epicycles

in the geocentric models of the solar system (universe?), planets were said to move in circular paths. it was observed that the planets did not appear to move in this way, as during certain periods they enter “retrograde” motion, appearing to move backwards for a while, then return to moving forwards. in order to explain this deviation from the explanation, the epicycle was added. the epicycle, as you might predict from the name, is a cycle on top of another cycle. in this case, a circle that the planet moves around, which itself moves around the sun.