<P> The geocentric model was eventually replaced by the heliocentric model . The earliest heliocentric model , Copernican heliocentrism , could remove Ptolemy 's epicycles because the retrograde motion could be seen to be the result of the combination of Earth and planet movement and speeds . Copernicus felt strongly that equants were a violation of Aristotelian purity , and proved that replacement of the equant with a pair of new epicycles was entirely equivalent . Astronomers often continued using the equants instead of the epicycles because the former was easier to calculate , and gave the same result . </P>
who challenged the aristotelian model of a geocentric universe
Copernicus