Once upon a time, astronomers thought that the planets, the Sun, and the Moon all moved around the Earth in uniform circular motion. The heavens must be perfect, right? And what could be more perfect than a circle!
One problem: every now and then, planets reverse the direction of their visible motion across the sky, which would be impossible if they turned around the Earth on circular orbits. This so-called retrograde motion of planets forced the introduction of epicycles. An epicycle was a smaller circle on which a planet would turn around a certain point, which itself would turn on a circular orbit (a deferent) around the Earth.
If this sounds complex, you have seen nothing yet. Although retrograde motion now became possible, the calculations still didn’t quite match the observations. Soon, epicycles on epicycles were invented, yet smaller circles on which the planets would turn around a certain point that would move along an epicycle that would move along a deferent around the Earth.
Where did all the original perfect simplicity go?