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Died probably Samarra, (Iraq), 829 - 869
Ḥabash al‐Ḥāsib (literally, “Ḥabash the calculator,” with the intended meaning of “mathematical astronomer”) was one of the most original and most influential Muslim astronomers of the formative period of Islamic astronomy. The dates of his birth and death are not known, but according to the bibliographer Ibn al‐Nadīm he died as a centenarian. Ḥabash was closely associated with the ʿAbbāsid court; he was active in Baghdad during the reign of Caliph Maʾmūn (813–833). Later, he lived and worked in Samarra, which in 838, became the new administrative capital of the ʿAbbāsid Empire.
Ḥabash's biography is yet to be definitively established. The bibliographer Ibn al‐Nadīm (died: 995) mentions Ḥabash as a scientist active at the time of Maʾmūn, and Ibn al‐Qifṭī (died: 1248) adds that he also lived under the reign of al‐Muʿtaṣim. In his own account of the achievements of the aṣḥāb al‐mumtaḥan – the group of scholars involved in the observational project sponsored by Caliph Maʾmūn whose objective was to check the parameters of Ptolemy's Almagest – Ḥabash does not present himself as one of their protagonists, although he was certainly in close contact with them. The earliest certain date associated with him is given by Ibn Yūnus, who reports an observation conducted by Ḥabash in Baghdad in the year 829/830 (i. e., 4 years before the death of Maʾmūn). This is also the date associated with many other mumtaḥanobservations and with the mumtaḥan star‐table.
Ibn al‐Qifṭī attributes a zīj (astronomical handbook) to Ḥabash. This was compiled when he was a young man in the tradition of the Indian Sindhind, and was based upon the zīj of Khwārizmī. Also ascribed to him is another smaller work, the Zīj al‐Shāh, probably following the same Pahlavi tradition as the eponym work by Fazārī. The composition of those two non‐Ptolemaic zījes must have occurred before 829/830, the year when the mumtaḥan observational program was inaugurated. But Ḥabash is best known to his contemporaries and successors for his authorship of a third zīj, whose content is almost entirely Ptolemaic, and which became known as “the” zīj ofḤabash.
In the introduction to this latter zīj, Ḥabash informs his readers that after Maʾmūn's death he took upon himself the task of revising the observational data gathered by the “mumtaḥan astronomers.” Hence, inspired by Ptolemy's methodology, he conducted his own observations of the Sun and Moon, and also made repeated observations of the remaining planets at specific times. The latest dates associated with Ḥabash are recorded in his zīj – 22 April 849, 17 November 860, and 15 September 868. These dates coincide with the reigns of Caliph al‐Mutawakkil (reigned: 847–861) and of his third short‐lived successor al‐Muʿtazz (reigned: 866–869). We can assume that thezīj was finalized after the year 869 and represented Ḥabash's ultimate achievement. A further indication of this is the fact that Ḥabash uses an obliquity of the ecliptic of 23° 35′, a value observed by the Banū Mūsā in Samarra in the year 868/869. He could not have been more than circa 75 years old at that time, which would then imply that he was not born before circa 796. The period 796–894, in fact, seems to be the most reasonable estimate for his life span, and this would make him belong to the same generation as Abū Maʿshar and Kindī. The usual modern references to him as flourishing circa 830 would seem to correspond in actuality to the earliest period of his life.
To summarize, we can divide Ḥabash's scientific career into the following four distinct periods:
1.The early, formative period in Baghdad (circa 815–829), during which he became acquainted with the Indian and Persian astronomical systems through the works of Fazārī and Khwārizmī, and composed two zījes based upon these systems.
2. The mumtaḥan period (829–834), during which he presumably had close contacts with the mumtaḥan group of astronomers in Baghdad and Damascus, and benefited from their new observations and insights. During this crucial period, the superiority of Ptolemy's system became gradually obvious to most specialists. With the resulting consensus in favor of Ptolemaic astronomy and the consequent abandonment of Persian and Indian theories, Islamic astronomy reached a new, stable phase of its development.
3. The post‐mumtaḥan period, beginning after the death of Maʾmūn in August 833, and possibly based in Damascus, during which Ḥabash pursued his own observational program following the mumtaḥan tradition.
4. The Samarra period, covering the last half of his career, during which he finalized his Ptolemaic zīj and composed most of his astronomical works that are now extant.
The Ptolemaic zīj of Ḥabash, the only one that is extant, is known under four different names – al‐Zīj al‐Mumtaḥan and al‐Zīj al‐Maʾmūnī (because it is based on the observational program of the mumtaḥan group under the sponsorship of Maʾmūn), al‐Zīj al‐Dimashqī (presumably because it was also based on observations conducted byḤabash in Damascus), and al‐Zīj al‐ʿArabī (because it is based on the Arabic Hijra calendar). There is absolutely no evidence to support the contention that the above appellations might refer to more than a single work. Every reference to “the zīj of Ḥabash” encountered in later sources (notably Bīrūnī and Ibn Yūnus) is in accord with the single version of the zīj by this author that is preserved for us. There is an instance where Bīrūnī mentions the zīj of Ḥabash in general terms, and later characterizes the same work with the epithet al‐mumtaḥan. This zīj is the earliest independently compiled Ptolemaic astronomical handbook in the Arabic language that is preserved in its entirety. Undoubtedly, it was also one of the most influential zījes of its generation. Indeed, Bīrūnī, in the early (Khwārizmian) period of his life, utilized it for his own astronomical practice. Although Ḥabash follows Ptolemy's models and procedures very closely, he does introduce several new, improved parameters as well as an impressive amount of original computational methods, some of them undoubtedly of Indian origin or inspiration. His zīj also contains a set of auxiliary trigonometric tables, called jadwal al‐taqwīm, which are of singular importance in the history of trigonometry.
Two copies of this zīj are available, one preserved in Istanbul, which preserves fairly well the original text, and a second one in Berlin. The latter is a recension of the original, mixed with materials due to various later astronomers. (A table of concordance with the Istanbul MS is appended to M. Debarnot's survey of the Istanbul MS.) Unfortunately, Ḥabash's zīj is yet to be published.
Another work of Ḥabash, his Book of Bodies and Distances, is in fact devoted to five different topics of scientific activity conducted under the patronage of Maʾmūn, including an interesting report on the geodetic expedition to determine the radius of the Earth (or equivalently the length of 1° of the meridian). Ḥabash also devoted several works to the topic of astronomical instrumentation. An important treatise on the construction of the melon astrolabe, which he probably invented and whose principle is based on an “azimuthal equidistant” mapping, has been published by E. Kennedy et al. (1999). An anonymous treatise on the construction of a highly original but still unexplained universal instrument for timekeeping with the stars, preserved in a unique and incomplete copy, has been published lately, and Ḥabash's authorship has been established. D. King recently suggested that this instrument could be a companion to the medieval European universal dial known as navicula de venetiis, which he hypothesizes to be, ultimately, of Islamic origin. Ḥabash also composed treatises on the use of the celestial globe, the spherical astrolabe, and the armillary sphere.
Ḥabash's graphical procedure (a so–called analemma construction) for determining the direction of Mecca (qibla) is preserved in a letter of Bīrūnī to an Abū Saʿīd (most probably Sijzī), in which the contents of Ḥabash's treatise – not extant in its original form but incorporated in his zīj – are summarized. Among several works of his that have not survived are treatises on the construction of the standard planispheric astrolabe, on the prediction of lunar crescent visibility, on the construction of sundials, and on some geometrical problem; also lost are his two critical reports on the observations conducted by the mumtaḥan group in Baghdad and Damascus.
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