ʿAlī ibn ʿĪsā al-Asṭurlābī was a 9th century Arab geographer and astronomer. 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. Differing reports state that they obtained a result of 56 miles (90 km), 56 and two-thirds, or 56 and one-quarter miles per degree.