Final answer:
The Copernican theory explained retrograde motion by suggesting that Earth and other planets orbit the Sun, causing the apparent backward or westward motion of the planets when Earth overtakes them in its faster orbit.
Step-by-step explanation:
Copernican theory explained retrograde motion through a heliocentric model, where planets, including Earth, orbited the Sun. Unlike the Ptolemaic system, which used epicycles and deferents, Copernicus proposed that the retrograde motion of planets is an apparent effect.
As Earth orbits the Sun, it sometimes overtakes other planets in their orbits. When this happens, from our perspective on Earth, these planets appear to move backwards, or westward, against the backdrop of stars. This westward movement is what is referred to as retrograde motion.
Copernicus's model eliminated the need for the complex system of epicycles used previously to account for this motion.