   In the Commentariolus Copernicus listed assumptions that he believed solved the problems of ancient astronomy. He stated that the earth is only the center of gravity and center of the moon’s orbit; that all the spheres encircle the sun, which is close to the center of the universe; that the universe is much larger than previously assumed, and the earth’s distance to the sun is a small fraction of the size of the universe; that the apparent motion of the heavens and the sun is created by the motion of the earth; and that the apparent retrograde motion of the planets is created by the earth’s motion. Although the Copernican model maintained epicycles moving along the deferrent, which explained retrograde motion in the Ptolemaic model, Copernicus correctly explained that the retrograde motion of the planets was only apparent not real, and its appearance was due to the fact that the observers were not at rest in the center. The work dealt very briefly with the order of the planets (Mercury, Venus, earth, Mars, Jupiter, and Saturn, the only planets that could be observed with the naked eye), the triple motion of the earth (the daily rotation, the annual revolution of its center, and the annual revolution of its inclination) that causes the sun to seem to be in motion, the motions of the equinoxes, the revolution of the moon around the earth, and the revolution of the five planets around the sun.