Ptolemy and Jābir b. Aflaḥ on Solar Eclipses

Abstract

Abū Muḥammad Jābir b. Aflaḥ, a 12th-century Andalusian mathematician and astronomer, is recognized for his influential work al-Kitāb fī l-Hay ̓a, a work today better known as Iṣlāḥ al-Majisṭī. Jābir b. Aflaḥ’s al-Kitāb fī l-Hay ̓a is a reedition of Ptolemy’s Almagest in which he also included a number of criticisms of Ptolemy’s work. The present study focuses on Jābir b. Aflaḥ’s criticisms of Ptolemy’s theory of solar eclipses, highlighting three key objections: two on Ptolemy’s choice of the mid-heaven instead of the mid-heaven of the ascendant to obtain the parallax in longitude and its effect on solar eclipses; and an additional one on Ptolemy’s treatment of the lunar parallax in latitude. While two of these criticisms appear unjustified, they offer insight into Jābir’s methodology and his reliance on defective or abridged manuscripts. Jābir’s novel approach, particularly in his use of the new trigonometry, together with his disregard for certain celestial motions, reveal both his mathematical strength and his limitations in practical astronomy. His failure to account for the Sun’s additional motion in the computations of solar eclipses further underscores his inexperience. Nevertheless, Jābir b. Aflaḥ emerges as a creative astronomer, whose work demonstrates a deep engagement with and unfrequent understanding of Ptolemy’s Almagest, albeit with notable oversights in the practical aspects of astronomy.