Retrogra

Meet Retrogra, a Mars who insists she is not really going backwards.

Most nights Mars drifts slowly eastward against the background stars. But for a few weeks around the time Earth passes it, Mars seems to stop, loop back westward, then carry on. The planet never reverses. We watch from a moving platform: Earth, on its smaller, faster inner orbit, overtakes Mars, and our line of sight to Mars swings backwards against the far stars, just as a slower car seems to slide backwards as you pass it.

Earth laps Mars about every 780 days, roughly every 26 months, so the backward loop repeats on that rhythm, centred on the time Mars is opposite the Sun in our sky.

Explaining this loop simply was a big argument for the Sun-centred model that Nicolaus Copernicus published in 1543, replacing the earlier system of epicycles. The same overtaking effect makes every outer planet show retrograde loops.

The sticker's loop is a schematic, drawn in code. Real loops tilt and vary in shape because the orbits are elliptical and slightly inclined to each other.