Abraham bar Hiyya

Abraham bar Ḥiyya ha-Nasi (;
– 1136 or 1145), also known as Abraham Savasorda, Abraham Albargeloni, and Abraham Judaeus, was a Catalan Jewish mathematician, astronomer and philosopher who resided in Barcelona.

Bar Ḥiyya was active in translating the works of Islamic science into Latin, and was likely the earliest to introduce Arabic algebra into Christian Europe. He also wrote several original works on mathematics, astronomy, Jewish philosophy, chronology, and land surveying. His most influential work is his ''Ḥibbur ha-Meshiḥah ve-ha-Tishboret'', translated in 1145 into Latin as ''Liber embadorum''. A Hebrew treatise on practical geometry and Islamic algebra, the book contains the first known complete solution of the quadratic equation $x^2 - ax + b = c$, and influenced the work of Leonardo Fibonacci.

Biography

Abraham bar Ḥiyya was the great-grandson of the Hezekiah Gaon. Bar Ḥiyya occupied a high position in the royal court, serving as minister of police, and bore the title of governor (). Scholars assume that Bar Hiyya would have obtained this title in the court of Banu Hud of Saragossa-Lerida; there is even a record of a Jewish Savasorda there in the beginning of the 12th century. In his travelogues, Benjamin of Tudela mentions bar Ḥiyya living in Barcelona in the 1160s.

According to Adolph Drechsler, bar Ḥiyya was a pupil of Rabbi Moshe haDarshan and teacher of Abraham Ibn Ezra. He was held in high consideration by the ruler he served on account of his astronomical knowledge, and had disputes with learned priests, to whom he demonstrated the accuracy of the Jewish calendar. Abraham bar Hiyya is said to have been a great astronomer and wrote some works on astronomy and geography. One tells about the form of the earth, the elements and the structure of the spheres. Other works included papers on astrology, trigonometry, and music.

Some scholars think that the Magister Abraham who dictated ''De Astrolabio'' (probably at Toulouse) to Rudolf de Bruges (a work that the latter finished in 1143) was identical with Abraham bar Ḥiyya. Although the title "Sephardi" is always appended to his name, Barcelona was at the time no longer under Muslim rule, and therefore not part of Sepharad. Abraham Albargeloni (i.e., from Barcelona) thus belonged to the community of the Jews of Catalonia. Catalonia joined Provence in 1112 and Aragon in 1137, and thus the County of Barcelona became the capital of the Catalan-Aragonese Confederation called the Crown of Aragon. The kings of the Crown of Aragon extended their domains to the Occitan countries in what is now southern France. Abraham Albargeloni spent some time in Narbonne where he composed some works for the Jews of Provence, in which he complains of their Provençal Jewry's ignorance of mathematics.

Work

Abraham bar Ḥiyya was one of the most important figures in the scientific movement which made the Jews of Provence, the Jews of Catalonia, Spain, and Italy the intermediaries between Arabic science and the Christian world, in both his original works and his translations.

Bar Ḥiyya's ''Yesode ha-Tebunah u-Migdal ha-Emunah'' (, usually referred to as the ''Encyclopedia'', was the first European attempt to synthesize Greek and Arabic mathematics. Likely written in the first quarter of the 12th century, the book is said to elaborate on the interdependence of number theory, mathematical operations, business arithmetic, geometry, optics, and music. The book draws from a number of Greek sources then available in Arabic, as well as the works of al-Khwarizmi and Al-Karaji. Only a few short fragments of this work have been preserved.

Bar Ḥiyya's most notable work is his ''Ḥibbur ha-Meshiḥah ve-ha-Tishboret'' (), probably intended to be a part of the preceding work. This is the celebrated geometry translated in 1145 by Plato of Tivoli, under the title ''Liber embadorum a Savasordo in hebraico compositus''. Fibonacci made the Latin translation of the ''Ḥibbūr'' the basis of his ''Practica Geometriae'', following it even to the sameness of some of the examples.

Bar Ḥiyya also wrote two religious works in the field of Judaism and the Tanach

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