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Retrograde Motion: Geocentric vs Heliocentric (2D)

Interactive 2D orbital simulator: watch Earth lap a slower outer planet on true circular orbits and see, plotted live against time, exactly when and why the outer planet's apparent position in Earth's sky reverses into a retrograde loop.

Space & Astronomy2DModerate60 FPS📱 Mobile-adapted⇄ 3D version

Earth and an outer planet both orbit the Sun on circular paths, but Earth — closer in, faster — periodically laps the outer planet. This 2D simulator integrates both real orbits forward in time and, every frame, computes the outer planet's true apparent bearing as seen from Earth using plain vector geometry. The top-down panel shows the real heliocentric orbits; the lower strip chart plots that apparent bearing against time, revealing the characteristic forward-retrograde-forward pattern precisely when Earth overtakes the outer planet — timed to match the synodic period formula exactly, the same phenomenon Ptolemy needed epicycles to fake and Copernicus explained with nothing more than two circles at different speeds.

⚙ Under the hood

Watch Earth and Mars orbit the Sun under Kepler's Third Law, then switch to Earth's own sky to see why Mars loops backward — the observation that helped overturn the Ptolemaic universe during the Scientific Revolution.

astronomyorbital-mechanicskeplerhistory-of-scienceretrograde-motion

2D · HTML5 Canvas 2D · 60 FPS target · runs fully client-side, no install