Cosmology and Astronomy

For two millennia, the Aristotelian physical cosmology of rotating spheres carrying the sun, moon, planets, and stars around the central earth permeated Western thought. The natural place for earthy material was at the center of the universe, and earthy material tended to move to its natural place. Planetary spheres rotated because that was their natural motion. Aristotle's teleological explanations, with the world fulfilling a purpose formed by a superhuman mind, would not necessarily be incompatible with Christian philosophy, which also envisioned the world as inherently meaningful and purposive. Concurrent with Aristotle's physical cosmology was Plato's geometrical cosmology, in which astronomers strove "to save the appearances": to explain apparently irregular planetary motions with combinations of circular motions at constant speeds.

In the context of modern science, saving the appearances with uniform circular motions is an arbitrary and absurd task. The task was, however, widely accepted and pursued for around two thousand years, from the Greeks in the fourth century B.C.E. through Copernicus and the European Renaissance in the sixteenth and seventeenth centuries C.E. The historical importance of a cosmology is not necessarily negated by its absurdity, especially when that label is applied in hindsight by different people in a different age with different standards and values.

Saving the appearances is plausible in the context of Plato's philosophy, which in turn can be understood as his reaction to the moral and political chaos of his age, which left him highly dissatisfied with the physical world. His concept of an ideal reality is simply illustrated. A circle drawn on paper is an imperfect representation in the visible world of experience of a perfect circle, which exists only in the world of thought. In his Republic, Plato wrote that the sky was part of the visible world, and the true revolutions of the planets, sun, and moon were to be discerned by reason and thought, not by sight.