Kûşyar B. Lebban

Mathematician, astronomer and astrologist (B.? – D. The first quarter of the XI. Century). His full name was Kiyâ Ebu’l-Hasen Kûşyâr b. Lebbân b. Bâşehrî el-Cilî. He was from a family that went to Bagdad from the region of Gilan (Cîlân) in the north of the Caspian Sea and there is no information about his life. The fact that he gave the position of some fixed stars in the year of 992 in his work “el-Medhal fî şinâ ati ahkâmi’n-nücûm” and that he attributed to his previous works “ez-Zîcü’l-câm” and “ez-Zîcü’I-bâliğ” demonstrate that he lived in those dates. It is assumed that Kûşyâr, who is thought to have spent most of his life in Bagdad, was at the peak of his professional career approximately in the years of 990-1000.

Kûşyâr was one of the notable names of his period in the field of mathematics, astronomy and astrology as his sobriquet “Kiyâ” having the meaning of “master” indicates. He processed systematically the calculation realized through the Hindu numerals and the numeration system to the base sixty (hisâb-ı sittinî) (this calculation was a sub-branch of the science of astronomy) in his written work “Kitâb lî uşûli’l-hisâbi’i-Hindî”. This work was one of the oldest written works in its field that arrived to the present day and its most important characteristic is that it transmitted the Hindu mathematics numbers and methods for the first time to the astronomic calculation system according to the identifications made until now. Within this context, he successfully implemented and developed the algorithmic calculation technique of Harizmî both in the Hindu calculation and in the numeration system to the base sixty (hisâb-ı sittînî), especially in the calculation of square and cube root. Besides, the fact that he indicated that the results in some divisions and square root operations were continuous, showed that he approached to the decimal fraction system.

The calculation techniques put forward in this work of Kûşyâr were developed by Ali b. Ahmed en-Nesevî, who was accepted as his student. Kûşyâr dealt with the topics of calendar, planar and globular trigonometric functions, star tables etc. in his written work named “ez-Zîcü’l-câmi”. Within this context, he reviewed the trigonometric information put forward before him. As a matter of fact, while Ebü’l-Vefâ el-Buzcânî gave only the table of sinus and Bettânî only the tables of sinus and cotangent, Kûşyâr has given the tables of sinus, contingent, tangent and versed-sinus together with the differences between them. As for the calculations of the trigonometric functions, he continued them until three digits in the system to the base sixty and calculated the angles by increasing them by one degree. Besides, Kûşyâr is accepted as the inventor and the eponym of “şekl-i muğnî”(T.N. “rich form”)in the Islamic trigonometry history as Birûnî firstly mentioned in his written work “Ma-kâlîdü ilmi’l-heye” and Nasirüddin-i Tusî repeated it in his book “Kitâ-bü Şekli’l-kattâ”. The written work of Kûşyâr named “Kitâbü’l-Medhal fî şınâ ati ahkâmi’n-nücûm” is one of the most famous written works of the East in the field of astrology, it was translated to Turkish, Persian and Chinese and it turned into a reference book. The written work was an exceptionally systematical work that contained Persian and Hindu astrology studies, as well as his own opinions, together with the book “Tetrabiblos” of Batlamyus.

Kûşyâr’s written works are: “Risâletü’l-eb ad ve el-ecrâm, Tecrîdü uşûli terkî-bi’I-cüyûb, Kitâbü’l-Usturlâb ve keyfiyyeti amelihî ve i’tibârihî ale’t-tamâm ve’I-kemâl (He also had some written works known with the name Kitâbü’l-Usturtâb and İrşâd-i Usturlâb (translated into Persian). Beyhaki indicated that some engineers had criticized Kûşyâr about the subject of the planet Mars and that he had written a work named “Is­la hu ta’dîli’l-Mirrîh”. Kâtip Çelebi recorded that Kûşyâr had owned a written work named “Kitâbü’l-Kiyâ fi’n-nücûm”. Bağdatlı İsmail Pasha attributed him a written work named “Kitâbü’l-Lâmi fî em-şileti’z-zîci’i-câmi”. This work was created with the explanation of each chapter of “ez-Zîcü’l-câmi” with examples. There was the record that Kûşyâr also had the written works named “Kitâbü’l-Kırânât”, “Risâle fî delâleti’l-kevâkib” and “Kitâbü’l-İhtiyârât” in some libraries.

WORKS:

ez-Zîcü'l-câmi (composed of four divisions and classified among the astronomic books named as “the Group of Bettanî”, because Kûşyâr, who didn’t make direct astronomic observation, completed many deficiencies of this written work by taking the principle information from the “ez-Zîcü’ş-Şâbî” of Bettanî as Şemsülmüneccim Muhammed Ali Hoca indicated in the preface of “Zîcü'l-muhakkı’s-sultânî ale’r-raşadi’l-İlhânî”), ez-Zîcü’l-bâliğ (only the copy including the first two chapters of the book survived until today. Edward S. Kennedy tells that the written work was the summarized copy of ez-Zîcü'l-câmi), Kitâbü’l-Medhal fî sinati ahkâmı’n nücûm (also known as el-Mücmel fî şınâ’âti’n-nücûm), Kitâb fî uşûli’l-hisâbi’l-Hindî (Şâlöm b. Joseph Anâbî made it translated to Hebrew language in the years of 1450-1460 in Istanbul with the name of Iyyûn hâikkârîm and made it expounded).

REFERENCE: Sâlih Zeki / Âsâr-ı Bâkıye (s. 166-167, 1911), İhsan Fazlıoğlu / TDV İslâm Ansiklopedisi (c.26, 2002), İhsan Işık / Ünlü Bilim Adamları (Türkiye Ünlüleri Ansiklopedisi, C. 2, 2013) - Encyclopedia of Turkey’s Famous People (2013).