Ptolemy’s Almagest was translated into Arabic in the mid-9th CE. Based on this attainment, Islamic scholars worked on the quantitative determination of planetary motion. Besides this work, they also articulated qualitative and physical cosmology by using composition of celestial spheres inspired by the Ptolemaic planetary models. This genre of astronomical research was called `ilm al-hay’a. One of the earliest books in this genre was Ibn al-Haytham (965-ca. 1040)’s On the Configuration of the World, and later it was canonicalized by Naṣīr al-Dīn al-Tūsī (1201-1274) in Tadhkira fī `ilm al-hay’a.

Development of the `ilm al-hay’a implies that Islamic scholars paid profound and systematic attention to physical cosmology. Then, how did these scholars foster the `ilm al-hay’a, after the Arabic Almagest till it was accumulated in Ibn al-Haytham’s work? Studying this early `ilm al-hay’a tradition is essential in obtaining the whole picture.

Due to the scarcity of available documents, research on the early `ilm al-hay’a tradition is not an easy task. While most of few materials available are fragmental, there is a rare source containing rich information on physical cosmology written before Ibn al-Haytham: Liber de orbe attributed to Mashā'allāh (ca. 815). Since this work had long been considered as remaining only in the Latin translation, however, its importance as an early `ilm al-hay’a work had not been well-recognized, probably except for D. Pingree and J. F. Ragep.

Owing greatly to my privilege as a member of the “Rational Sciences in Islam” project (McGill) until 2012, I managed to find two manuscripts of the Arabic original of the Liber de orbe: Berlin, Staatsbibliothek zu Berlin, Ms. or. oct. 273, and Philadelphia, Pennsylvania University Library, MS LJS 439. Moreover, in the Arabic text, I found that the author gave an example of solar eclipse occurred in 938. This contradicts the attribution to Mashā'allāh, and instead, makes it plausible to be written just after the eclipse in 938, for he chose this example as an incident still freshly remembered by its readers, to support his explanation about time difference of occurrence of solar eclipse in different places. It should also mean that the Liber de orbe belongs to the early `ilm al-hay’a tradition.

In this presentation, I attempt to illustrate the details of planetary models explained in the Liber de orbe by analyzing the Arabic original, and to determine the significance in the early `ilm al-hay’a tradition.