INTERLUDE 2-1 The Foundations of the Copernican Revolution
The following seven points are essentially Copernicus's own words. The italicized material is additional explanation.
  1. The celestial spheres do not have just one common center. Specifically, Earth is not at the center of everything.
  2. The center of Earth is not the center of the universe but is instead only the center of gravity and of the lunar orbit.
  3. All the spheres revolve around the Sun. By spheres, Copernicus meant the planets.
  4. The ratio of Earth's distance from the Sun to the height of the firmament is so much smaller than the ratio of Earth's radius to the distance to the Sun that the distance to the Sun is imperceptible when compared with the height of the firmament. By firmament, Copernicus meant the distant stars. The point he was making is that the stars are very much farther away than the Sun.
  5. The motions appearing in the firmament are not its motions but those of Earth. Earth performs a daily rotation around its fixed poles while the firmament remains immobile as the highest heaven. Because the stars are so far away, any apparent motion we see in them is the result of Earth's rotation.
  6. The motions of the Sun are not its motions but the motion of Earth. Similarly, the Sun's apparent daily and yearly motion are actually due to the various motions of Earth.
  7. What appears to us as retrograde and forward motion of the planets is not their own but that of Earth. The heliocentric picture provides a natural explanation for retrograde planetary motion, again as a consequence of Earth's motion.