Ibn Yunus (950?-1009) was born in Islamic Egypt and served the Fatimid dynasty for twenty-six years. His most famous work, al-Zij al-Hakimi al-kabir, is notable for its very accurate tabulated results. These may have been obtained using very large instruments.
Abu’l-Hasan ‘Ali Ibn ‘Abd al-Rahman Ibn Ahmad Ibn Yunus al-Sadafi came from a respected family in Fustat, his great grandfather having been a companion of the famous legal scholar al-Sahfi and his father being a distinguished historian and scholar of hadith (the sayings of Muhammad). Little is known about his early life or education. Indeed, his date of birth is not known, although 950 have been suggested. As a young man Ibn Yunus witnessed the Fatimid conquest of Egypt and the foundation of Cairo in 969 (Fustat was just outside the new city of Cairo). He served two Caliphs of the dynasty, al-Aziz and al-Hakim, making astronomical observations for them between 977 and 1003. To the second, al-Hakim, he dedicated his major work al-Zij al-Hakimi al-kabir (a zij is an astronomical handbook with tables). He died in 1009.
In astrology, noted for making predictions and having written the Kitab bulugh al-umniyya (“On the Attainment of Desire”), a work concerning the heliacal risings of Sirius, and on predictions concerning what day of the week the Coptic year will start on.
Ibn Yunus’ most famous work in Islamic astronomy, al-Zij al-Kabir al-Hakimi (c. 1000), was a handbook of astronomical tables which contained very accurate observations, many of which may have been obtained with very large astronomical instruments. According to N. M. Swerdlow, the Zij al-Kabir al-Hakimi is “a work of outstanding originality of which just over half survives”
Ibn Yunus described 40 planetary conjunctions and 30 lunar eclipses. For example, he accurately describes the planetary conjunction that occurred in the year 1000 as follows:
A conjunction of Venus and Mercury in Gemini, observed in the western sky: The two planets were in conjunction after sunset on the night [of Sunday 19 May 1000]. The time was approximately eight equinoctial hours after midday on Sunday…. Mercury was north of Venus and their latitude difference was a third of a degree.
In many respects his astronomical works have a modern appearance; many of the parameters which he used in his Zij are much superior to those of his predecessors and he is also known for his meticulous calculations and attention to detail. For example, where applicable his calculations took into account the atmospheric refraction of the Sun’s rays at the horizon, and his figure of forty minutes of arc between the observed and ‘true’ (level) horizon is probably the earliest specific figure recorded for this quantity. His observations are considered so reliable that some of the thirty eclipses reported by him were used by Simon Newcomb in the nineteenth century, in determining the secular acceleration of the moon.
Modern knowledge of the positions of the planets confirms that his description and his calculation of the distance being one third of a degree are exactly correct. In the 19th century, Simon Newcomb found Ibn Yunus’ observations on conjunctions and eclipses reliable enough to use them in his lunar theory to determine the secular acceleration of the moon. Ibn Yunus’ other observations also inspired Laplace’s Obliquity of the Ecliptic and Inequalities of Jupiter and Saturn’s. Ibn Yunus also observed more than 10,000 entries for the sun’s position for many years using a large monumental astrolabe with a diameter of nearly 1.4 metres
Ibn Yunus was renowned as a poet, and has been associated with some large instruments. One was an armillary sphere with 9 rings, each of which weighed 2,000 pounds, and was large enough for a horseman to pass through, and the other was a copper instrument resembling an astrolabe, three cubits across.