This animation represents the movement of Earth (inside, blue) and Mars (outside, red) around the Sun, from January 1, 2000 to December 31, 2029. One white line is drawn from one planet’s position to the other’s every five days.

The circles represent the orbit of each planet if they were perfectly circular—the difference is slightly noticeable for Earth’s orbit, but really stands out for Mars. This fact helped early seventeeth–century astronomer Johannes Kepler to figure out, based on very accurate observations by Tycho Brahe, that planetary orbits are ellipses and not circles, as was previously thought.

Seeing how close Mars gets to Earth orbit in the bottom-right portion of the animation, and how distant it is on the upper left, helps understand why some Mars oppositions in Earth’s skies are more “favourable” than others.