Ali ibn Isa al-Asturlabi
ʿAlī ibn ʿĪsā al-Asṭurlābī (Arabic: علي بن عيسى) was an astronomer and geographer of the 9th century. He wrote a treatise on the astrolabe and was an opponent of astrology. During the reign of al-Ma'mun, and together with Khālid ibn ʿAbd al‐Malik al‐Marwarrūdhī, he participated in an expedition to the Plain of Sinjar to measure the length of a degree, or the circumference of the Earth. He measured the Earth's circumference, getting a result of 40,248 km (or, according to other sources, 41,436 km).