The treatise entitled “Arithmetic in astronomy”, which is edited and commented upon in this paper, is part of an old manuscript collection preserved in the National Library of Bosnia and Herzegovina. At the end of this treatise there are some phrases in Arabic which show that the treatise was originally part of a Persian book written by ʿAlī al-Qūshchī. This manuscript is, in fact, an abridged form of the second chapter of Qūshchī’s Book prepared by Naṣrullāh al-Naṣīr bin Naṣīr nicknamed as Wāqif al-Khalkhālī and copied by Ḥājī Uthmān bin Ḥajī ʿUmar. Following the well-known practice of arithmetic books, the treatise contains a breaf discussion of topics such as addition, substraction, multiplication, division and extractactin of the second root in sexagesimal system. Some astronomical applications are also mentioned.
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