ab " ▁meaning ▁marked ▁with ▁s ine . ▁ ▁It ▁was ▁described , ▁according ▁to ▁King , ▁by ▁Muhammad ▁ibn ▁M ū s ā ▁al - K hw ā riz m ī ▁in ▁ 9 th ▁century ▁Bag hd ad . ▁ ▁Description ▁ ▁The ▁instrument ▁is ▁a ▁quarter ▁of ▁a ▁circle ▁made ▁of ▁wood ▁or ▁metal ▁( us ually ▁br ass ) ▁divided ▁on ▁its ▁arc ▁side ▁into ▁ 9 0 ▁equal ▁parts ▁or ▁degrees . ▁ ▁The ▁ 9 0 ▁divisions ▁are ▁gathered ▁in ▁ 1 8 ▁groups ▁of ▁five ▁degrees ▁each ▁and ▁are ▁generally ▁number ed ▁both ▁ways ▁from ▁the ▁ends ▁of ▁the ▁arc . ▁ ▁That ▁is , ▁one ▁set ▁of ▁numbers ▁begins ▁at ▁the ▁left ▁end ▁of ▁the ▁arc ▁and ▁goes ▁to ▁ 9 0 ▁at ▁the ▁right ▁end ▁while ▁the ▁other ▁set ▁the ▁zero ▁is ▁at ▁the ▁right ▁and ▁the ▁ 9 0 ▁is ▁at ▁the ▁left . ▁ ▁This ▁double ▁number ing ▁enables ▁the ▁instrument ▁to ▁measure ▁either ▁cel est ial ▁alt itude ▁or ▁z en ith ▁distance ▁or ▁both ▁simultaneously . ▁ ▁At ▁the ▁a pe x ▁where ▁the ▁two ▁graduated ▁straight ▁sides ▁of ▁the ▁grid ▁pattern ▁meet ▁in ▁a ▁right ▁angle ▁there ▁is ▁a ▁pin ▁hole ▁with ▁a ▁cord ▁in ▁it ▁and ▁a ▁small ▁weight ▁on ▁the ▁free ▁end ▁with ▁a ▁small ▁be ad ▁that ▁sl ides ▁along ▁the ▁cord . ▁The ▁cord ▁is ▁called ▁“ K h ait ” ▁and ▁is ▁used ▁as ▁a ▁pl umb ▁line ▁when ▁meas uring ▁cel est ial