Abū al-ʿAbbās Aḥmad ibn Muḥammad ibn Kathīr al-Farghānī;
(800/805-870) also known as Alfraganus in the West, was an astronomer in the Abbasid court in Baghdad, and one of the most famous astronomers in the 9th century. The lunar crater Alfraganus is named after him.
He was involved in the calculation of the diameter of the Earth by the measurement of the meridian arc length, together with a team of scientists under the patronage of the ʿAbbāsid caliph al-Ma’mūn in Baghdad. Later he moved to Cairo, where he composed a treatise on the astrolabe around 856. There he also supervised the construction of the large Nilometer on the island of al-Rawda (in Old Cairo) in the year 861. Some modern sources describe him as Arab or Persian.