Ibn Yunus

Abu al-Hasan Ali ibn Abd al-Rahman ibn Ahmad ibn Yunus al-Sadafi was born around 950 CE in Cairo, Egypt. One of the greatest observational astronomers of the medieval Islamic world, he spent his career under Fatimid patronage and produced a monumental astronomical handbook whose precision surpassed anything previously compiled.

Ibn Yunus conducted systematic astronomical observations from the Muqattam Hills near Cairo over several decades, working with some of the largest and most accurate instruments available in his time. He made careful measurements of solar and lunar positions, recorded numerous eclipses with precise timings, and computed planetary positions with exceptional accuracy. His work corrected many errors in the astronomical tables inherited from Ptolemy and earlier Islamic astronomers.

His masterpiece, the Zij al-Kabir al-Hakimi (The Great Astronomical Handbook of al-Hakim), written around 1007 CE and dedicated to the Fatimid Caliph al-Hakim bi-Amr Allah, compiled decades of original observations alongside sophisticated mathematical tables. It contained important contributions to trigonometry, including what may be the first use of a prosthaphaeresis formula — a technique for converting products of trigonometric functions into sums or differences — which later proved invaluable for astronomical computation and prefigured the development of logarithms.

Ibn Yunus is also noted for his observations of the pendulum and his measurements of time, which some historians consider precursors to the use of the pendulum in clocks. He died in 1009 CE in Cairo, reportedly while in the middle of composing a poem, serene and absorbed in contemplation. His zij remained one of the most authoritative astronomical references for centuries after his death.