The first two books of Aristotle's On the Heavens (De Caelo) offer perspectives on the cosmology of a thinker whose ideas in this area were considered authoritative by many until the seventeenth century. This new translation and commentary provide the modern reader with the opportunity to appraise Aristotle's ideas in relation to the cosmologies of his predecessors. While tied to the thinking of the day, Aristotle nevertheless succeeded in placing cosmology on a new footing; indeed, as the commentary in this volume shows, his use of mathematics-style demonstration, along with his appeal to observation, in avoiding more standard forms of argument, resulted in a methodology that often shares common ground with today's cosmologies. Greek text, with facing-page English translation, introduction, notes and commentary.