise that defined the family: the missions to Byzantium for Greek manuscripts, the salaried circle of translators, the geodesic measurement of a degree of latitude near Sinjar, and the observational programme recorded from Baghdad between about 847 and 869. ¹ The brothers' joint Book on the Measurement of Plane and Spherical Figures, which recast Greek ratio geometry in numerical terms and was read for centuries in Gerard of Cremona's Latin translation, came out of the common workshop in which the sources rarely distinguish one brother's hand from another's. ^{1 5} That is part of why his individual outline is the faintest of the three: the historical sources, as the English article notes, often did not distinguish the brothers individually at all. ¹ No source consulted names a religion or legal school for al-Hasan, and his faith is recorded as unattested. ^{1 3 6}