| From: Thomas Hockey et al. (eds.). The Biographical Encyclopedia of Astronomers, Springer Reference. New York: Springer, 2007, p. 1004 |
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Ṣāghānī: Abū Ḥāmid Aḥmad
ibn Muḥammad al‐Ṣāghānī [al‐Ṣaghānī] al‐Asṭurlābī
Roser Puig
Flourished Ṣāghān (near Merv, Turkmenistan)
Died Baghdad, (Iraq),
990
Ṣāghānī was a mathematician,
astronomer, and astrolabe maker. The 13th‐century biographer al‐Qifṭī
reports that Ṣāghānī
was an expert in geometry and cosmology (ʿilm
al‐hayʾa) and was the inventor and maker of instruments of observation.
He had a number of students in Baghdad. He was also one of the outstanding
astronomers at the observatory (bayt al‐raṣd)
built by order of the Būyid ruler Sharaf al‐Dawla (982–989) at
the extremity of the garden of the royal palace.
The Sharaf al‐Dawla Observatory was the first in the history
of Islam to have official status of some kind. According to al‐Qifṭī,
its program included the observation of the seven planets. This task was entrusted
by Sharaf al‐Dawla to Wījan ibn Rustam
al‐Kūhī, the director (ṣāḥib) of the observatory and
the leader of the astronomers working at the institution in 988. One of the
project's achievements was the observation of the Sun's entrance into two
signs (the sign of Cancer and about three months later the sign of Libra).
Two official documents were drawn up to testify to the accuracy of the procedures,
and Ṣāghānī was one of the signatories.
According to Bīrūnī,
Ṣāghānī
used a ring with subdivisions into 5 min and diameter of 6 shibr, i.
e., about 145 cm, for the determination of the obliquity of the ecliptic
and also for measuring the latitude of Baghdad. The date of the observation
is given as 984/985, and the site is specified as “Birka Zalal” in western
Baghdad. Bīrūnī also mentions that Ṣāghānī determined the lengths of the seasons using similar
methods.
Ṣāghānī is frequently associated
with a determination of the obliquity of the ecliptic by an observation using
a 21‐ft. quadrant in the year 995. However, this observation with a
quadrant of a very similar size has also been attributed to Ṣāghānī's contemporary, the great astronomer and mathematician Abū
al‐Wafāʾ al‐Būzjānī, who died
in 997 or 998. As Ṣāghānī died in 990, the latter attribution must be the
correct one.
Ṣāghānī's work on
the astrolabe, entitled Kitāb fī kayfiyyat tasṭīḥ al‐kura ʿalā saṭḥ al‐asṭurlāb, was dedicated to ʿAḍūd
al‐Dawla (977–983). In this treatise in 12 sections, Ṣāghānī describes his own
method, which he claims to be new, of projecting the sphere onto the plane
of the astrolabe. With this technique, conic sections (ellipse, parabola,
and hyperbola), in addition to points, straight lines, and circles, are formed
by taking as the “pole of projection” not one of the poles but some other
point on the line joining them. In his book Kitāb fī istīʿāb al‐wujūh
al‐mumkina fī ṣanʿat al‐asṭurlāb, Bīrūnī
states that no one can deny that Ṣāghānī
is the inventor of this projection. Ṣāghānī seems to have encouraged
Bīrūnī to develop a special type of projection, the orthographic
or cylindrical.
Ṣāghānī's treatise,
Risāla fī al‐sāʿāt al‐maʿmūla
ʿalā ṣafāʾiḥ
al‐asṭurlāb, of which only the first
chapter is extant, deals with the circular arcs that represent the hour lines
on an astrolabe plate. Ṣāghānī
states that many people in his time believed that these arcs pass through
the projections of the north and south points. With a very clear and practically
oriented explanation, he then proves that on astrolabe plates for the temperate
latitudes the circular arcs for the ends of the first, second, and third seasonal
hour cannot all pass through the projections of the north and south points.
Ṣāghānī also wrote a work
in three parts on planetary sizes and distances.
Selected References
Al‐Qiftī, Jamāl al‐Dīn (1903). Taʾrīkh
al‐ḥukamāʾ, edited by J. Lippert, p. 79. Leipzig:
Theodor Weicher.
Hogendijk,
Jan P. (2001). “The Contributions by Abū Naṣr ibn ʿIrāq and
al‐Ṣāghānī to the Theory of Seasonal Hour Lines
on Astrolabes and Sundials.” Zeitschrift für Geschichte der Arabisch‐Islamischen
Wissenschaften 14: 1–30. (Hogendijk gives an edition, translation, and
commentary of Ṣāghānī's only extant chapter from his
Risāla fī al‐sāʿāt
al‐maʿmūla ʿalā safāʾih
al‐asturlāb.)
Lorch,
Richard (1987). “Al‐Ṣaghānī's Treatise on Projecting
the Sphere.” In From Deferent to Equant: A Volume of Studies in the History
of Science in the Ancient and Medieval Near East in Honor of E. S. Kennedy,
edited by David A. King and George Saliba, pp. 237–252. Annals of the New
York Academy of Sciences, Vol. 500. New York: New York Academy of Sciences.
(Reprinted in Lorch, Arabic Mathematical Sciences, XVII. Aldershot:
Ashgate, 1995.) (Study of the Kitāb fī kayfiyyat tasṭīḥ al‐kura ʿalā
saṭḥ al‐asṭurlāb.)
Puig,
Roser (1996). “On the Eastern Sources of Ibn al‐Zarqālluh's Orthographic
Projection.” 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. 2, pp. 737–753. Barcelona: Instituto “Millás Valicrosa”de
Historia de la Ciencia Árabe.
Sayılı, Aydın (1960). The Observatory in Islam.
Ankara: Turkish Historical Society.
Sezgin, Fuat. Geschichte des arabischen Schrifttums.
Vol. 5, Mathematik (1974): 311; Vol. 6, Astronomie (1978): 217–218.
Leiden: E. J. Brill.