Since primitive ages, man has been fascinated by the pageant of the skies?the sun, the moon, the planets, the stars, and their ceaseless movements. The rising and setting of the sun, the waxing and waning of the moon, the succession of the seasons, the advance and retreat of the planets were not only observable facts, but in numerous respects affected mankind?s daily existence. It is not strange, therefore, that a vast body of legends, and even religions, grew up around celestial phenomena.
As civilizations advanced, philosophers attempted to explain the moving heavens in rational terms. Most advanced of the ancient scientists and thinkers on matters of astronomy were the Greeks, starting with Pythagoras in the fifth century B.C. and Aristotle in the next century. An Egyptian, Claudius Ptolemy, living in Alexandria about A.D. 150, organized and systematized the classical learning of his own and preceding eras into a comprehensive set of theories. For nearly 1,500 years, the Ptolemaic system, as set forth in ?The Almagest,? dominated the minds of men and was universally accepted as the true conception of the universe.
Ptolemy?s theory was built around the notion that the earth was a fixed, inert, immovable mass, located at the center of the universe, and all celestial bodies, including the sun and the fixed stars, revolved around it. The earth, it was believed, was the hub of a system of spheres. To these the planets were rigidly attached. The stars were attached to another sphere outside this system, and all rotated each twenty-four hours. The complicated motions of the planets were explained by epicycles, with the planetary spheres rotating in the opposite direction from the sphere of stars, but pulled along by a stronger force. Saturn was consid?ered the most distant planet, nearest to stellar space, and conse?quently took longest to complete a revolution. The moon, being nearest the center, completed a revolution in the briefest time. Edward Rosen describes the Ptolemaic conception in these terms: