All planets move about the sun in an elliptical orbit with the sun at one foci.
This empirical expression sets the scale of the solar system. For instance, if I observe an object to have an orbital period of 8 years then I know that it must have a semi-major axis of 4 AU, by the above expression. The discovery of retrograde motion in the orbit of mars was a serious challenge to the standard geocentric cosmology which demanded that all the planets orbit the earth in the same direction. While retrograde motion has a natural explanation in a heliocentric cosmology, Ptolmey came up with an elaborate machine to both explain the observed retrograde motion and to keep the earth at the center of the Universe.