From: Thomas Hockey et al. (eds.). The Biographical Encyclopedia of Astronomers, Springer Reference. New York: Springer, 2007, pp. 558-559

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Ibn Isāq: Abū al‐ʿAbbās ibn Isāq al‐Tamīmī al‐Tūnisī

Julio Samsó

FlourishedTunis (Tunisia) and Marrakech (Morroco), circa 1193–1222

Ibn Isāq was a Tunisian astronomer who left an unfinished zīj (an astronomical handbook with tables) with a few canons and instructions for their use; this marked the first of a family of Maghribī astronomical works of this kind. The zīj was heavily influenced by the Toledan astronomer Ibn al‐Zarqālī, and therefore characteristically contained sidereal mean motion tables, a model for the trepidation of the equinoxes, a solar model with variable eccentricity, and Zarqālī's correction of the Ptolemaic lunar model as well as some of his parameters. Before Ibn Isāq, we only know for the Maghrib that at the beginning of the 11th century the famous astrologer Ibn Abī al‐Rijāl al‐Qayrawānī composed a zīj, which, unfortunately, has been lost.

Until recently the only known references to Ibn Isāq were from:


the famous historian Ibn Khaldūn (1332–1382), who says, in his Muqaddima, that he was an astronomer at the beginning of the 13th century who composed his zīj using (his own) observations as well as the information he obtained through correspondence with a Sicilian Jew who was competent in astronomy and a good teacher; and


Ibn al‐Bannāʾ al‐Marrākushī (1256–1321) who states in his Minhāj al‐ālib fī taʿdīl al‐kawākib that Ibn Isāq made observations in Marrakech, that his book was written on cards or independent sheets (baāʾiq), and (in one manuscript) that some of his tables were calculated for the year 1222.

Much more information on Ibn Isāq has been gathered due to the discovery, by David A. King, of Hyderabad, Andhra Pradesh State Library MS 298, copied in Homs (Syria) in 1317, which contains the most important collection of materials derived from Ibn Isāq as well as from other (mainly Andalusian) sources. This compilation was made by an anonymous Tunisian astronomer who flourished circa 1267–1282. It contains a strange table with the names and dates of astronomers who established, purportedly by observation, the position of the solar apogee and the obliquity of the ecliptic. One of them is Ghiyām ibn Rujjār in 1178, who can be identified as William II (who reigned in Sicily between 1166 and 1189), the son of William I and grandson of Roger II. William II is undoubtedly the patron of the unnamed Jewish astronomer mentioned by Ibn Khaldūn. Another of the “observers” is Ibn Isāq himself, and the date given is 1193. The date (1222) mentioned in one manuscript of Ibn al‐Bannāʾs Minhāj is confirmed by Ibn Isāq's table of the solar equation that reaches a maximum value of 1° 49' 7''. This amount can be calculated (using Ibn Isāq's own tables based on a Zarqālian solar model with variable eccentricity) precisely for the year 1222.

Ibn Isāq seems to have left only one set of numerical tables (nos. 6–58 of the Hyderabad manuscript) for the computation of planetary longitudes, eclipses, equation of time, parallax and, probably, solar and lunar velocity. These tables were not accompanied by an elaborate collection of canons, although they contained instructions of some kind for the use of a few tables. His zīj, therefore, was unfinished and not ready to be used. This is why the anonymous compiler of the Hyderabad manuscript tried to finish this work and to “edit” Ibn Isāq's zīj by adding both canons and numerical tables. The whole constitutes an impressive collection of materials in which the predominant influence is clearly Andalusian, but we do not know yet to what extent Ibn Isāq's contributions are original. His solar tables are clearly Zarqālian in origin; the maximum equations of the center for the planets are Ptolemaic for Mars, Mercury, and the Moon; and the case of Venus (1° 51') may derive from a new computation of the solar eccentricity using Zarqālī's solar model with variable eccentricity. On the other hand, the values for Saturn (5° 48') and Jupiter (5° 41') seem new.

This unknown Tunisian compiler was not the only “editor” of the tables of Ibn Isāq. Two other contemporaries prepared “editions” of the same work. One of them was Ibn al‐Bannāʾ who wrote his Minhāj with the same purpose. The other was Muammad ibn al‐Raqqām of Tunis and Granada, who is the author of three different versions of Ibn Isāq's zīj.

The zījes derived from Ibn Isāq were used in the Maghrib until the 19th century, for they allowed the computation of sidereal longitudes that were used by astrologers. We have a limited amount of information about the observations made in the Maghrib in the 13th and 14th centuries, which established that precession exceeded the amounts fixed in Andalusian trepidation tables and that the obliquity of the ecliptic had fallen below the limits of Zarqālī's model and tables. This explains the introduction of eastern zījes in the Maghrib from the 14th century onward: Those of Ibn Abī al‐Shukr al‐Maghribī and Ibn al‐Shāir were known in the late 14th century, while the Zīj‐i jadīd of Ulugh Beg did not reach the Maghrib until the end of the 17th century. In them, mean motions were tropical and constant precession was used instead of trepidation, and there were no tables to compute the obliquity of the ecliptic. They were used by astronomers while astrologers stuck to Ibn Isāq's tradition.

Selected References

Comes, Mercè (2002). “Some New Maghribī Sources Dealing with Trepidation.” In Science and Technology in the Islamic World, edited by S. M. Razaullah Ansari, pp. 121–141. Turnhout: Brepols.

Kennedy, E. S. (1997). “The Astronomical Tables of Ibn al‐Raqqām a Scientist of Granada.” Zeitschrift für Geschichte der Arabisch‐Islamischen Wissenschaften 11: 35–72.

King, David A. (1998). “On the History of Astronomy in the Medieval Maghrib.” In Études philosophiques et sociologiques dédiées à Jamal ed‐Dine Alaoui, pp. 27–61. Fez.

Mestres, Angel (1996). “Maghribī Astronomy in the 13th Century: A Description of Manuscript Hyderabad Andra Pradesh State Library 298.” In From Baghdad to Barcelona: Studies in the Islamic Exact Sciences in Honour of Prof. Juan Vernet, edited by Josep Casulleras and Julio Samsó, Vol. 1, pp. 383–443. Barcelona: Instituto “Millás Vallicrosa” de Historia de la Ciencia Árabe.

——— (1999). “Materials andalusins en el Zīj d'Ibn Isḥāq al‐Tūnisī.” Ph.D. diss., University of Barcelona. (It includes an edition and English commentary of all the canons of Hyderabad, Andhra Pradesh State Library MS 298 and part of the numerical tables.)

Samsó, Julio (1998). “An Outline of the History of Maghribī Zijes from the End of the Thirteenth Century.” Journal for the History of Astronomy 29: 93–102.

——— (2001). “Astronomical Observations in the Maghrib in the Fourteenth and Fifteenth Centuries.” Science in Context 14: 165–178.

——— (1994). Islamic Astronomy and Medieval Spain. Aldershot: Variorum, papers VI and X.

Vernet, Juan (ed.) (1952). Contribución al estudio de la labor astronómica de Ibn al‐Bannāʾ. Tetouan: Editora Marroquí. (For the reference of Ibn al‐Bannāʾ to Ibn Isḥāq, see p. 13 in Arabic and pp. 21, 57 in Spanish.)