Retrograde Motion: Geocentric vs Heliocentric

Earth and Mars orbit the Sun on circular paths whose periods obey Kepler's Third Law — Mars, farther out, takes 1.88 years to Earth's 1. This simulator renders both orbits in true 3D and lets you switch reference frames: the Sun-centered Heliocentric view shows the simple mechanism, while the Earth-centered Geocentric view plots Mars's apparent position in Earth's sky, tracing out the looping retrograde motion that puzzled astronomers for over a millennium. Live readouts track elapsed time, the Earth–Mars distance, and whether Mars is currently moving direct or retrograde against the stars — the exact observation that helped drive the shift from Ptolemy's epicycles to Copernican, and eventually Newtonian, astronomy.