Abstract

With the introduction of Greco-Islamic science and natural philosophy, medieval natural philosophers were confronted with three distinct astronomical systems: Aristotelian, Ptolemaic, and the system of al-Bitruji. A fundamental problem that each had to confront was how to explain simultaneous contrary motions in the heavens -for example, the sun's motion, which moves east to west with a daily motion while simultaneously moving west to east along the ecliptic- within an Aristotelian physical system that assumed that a simple body could have only one proper motion. How medieval natural philosophers resolved this problem is the focus of the article.