15. Ibn Yunes
(Died in 399H/1009AD)

Ibn Yunus was a prominent observer of the astronomical phenomena and a top level theoretician(93). In the words of Sarton, he might have the greatest Muslim astronomer.

His complete name was Abu al-Hassan Ali Ibn Abu Said Abdurrahman Ibn Ahmed Ibn Yunus Ibn Abd al-Ali as-sadafi al-Masri. The date of his birth is unknown, while his death occurred in 399 H/1009 AD in Cairo. He belonged to a family known for its lore. His father was an eloquent narrator and a great historian, and his grand-father was an expert in astronomy.

Ibn Yunus gained the favors of the Fatimid monarchs who encouraged him to persevere in his research in astronomy and mathematics, and erected for him an observatory near al-Fostat (Cairo), and equipped it with all the necessary tools and instruments(94).

Scientific Contributions

Ibn Yunus excelled in trigonometry; his valuable treatises contributed to the development of this science. He was the first to develop a law for the calculation of spherical triangles, which had a great importance for astronomers well before the discovery of logarithms. Thanks to this law, the operations of multiplication could be transformed in the calculation of triangles into addition. This facilitated much of the long and complicated equations. Ibn Yunus also invented the pendulum of the clock. Besides, Ibn Yunus was very skilled in solving many of the equations in astronomy(95):

He observed the Sun and Moon eclipses in Cairo towards 978 AD. His calculation constituted the more accurate findings ever known until the apparition of modern observation instruments.

Major Works

The most interesting book of Ibn Yunus is :

-“Zij al-kabir al-Hakimi" (The Big al-Hakim Ephemeris) : He began its compilation at the request of the Fatimid Caliph Aziz in 380 H/990 AD, and completed it in 1007 under the reign of the Caliph al-Hakim. The treatise was named under the Caliph al-Hakim. Parts of this book exist in several libraries around the world such as Oxford, Paris, El Escorial, Berlin and Cairo. Caussin published and translated part of this book, which contain observations of ancient astronomers as well as Ibn Yunus's observations about solar and lunar eclipses, and planets conjunction. In compiling this book, Ibn Yunus wanted to correct the observations and sayings of his predecessors, and at the same time complete them(96).