Complete Works of Claudius Ptolemy, Vol. II

Introduction to Book II of the Hypotheses by Paulus Heegaard.

Just as it is probable that Ptolemy himself, being of a declining age, expounded these matters more verbosely than clearly, so it cannot be doubted that the obscurity of the work must in large part be attributed to the Arabic translator, for whom it was most difficult to express the technical vocabulary of Greek astronomy in his own language.

In the first place, Ptolemy, following the opinion of the Pythagoreans and Plato regarding simple motion and the doctrine of Aristotle regarding natural motions, explains rather obscurely from what principles he attempts to explain the celestial motions. The core of his system is a slightly altered version of Eudoxus. He imagines certain spheres, partly solid and partly hollow, placed one inside the other, so that each rotates with uniform motion around its own diameter. This happens most easily if individual spheres are imagined as suspended within a containing sphere from two mutually opposite poles, around which they move uniformly. Ptolemy discusses the nature and reason for these poles at greater length than was necessary. His system differs from similar systems of the ancients, such as those of Eudoxus and Calippus, in that some of the spheres are imagined to be eccentric, either containing the center of the Earth and the universe or not containing it, such as epicycles. Furthermore, Ptolemy requires somewhat fewer spheres than his predecessors. In particular, he rejects the spheres of Aristotle, which are called ἀνελίττουσαι unrolling. The eight spheres, which are called the movers, are carried from east to west around the axis of the universe. Their task is to impart the daily motion to the celestial bodies. The last of these contains the sphere of the fixed stars, and within it, around the same center, are placed the other movers carrying the seven spheres of the planets. Ptolemy now proposes to arrange the axes around which all these spheres rotate so that those eight movers can be carried around the axis of the universe toward the west.