How did Islamic astronomers preserve and use the Almagest?

Islamic astronomers played a vital role in preserving and advancing the astronomical knowledge contained in Ptolemy’s Almagest. During the Abbasid caliphate, the text was translated into Arabic at least four times, making its complex geometrical planetary theory accessible to scholars.1

Islamic astronomers used the Almagest as a foundational reference to create zijes—handbooks of astronomical tables used to calculate the positions of the Sun, Moon, and planets. While most of these tables followed the tradition of the Almagest and Ptolemy’s Handy Tables, scholars also engaged in critical analysis. They improved upon Ptolemy’s parameters, such as the magnitude and direction of the Sun’s eccentricity, and noted discrepancies in the obliquity of the ecliptic and the length of seasons.1

Furthermore, astronomers like Ibn al-Haytham and the Marāgheh school criticized Ptolemy’s reliance on non-physical points, such as the equant point. This led to innovative mathematical modeling, including the "al-Ṭūsī couple," which sought to reconcile planetary motions with more physically plausible mechanisms.1