From: Thomas Hockey et al. (eds.). The Biographical Encyclopedia of Astronomers, Springer Reference. New York: Springer, 2007, pp. 573-574 |
Courtesy of |
Ibn Yūnus:
Abū al‐Ḥasan ʿAlī ibn
ʿAbd al‐Raḥmān ibn Aḥmad ibn Yūnus al‐Ṣadafī
David A. King
Died (Egypt), 1009
Ibn
Yūnus was one of the greatest astronomers of medieval Islam and the most
important astronomer of medieval Egypt. Unfortunately, nothing of consequence
is known about his early life or education. As a young man he witnessed the
Fatimid conquest of Egypt and the founding of the new city of Cairo in 969.
In the period up to the reign of Caliph al‐ʿAzīz (975–996),
he made astronomical observations that were renewed by order of Caliph al‐Ḥākim, who succeeded al‐ʿAzīz in 996 at
the age of 11 and was much interested in astrology. Ibn Yūnus's recorded
observations continued until 1003.
Ibn
Yūnus's major work was a monumental zīj or astronomical handbook
with tables. Three substantial fragments of it survive in three manuscripts
in Leiden, Oxford, and Paris. The Ḥākimī Zīj, dedicated to the caliph,
is distinguished from all other extant zījes by beginning with
a list of observations made by Ibn Yūnus and others made by some of his
predecessors. Despite his critical attitude toward these earlier scholars
and his careful recording of their observations and some of his own, he completely
neglects to describe the observations that he used in establishing his own
planetary parameters; nor does he indicate whether he used any instruments
for these observations. In view of the paucity of this information, it is
remarkable that the statement that Ibn Yūnus worked in a “well‐equipped
observatory” is often found in popular accounts of Islamic astronomy. A. Sayılı
has shown how this notion gained acceptance in Western literature.
Ibn
Yūnus's Zīj was intended to replace the Mumtaḥan
Zīj of Yaḥyā ibn Abī Manṣūr, prepared for
the ʿAbbāsid Caliph Maʾmūn
in Baghdad almost 200 years earlier. When reporting his own observations,
Ibn Yūnus often compared what he observed with what he had computed using
the Mumtaḥan tables.
The
observations Ibn Yūnus described are of conjunctions of planets with
each other and with Regulus, solar and lunar eclipses, and equinoxes; he also
records measurements of the obliquity of the ecliptic (Chapter 11) and of
the maximum lunar latitude (Chapter 38).
In
spherical astronomy (Chapters 12–54), Ibn Yūnus reached a very high level
of sophistication. Although none of the several hundred formulae that he presents
is explained, it seems probable that most of them were derived by means of
orthogonal projections and analemma constructions, rather than by the application
of the rules of spherical trigonometry that were developed by Muslim scholars
in Iraq and Iran during the 10th century.
The
chapters of the Zīj dealing with astrological calculations (77–81),
although partially extant in an anonymous abridgment of the work preserved
in Paris, have never been studied. Ibn Yūnus was famous as an astrologer
and, according to his biographers, devoted much time to making astrological
predictions.
Ibn
Yūnus's second major work was part of the corpus of spherical astronomical
tables for timekeeping used in Cairo until the 19th century. It is difficult
to ascertain precisely how many tables in this corpus were actually computed
by Ibn Yūnus. Some appear to have been added in the 13th and 14th centuries.
The corpus exists in numerous manuscript sources, each containing different
arrangements of the tables or only selected sets of tables. The best copies
are two manuscripts now in Dublin and Cairo. In its entirety the corpus consists
of about 200 pages of tables, most of which contain 180 entries each. The
tables are generally rather accurately computed and are all based on Ibn Yūnus's
values of 30° 0′ for the latitude of Cairo
and 23° 35′ for the obliquity of the ecliptic. The main
tables in the corpus display the time since sunrise, the time remaining to
midday, and the solar azimuth as functions of the solar altitude and solar
longitude; entries are given for each degree of both arguments, and each of
the three sets contains over 10,000 entries. The remaining tables in the corpus
are of spherical astronomical functions, some of which relate to the determination
of the five daily prayers of Islam. The impressive developments in astronomical
timekeeping in 14th‐century Yemen and Syria, particularly the tables
of Abū al‐ʿUqūl for Taiz and Khalīlī
for Damascus, also owe their inspiration to the main Cairo corpus.
It is clear
from a contemporaneous biography of Ibn Yūnus that he was an eccentric,
careless, and absent‐minded man who dressed shabbily and had a comic
appearance. One day in the year 1009, when he was in good health, he predicted
his own death in 7 days. He attended to his personal business, locked himself
in his house, and washed the ink off his manuscripts. He then recited the
Quran until he died – on the day he had predicted. According to his biographer,
Ibn Yūnus's son was so stupid that he sold his father's papers by the
pound in the soap market.
Anon.
“An Abridgment of Ibn Yūnus's al‐Zīj al‐kabīr
al‐Ḥākimī.” Paris Bibliothèque nationale MS ar. 2496.
(The
sole source for some additional chapters of Ibn Yūnus's Zīj.)
Anon.
“Cairo Corpus of Tables for Timekeeping.” Dublin, Chester Beatty MS 3673 and
Cairo, Dar al‐Kutub MS mīqāt 108. (Complete copies
of this corpus.)
Caussin
de Perceval, A. P. (1804). “Le livre de la grande table Hakemite.” Notices
et extraits des manuscrits de la Bibliothèque nationale 7: 16–240.
Ibn Yūnus. al‐Zīj al‐kabīr al‐Ḥākimī.
Leiden, MS Cod. Or. 143; Oxford, MS Hunt. 331. (Contains major fragments.)
King, David A. “The
Astronomical Works of Ibn Yūnus.” Ph.D. diss., Yale University, 1972.
(Deals with spherical astronomy only.)
——— (1973). “Ibn Yūnus' Very Useful Tables for Reckoning
Time by the Sun.” Archive for History of Exact Sciences 10: 342–394.
(Reprinted in King, Islamic Mathematical Astronomy, IX. London: Variorium
Reprints, 1986; 2nd rev. ed., Aldershot: Variorum, 1993.)
——— (1976). “Ibn Yūnus.”
In Dictionary of Scientific Biography, edited by Charles Coulston Gillispie,
Vol. 14, pp. 574–580. New York: Charles Scribner's Sons.
——— (1999). “Aspects
of Fatimid Astronomy: From Hard‐Core Mathematical Astronomy to Architectural
Orientations in Cairo.” In L'Égypte fatimide: Son art et son histoire, edited
by Marianne Barrucand, pp. 497–517. Paris: Presses de l'Université de Paris‐Sorbonne.
——— (2004). In
Synchrony with the Heavens: Studies in Astronomical Timekeeping and Instrumentation
in Medieval Islamic Civilization, Vol. 1, The Call of the Muezzin
(Studies I–IX). Leiden: E. J. Brill I–2.1.1, 5.1.1, and II–4.5.
Sayılı, Aydin (1960). The Observatory in Islam,
pp. 130–156, 167–175. Ankara: Turkish Historical Society
Stevenson, F. R. and S. S. Said (1991). “Precision of Medieval Islamic Eclipse Measurements.” Journal for the History of Astronomy 22: 195–207.