Ali ibn Isa al-Asturlabi

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For other people of the same name, see Ali ibn Isa.

ʿAlī ibn ʿĪsā al-Asṭurlābī (Arabic: علي بن عيسى‎) was an Afro-Arab[citation needed] 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.[1] He measured the Earth's circumference, getting a result of 40,248 km (or, according to other sources, 41,436 km).[citation needed]

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