The linear astrolabe was invented by Sharaf al-Dīn al-Ṭūsī (fl. c. 1180) and although several Arabic monographs on the subject have reached us, none have received sufficient scholarly attention. As a result, one of the most important features of Ṭūsī’s invention, the gnomonic application, remains unknown. Based on an analysis of all known monographs on the linear astrolabe, this paper will discuss the gnomonic application of the linear astrolabe, which is an alternative to the azimuth lines typical of the plane astrolabe. We will also consider the potential role of Ṭūsī’s pupil, Kamāl al-Dīn ibn Yūnus (d. 1242), in the improvement of this instrument. Finally, after surveying the available manuscript copies of the above-mentioned monographs, we present a full edition and translation of one such text and an edition and translation of one chapter, on the gnomonic application, from another text.
