The book argued heliocentrism and ellipses for planetary orbits instead of circles modified by epicycles. The Copernican model replaced Ptolemy's equant circles with more epicycles. Deferents and epicycles in the ancient models did not represent orbits in the modern sense. The book also contains rules for computing the longitudes of planets using eccentrics and epicycles. Eccentrics and epicycles came to be thought of as useful fictions. Ptolemy described lunar motion by using a well-defined geometric model of epicycles and evection. Avempace's model of the cosmos consists of concentric circles, but no epicycles. Copernicus eliminated Ptolemy's somewhat-maligned equant but at a cost of additional epicycles. Various 16th-century books based on Ptolemy and Copernicus use about equal numbers of epicycles. Ptolemy discussed the idea of epicycles and center of the world. This is the main reason that Copernicus' system had even more epicycles than Ptolemy's. Only after Kepler's refinement of Copernicus' theory was the need for deferents and epicycles abolished. Ptolemy proposed a complex motion called Epicycles. Epicycles are described as an orbit within an orbit. Mercury, just like Venus, has two epicycles, one greater than another.