Planes. The Timaeus is not limited to mathematical cosmology. It also covers issues of astronomy, biology, and human physiology. In Plato's astronomical scheme, for instance, a spherical earth lies at the center of a greater sphere of the heavens, on whose inner surface the stars are embedded like bright nails. As the outer sphere turns, the stars are carried around the earth in a daily rotation. The sun, moon, and the other planets revolve at different speeds around what is known as the ecliptic. If the earth's equator is extended outward to the sphere of the fixed stars, it forms a plane called the celestial equator. The ecliptic is an imaginary circular plane tilted about twenty-three degrees to the celestial equator. The path of the sun along the ecliptic intersects the celestial equator at the fall and spring equinoxes...