ʿAlī ibn ʿĪsā al-Asṭurlābī, author of an early Arabic treatise on the astrolabe and an opponent of astrology, enjoyed renown as an astronomical instrument maker and contributed to observations initiated by the ʿAbbāsid caliph Maʾmūn. He took part with Khālid ibn ʿAbd al-Malik al-Marwarrūdhī and others in an expedition to the Plain of Sinjār to measure 1° of latitude and, thus, the size of the Earth. ʿAlī ibn ʿĪsā made astronomical observations at Baghdad in 829/830 and at Damascus in 832–833. He divided the mural quadrant used for the Damascus observations to confirm results of the earlier missions.
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