YAḤYā IBN ABī MANṢūR

(d. near Aleppo, Syria, 832)

astronomy

Yahyā was a member of an important family of Persian scientists. His father, Abū Mansur Aban, was an astrologer; his on ’All bin Yahaya (d. 888) was eminetn in Baghdad and had a great library in which Abu Ma ʿshar studied; his grandson, Harun ibn ’All (d.900), also was an sastronomer.

Yahyā spent his life casting horoscopes (one is given by Ibn al–QiftĪ, Ta ʿ rikh al–hukamā pp. 358–359)and seeking methods to determine the positions of the stars with maximum oprecision. His first work as an astrologer was int eh service of al–Fadl ibn Sahl, vizier of Caliph al–Maʿ mun. After al–Fadl’s assassination in February 818, he entered the service of al–Maʿ mun and converted to Islam. We know that Yahya was an official at bayt al–hikma, who may have controlled funds for astronomy. who may have controlled funds for astronomy. He taught the Banū Musā and died while accompanying the caliph on an expedition against Tarsus.

yahya was appointed director of the grop of scholars who by order of al–Maʿ mun(828) established an observatory in the Shamasiya quarter of Baghdad and the observatory at the monastery of Dayr Murran in Damascus. These centers were intended to make observations that would improve and correct existing astronomical tables. The Damascus observatory was headed by Habvash al–Hʿsib (d.864/874), who sent the results obtained there to Baghdad for further elaboration. This would explain why the tables attributed to Habash are closely connected with ZĪj almumtahan.

Yahyā’s team of scientists included al–Marwarrhdhi; al–Khwārizmi, who collaborated with Yahya in 828; and Sanad ibn’ All (d. ca.864), who was in charge of improving the observational instruments, some of which were unusally large. Most of the instruments were graduated by the leading expert of the time, ’All ibn Isa al–Astrulabi, but were not very dependable. The group also included the Banu Musa and al–Jawhari; the latter corrected the data concering the positions of the planents nd of the sun and moon incorporationg the results into this own tables, on the margin of he ZĪj almumtahan.

This same group measured one degree of the meridian by using ttwo different processes: the measurement in situ of one degree on the earth’s surface, and the corroboration of thsi value by means of a trigonometric process based on measuring the dip of the horizon, with an astrolabe, from a mountaintop. The latter method seems to have been used for the first time by Sanad ibn ’All.

The numerical results of the observations were recorded in the ZĪj al–nunntahan (Tabulae probataein Latin). It should be noted that the words mumtahan and probatae are generic and indicate any table based on observation; thus the tables of Yahyā’s group are not the only ones to be so designated. the observations terminated abruptly at the almost simultaneous deaths of the caliph and of Yahyā. A written copy of the completed work was deposited at the library of the palace at Baghdad. Only one manuscript (Escorial 1927) is known to contain these tables, but it is badly bound and comprises many folios that are not from Yahyā’s work and are explicitly attributed to astronomers of the tenth and eleventh centuries. internal criticism seems to indicated that the first folios are by yahyā’s grou7p and the later ones are intermingled with works written long afterward.

Because of this disorder the manuscript can be analyzed only by means of an arbitrary arrangement. It contains a full explanation of calendars (Coptic, Greek Jewish, Muslim) and chronological eras, the majority of which are primitive. Many of the tables were compiled before those of Yahya,the date being indicated at the top of each. it is difficult to state precisely to what extent they used all the trigonometric functions, as Habash did in his works. the elements for the calculation of ephemerides were generally primitive, as were two of the tables of star positions. The elements for the calculation of ephemerides were generally primitive,m as were two of the tables of star postitions. The inferior planets are treated numerically (without theoretical explanations) as satellites of the sunan approach eqauvalent to the system of Heraclides and Tycho Brahe. This model may have been suggested by an ancient text, perhaps one by Then of Smyrna. The margins contain tables and rules that are very difficult to group and date because of their lack of unity.

Yahyā’s tables exerted a great influence on astronomy: Thabit ibn Qurra (d. 901) wrote an introduction to them that drew on data supplied by Habash, and took these tables into consideration in his works on eclipses. Ibn Yunus adapted them for use in Egypt; and al–ZārqalĪ derived from them the value of the inclination of the ecliptic and certain other values used in the calulation of ephemerides.

BIBLIOGRAPHY

See E. S.Kennedy, “A Survey of Islamic Astronomical Tables,” in Transactions of the American Philosophical Society, n.s. 46, no. 2 (1956), nos. 15, 51. MS Escorial 927 is analyzed by J Vernet, in “Las’Tabulae probatae,’ “in Homenaje a Millas–Vallicrosa, II (Barcelona, 1956), 501–522. Vernet has also published partial studies of this MS: “Los simbolos planetarios rumies, “in al–Andalus,16 (1951), 493; and “Un antiguo tratado sobre el callendario judio en las Tabulae probatae,” ub Sefarad,14 (1954), 59–78.

The Arabic sources are listed in W.Hārtner, “Habash al–Hasib al–MarwazĪ,” in Encyclopedia of Islam, new ed., III(Leiden–London, 1971), 8–9; Aydin Sayili, The Observatory in Islam (Ankara, 1960), 50–87and index; and H. Suter, Die Mathematiker und astronomen der Araber und ihre Werke (Leipzig, 1900), nos. 14, 22.

J. Vernet