Muhammad ibn Musa ibn Shakir

Abu Ja'far Muhammad ibn Musa ibn Shakir was the eldest of the Banu Musa, the three sons of the astronomer Musa ibn Shakir who made Baghdad's House of Wisdom a centre of translation and original science in the ninth century. ^{1 2} The sources date him only "before 803" for his birth; his death in 873 is the one firm year in the brothers' lives — Ibn al-Nadim's Fihrist places it in Rabi' I of 259 AH. ^{1 3} The museum's queue figure of 805 for his birth is attested in no source consulted: the articles give the terminus "before 803", and his Wikidata item offers two competing unsourced values, 800 and 803, while recording no death date at all. ^{4 5} Muhammad was the most productive of the brothers and, late in life, the most political. ¹

The brothers' father Musa had been a highwayman and then an astronomer in Khurasan, where he befriended the future caliph al-Ma'mun at Merv and entered his service as astrologer and astronomer. ^{1 2} When Musa died, al-Ma'mun took charge of the orphaned boys, placed them under the guardianship of the official Ishaq ibn Ibrahim al-Mus'abi, and enrolled them in the House of Wisdom, where they were trained by the court astrologer Yahya ibn Abi Mansur and lived in Baghdad for the rest of their lives. ¹

Under al-Ma'mun and his successors the brothers used their wealth to hunt Greek science: they sent agents to Byzantium, travelled there themselves, and paid a circle of translators — Hunayn ibn Ishaq among them — at rates the sources put near 500 dinars a month. ^{1 2} On a journey back from Byzantium Muhammad met a Harran money-changer named Thabit ibn Qurra, recognised his talent and brought him to Baghdad, where Thabit became one of the century's leading mathematicians and astronomers. ¹ The brothers' own fieldwork included the measurement of a degree of latitude in the desert near Sinjar, repeated at Kufa, from which they obtained a value of about 24,000 miles for the Earth's circumference, and a programme of observations recorded between about 847 and 869 — stellar altitudes, the sun's extremes, and simultaneous sightings of a lunar eclipse from Samarra and Nishapur to fix their difference of latitude. ^{1 2}

Employment on caliphal engineering projects drew the brothers into politics. Under al-Mutawakkil they were given the Ja'fariyya canal; when their agent's error threatened to drain it, the caliph's engineer Sanad ibn Ali certified the work sound — knowing the caliph, whose death the astrologers had predicted, would not live to see the truth — and the assassination of al-Mutawakkil saved them. ¹ They also turned court influence against al-Kindi, whose library was confiscated through their hostility before Sanad ibn Ali secured its return. ¹ Muhammad's own peak came after al-Mutawakkil's death, when Turkish commanders were taking control of the state: he helped al-Musta'in secure the caliphate, and when al-Musta'in's brother besieged Baghdad, Muhammad was sent first to estimate the besieging army and then to learn the terms on which al-Musta'in would abdicate. ^{1 2}

Of the nearly twenty books the brothers wrote, most are lost. ¹ Their joint Book on the Measurement of Plane and Spherical Figures — which treated area and circumference numerically where the Greeks had used ratios, and added theorems the Greeks did not have — was used by medieval mathematicians for centuries, in Gerard of Cremona's Latin version among others. ^{1 6} Muhammad's own writings include a recension of Apollonius's Conics, the Book on the First Motion of the Celestial Sphere, a treatise on the visibility of the crescent that cites his kunya Abu Ja'far, and a lost Book on the Beginning of the World. ^{1 3} He died in 873 — the Fihrist gives Rabi' I 259 AH; the English article's lead says February of that year and its Deaths section, following Bir, says January, so this record keeps the year. ^{1 3} No source consulted names a religion or legal school for him, and his faith is recorded as unattested. ^{1 4}