In the heliocentric model, the Earth and other planets orbit the Sun. The prograde and retrograde motions are apparent effects due to relative motions of the Earth and the planets. The heliocentric model of Copernicus is not intrinsically more accurate. Calculation is easier with the Copernicus model. Copernicus was able to determine the orbital radii (relative to Earth orbit) of all six planets, while in Ptolemy model the lengths are incorrect. Heliocentric models predict stellar parallax, while geocentric models predict otherwise.