Copernicus argued that the Sun rather than the Earth lies in the center of the universe. The Earth moves as a planet around the Sun, carrying its Moon along as a satellite. In 1543 little proof was available that the Earth moves; there were many reasons not to accept it. Ptolemaic astronomy was neither overly complex nor inaccurate. The most important advantage offered by Copernicus was a vision of the universe as a coherent and integrated system, where all the planets move together in elegant harmony. This copy is the 2d ed.; the 1st ed., published in 1543, is on display in Bizzell Memorial Library.