Plot the sky first
Weekly points reveal Mars slowing, reversing westward, then resuming its usual eastward drift. The loop is what a patient observer can measure without choosing a model in advance.
Interactive AstronomyCanvas · Kepler model
Planetary Motion Lab
Stand on Earth and watch Mars trace its path against the stars. Then test two ways of explaining the same observation: a Sun-centred orbit and an Earth-centred model.
From observation to model
Why does Mars turn around?
Change the reference frame
In the Sun-centred model, Earth moves faster on its inner orbit and overtakes Mars. Re-centre the same coordinates on Earth and the apparent loop emerges from the moving line of sight.
One sky, two diagrams
Both diagrams can describe the observations, but the heliocentric model produces the loop from two simple planetary orbits instead of adding a special reversal to Mars.
Orbital positions use compact JPL Keplerian elements for education; scale and planet sizes are adjusted for legibility.