Ironically, the characteristic of the Copernican version of the heliocentric model that made it acceptable was the one which was actually wrong. The strongest argument in favour of the heliocentric model was that it followed the Aristotelian dictum that celestial objects moved in perfect circles and dispensed with the awkward system of epicycles found in the Ptolemaic model. It also had the virtue of mathematical and conceptual simplicity. Since, especially in the wake of the great discoveries of the explorers, the earth was increasingly assumed to be spherical, the notion that all planets were spheres revolving around the sun looked less improbable than it would have a few centuries earlier.