Abu'l-Hasan 'Ali Ibn 'Abd al-Rahman Ibn Ahmad Ibn Yunus al-Sadafi

Abu al-Hasan 'Ali ibn 'Abd al-Rahman ibn Ahmad ibn Yunus al-Sadafi al-Misri (Arabic: ابن يونس) (c. 950-1009) was an important Egyptian Muslim astronomer and mathematician, whose works are noted for being ahead of their time, having been based on meticulous calculations and attention to detail.

The crater Ibn Yunus on the Moon is named after him.

Information regarding his early life and education is uncertain. He was born in Egypt between 950 and 952 and came from a respected family in Fustat. His father was a historian, biographer and scholar of hadith, who wrote two volumes about the history of Egypt—one about the Egyptians and one based on traveler commentary on Egypt. A prolific writer, Ibn Yunus' father has been described as "Egypt's most celebrated early historian and first known compiler of a biographical dictionary devoted exclusively to Egyptians". His great grandfather had been an associate of the noted legal scholar Imam Shafi.

Early in the life of Ibn Yunus, the Fatimid dynasty came to power and the new city of Cairo was founded. In Cairo, he worked as an astronomer for the Fatimid dynasty for twenty-six years, first for the Caliph al-Aziz and then for al-Hakim. Ibn Yunus dedicated his most famous astronomical work, al-Zij al-Kabir al-Hakimi, to the latter.

As well as for his mathematics, Ibn Yunus was also known as an eccentric and a poet.

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.

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