The challenge of calculating the positions and movements of celestial bodies for the purpose of preparing astronomical tables helped to stimulate the development of very sophisticated mathematical tools at least as far back as the Middle Ages. The link between mathematics and astronomy was so strong that important authors in the field of astronomy were often distinguished mathematicians and vice versa. This was the case with Badr al-Dīn Muhammad ibn Muhammad ibn Ahmad ibn Muhammad ibn al-Ġazal (1423–1506), also known as Sibt al-Māridīnī, who, according to contemporary sources, produced an impressive number of works on arithmetic and algebra, the division of inheritances, and astronomy. He compiled astronomical tables for the latitude of Cairo and wrote several commentaries on earlier works, including a mathematical poem by the 12th-century Moroccan or Andalusian scientist Ibn al-Yasamin. Sibt al-Māridīnī’s work as muwaqqit (timekeeper) at the Al-Azhar Mosque in Cairo drew his attention to the study of astronomical instruments for the calculation of time. The short treatise preserved in this manuscript describes the mathematical calculations that are implied in the construction of a sundial. The rubricated headings of the paragraphs indicate the mathematical and practical problems tackled by Sibt al-Māridīnī, which include calculating the correct inclination of sundials and the proper setting of sundials.