Copernicus continued the traditional use of concentric, circular shells moving about a fixed center, but in his new model the center is the sun. The earth moves. The same observational data on stellar positions can be modeled, conveniently still using the well-understood geometry of rotating circles.

The insight of Copernicus was to realize that the appearance of the retrogression of Mars is merely a phenomenon of perspective. Mars and the Earth both move about the Sun, but the orbital period of Mars is more than one Earth year (it’s 687 days). It is viewed against the fixed background of the stellar sphere, but Copernicus noted that we view it from an Earth that is itself moving. As the Earth “passes” Mars on an inside track (from position #1 – #7), the angle of observation changes, and the position of Mars appears to retrogress. This was one of Copernicus’s most powerful empirical and philosophical arguments.