Late medieval Islamic commentaries are a potentially rich but little-used source for historians to explore how ideas about science and religion were transmitted, criticized, and revised both in and outside of formal educational institutions. In this paper, I discuss examples taken from the previously unstudied Arabic manuscript commentaries of Fatḥallāh al-Shirwānī, a 15th century astronomer and theologian who was educated at Ulugh Beg’s observatory in Samarqand before settling in Anatolia around the conquest of Constantinople, where he dedicated commentaries to prominent individuals including the Ottoman sultan Mehmed II. Shirwānī’s massive commentary on Naṣīr al-Dīn al-Ṭūsī’s Tadhkira fī ʿilm al-hayʾa (Memoir on Astronomy), along with commentaries on other works of mathematical astronomy as well as the Qurʾān, demonstrates an abiding interest in the utility of science and human rationality for understanding God’s creation. My research uncovers examples of the dynamic nature of Shirwānī’s commentaries as sites of interaction between rival scholarly traditions, which reflect his experiences as a student at Samarqand and career as a teacher and practicing astronomer. Rather than limiting himself to a technical discussion of astronomy, Shirwānī’s work covers related fields such as optics and geography, and includes discussions of physical and philosophical principles. From engaging with his predecessors’ attempts to create physically plausible models for planetary motion to relating knowledge of the stars, the heavens and the earth to broader cultural interests in astrology, historical chronology and messianism in the post-Mongol Islamic world, Shirwānī’s work provides an important case study of science in Islamic society during a formative period in the creation of the modern world.