Al-Jawhari

Al-Abbas ibn Said al-Jawhari was a mathematician and astronomer of the first half of the ninth century. Sources disagree on where he was born: Russian and Uzbek reference works name Farab (today in the Turkistan Region of Kazakhstan), while English-language ones say Baghdad. His dates are approximate too: about 800 to about 860.

Al-Jawhari worked at the House of Wisdom (Bayt al-Hikma) in Baghdad under the caliph al-Ma’mun, at the same time as al-Khwarizmi. He had an excellent command of Greek, which was unusual among Muslim scholars of his day. He is also credited with translating into Arabic the Book of Poisons by the Indian author Shanaq.

Around 830 in Baghdad and around 833 in Damascus he took part in the astronomical observations commissioned by al-Ma’mun, together with Yahya ibn Abi Mansur, al-Marwarrudhi and Sanad ibn Ali. Their results formed the basis of the “Verified Tables of al-Ma’mun” (al-Zij al-Ma’muni al-Mumtahan). According to later accounts, the same scholars measured the length of one degree of the Earth’s meridian on the Sinjar plain, and al-Jawhari was among them.

His main work is a commentary on Euclid’s Elements, which added nearly fifty extra propositions, new figures for the first book and a revision of the theory of proportion in Book V: he tried to define the equality of two ratios as the equality of the partial quotients that arise when Euclid’s algorithm is applied to both.

He also attempted to prove Euclid’s fifth (parallel) postulate, as reported by Nasir al-Din al-Tusi. The proof rests on a tacit assumption, but along the way al-Jawhari proves that through any point inside an angle a line can be drawn that meets both of its sides. Legendre tacitly relied on the same statement in his own proof in 1800. Al-Jawhari is also credited with a treatise on determining the distance from the centre of the Earth to the Sun, and with a zij. He died around 860, probably in Baghdad.