It is easy to understand why the Greeks found geocentric cosmology so appealing. Night after night, we see celestial objects moving across the sky from east to west. Wouldn’t it be natural to assume that the Earth lies motionless at the center of the universe and that the Sun, the moon, planets, and stars revolve around it? In the 4th century B.C., Plato postulated a world system with the stationary Earth as its hub and a huge outer space that carried the planets and stars in its daily revolution. But his two-sphere configuration could not account for a particular motion of certain planets. As a rule, the planets and stars traveled together across the sky every night from east to west. But now and then, inexplicably, some planets seemed to drift backwards. Today we know that retrograde motion is an illusion caused by the fact that we observe planetary motion from a planet that is itself in motion. It never entered the mind of Eudoxus, a young contemporary of Plato’s, that the Earth could actually move. But he did try to accommodate planetary retrogression to the motionless earth ..