Hisam al-Din Salar and his Compendium of the Laws of the Science ofAstronomy

The extant work of the mathematician and philosopher Hisam al-Din Ali ibn Fadl- Allah al-Salar is limited to some small mathematical treatises and a book named Jami Qawanin Ilm al-Hayʾa (Compendium of the Laws of the Science of Astronomy). The latter has been used by Nasir al-Din al-Tusi in writing his Kashf al-Qinaʾ an Asrar al-Shakl al-Qattaʿ (Removing the Veils of the Secrets of Sector Figure). Nevertheless, he is rarely mentioned in ancient sources and what we find about him in modern scholarship is largely false and contradictory. Some place him at the beginning of the sixth century AH, others at the middle of the seventh century AH. Still others, while accepting that the Compendium of the Laws of the Science of Astronomy has been written about the middle of the sixth century AH, believe that it belongs to another author, because they consider Hisam al-Din as an author of the seventh century AH. In this article, we show that the Compendium of the Laws of the Science of Astronomy has been written about the middle of the fifth century AH, its author is no one but Hisam al-Din al-Salar and all that has been written to the contrary is due to the confusion between Hisam al-Din al-Salar and two other persons named Hisam al-Din Munajjim and Hisam al-Din Shami. We will also introduce another work by Hisam al-Din al-Salar and discuss his place in the history of trigonometry.