Mars caused the most trouble for Copernicus’s circular model of the planets. Its apparent motion across the sky, especially its pronounced retrograde loops, was difficult to reproduce accurately using only uniform circular orbits and epicycles.
Copernicus placed the Sun near the center and made Earth another moving planet, explaining retrograde motion as an effect of changing viewpoints. When Earth overtakes Mars, Mars appears temporarily to move backward against the stars. The explanation was conceptually powerful, but matching Mars’s exact position required a complicated arrangement of circles.
Mars was especially revealing because its orbit is noticeably eccentric and its distance from the Sun changes substantially. Copernicus retained the ancient assumption that heavenly bodies moved in perfect circles, so his system still needed additional circular devices and was not dramatically more accurate than Ptolemy’s model.
Johannes Kepler eventually solved the problem using Tycho Brahe’s exceptionally precise observations of Mars. In Astronomia Nova, published in 1609, Kepler showed that Mars follows an ellipse with the Sun at one focus and moves at varying speeds. That result helped replace the circular-orbit ideal with the laws of planetary motion.