Stephen of Pisa’s theory of the oscillating deferents of the inner planets (1h. 12th C.)

Abstract

Earlier than the Arabic-Latin transfer of Ptolemaic astronomy via the Iberian peninsula, a serious occupation with Arabic astronomy by Latin scholars took place in crusader Antioch in the first half of the twelfth century. One of the translators of Arabic science in the East was Stephen of Pisa, who produced a commented Latin version, entitled Liber Mamonis, of Ibn al-Haytham’s cosmography, On the Configuration of the World. Stephen’s considerations about the physical universe in relation to the doctrines of Ptolemaic astronomy have hitherto received but little attention. The present paper discusses Stephen of Pisa’s treatment of the planetary spheres in regard to Ptolemy’s theory of oscillating deferents. Emphasis is given to geometric arguments in Stephen’s criticism of Ibn al-Haytham’s spherical model of the inner planets and to Stephen’s own attempt at an improved theory based on additional spheres. The paper argues that astronomical studies in Antioch were of an advanced level, involving independent judgement as well as an influence of contemporary trends in Arabic astronomy.