Ibn Yunus

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 has 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.

Ibn Yunus' importance in the history of astronomy stems mainly from this work, which is a particularly fine example of this class of astronomical handbook, the compilation of which concerned most Islamic astronomers. Ibn Yunus' Hakimi Zij is distinguished from other surviving zijes in that it begins with a list of observations, made by both Ibn Yunus and some of his predecessors. 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.

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. However, both associations are uncertain.

Recommended Reading

D.A. King, 'Ibn Yunus', in The Dictionary of Scientific Biography, New York 1970

John North, The Fontana History of Astronomy and Cosmology, London 1994

Full Bibliography