An epicycle is a circle whose center travels upon a circle.

Here a random number of random epicycles upon random epicycles move Mars on some insane, nonsensical path around, or perhaps through, the Sun.

Epicycles upon epicycles can be used to compose any arbitrary curve, as
discussed in detail in Norwood Hanson's
famous paper where he pointed out that
epicycles are just terms in a
Fourier series.
In that sense this ancient device is still used every day by modern scientists
to solve complex problems.

They're also the perfect tool for modelling the heavens in a geocentric coordinate frame, or indeed any other coordinate frame. If you wanted to model a universe that revolves around yourself you just need the right epicycles.