Two Treatises on the Astrolabe by Abū al-Rayḥān al-Bīrūnī

This article concerns two treatises by Abū al-Rayḥān Muḥammad ibn Aḥmad al-Bīrūnī (973 - ca. 1050 A.D.) entitled the Book on Bringing what is in the Potentiality of the Astrolabe into Actuality (Arabic: Kitāb Ikhrāj mā fī Quwwat al-Usṭurlāb ilā al-Fiʿl) and the Treatise on Opening the Way towards Using the Different Kinds of Astrolabes (Arabic: Maqāla fī al-Taṭrīq ilā Istiʿmāl Funūn al-Usṭurlābāt). The two treatises are user manuals for the astrolabe. In the first treatise, al-Bīrūnī deals with the applications of the standard astrolabe and in the second one, which is more advanced than the first one and was written for the readers with a higher level of knowledge in this field, treats the applications and structure of standard as well as some non-standard astrolabes. Both treatises contain technical and historical information some of which cannot be found in any other work with the same subject. In this article, after a glance at al-Bīrūnī՚s biography, I give some explanations on the stereographic projection as the principle of the astrolabe, then explain the difference between the standard and non-standard astrolabes (including mixed ones) mentioning those non-standard astrolabes which are discussed in the Taṭrīq by al-Bīrūnī. After that I give an overview of both treatises. This part is followed by a special attention to the reconstruction of the “crescent-shaped” astrolabe based on al-Bīrūnī՚s explanations in the Taṭrīq. The crescent-shaped astrolabe is a non-standard astrolabe and was discussed only in the Taṭrīq and al-Sijzī՚s Book on the Operation of the Astrolabe (Arabic: Kitāb fī ʿAmal al-Usṭurlāb; which contains brief explanations on the construction of this astrolabe) among the medieval sources. Then, I analyze what al-Bīrūnī wrote on the “boat-shaped astrolabe” in the Taṭrīq. The boat-shaped astrolabe is another example of non-standard astrolabes on which, unlike the standard astrolabes, the projected image of the ecliptic doesn՚t move, but it is equipped with a movable boat-shaped piece simulating the movement of the horizon. I then discuss al-Bīrūnīʼs authorship and the dates of these two texts, as well as the place of these two treatises among the other astrolabe works by al-Bīrūnī. I also describe the available manuscripts of these two texts.