| From: Thomas Hockey et al. (eds.). The Biographical Encyclopedia of Astronomers, Springer Reference. New York: Springer, 2007, pp. 558-559 |
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Courtesy of 
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Ibn Isḥāq: Abū al‐ʿAbbās ibn Isḥāq al‐Tamīmī
al‐Tūnisī
Julio Samsó
Flourished Tunis (Tunisia)
and Marrakech (Morroco), circa 1193–1222
Ibn
Isḥāq 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
Isḥāq, 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 Isḥāq
were from:
| (1) |
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 |
| (2) |
Ibn
al‐Bannāʾ al‐Marrākushī (1256–1321) who states
in his Minhāj al‐ṭālib fī taʿdīl
al‐kawākib that Ibn Isḥāq
made observations in Marrakech, that his book was written on cards or
independent sheets (baṭāʾiq), and (in one manuscript) that some
of his tables were calculated for the year 1222. |
Much more information on Ibn Isḥāq 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 Isḥāq
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 Isḥāq himself, and the date given is 1193. The date (1222)
mentioned in one manuscript of Ibn al‐Bannāʾs Minhāj
is confirmed by Ibn Isḥāq's table of the solar
equation that reaches a maximum value of 1° 49' 7''.
This amount can be calculated (using Ibn Isḥāq's
own tables based on a Zarqālian solar model with variable eccentricity)
precisely for the year 1222.
Ibn
Isḥāq 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 Isḥāq'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 Isḥāq'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 Isḥāq.
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 Muḥammad ibn al‐Raqqām of Tunis and Granada,
who is the author of three different versions of Ibn Isḥāq's
zīj.
The zījes derived from Ibn Isḥāq 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‐Shāṭir
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 Isḥāq'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.)