How Did the Copernican Theory Explain Retrograde Motion?

Copernicus’ heliocentric model shows how an observer on Earth orbiting the sun would see a planet with a longer orbital period appear to move backward and then forward again. Ptolemy’s model, with the Earth at the center, required complex additional mechanics to explain retrograde motion that never matched the observed motion.

Copernicus resolved the problem of the “wandering stars” by proposing a heliocentric system. An observer on Earth, orbiting the sun, would see a planet in an outer orbit apparently start to slow down and then reverse motion, and then continue forward again. The observer on Earth moves faster or slower than the planet being observed, so it appears the planet goes in reverse for a period. As the planet catches up with the observer’s orbit, it appears to move forward again.

Although Aristarchus of Samos proposed a heliocentric model in 200 B.C., Aristotle said common sense dictated a geocentric model. In 2 A.D., Ptolemy refined Aristotle’s model and added some key elements to explain retrograde motion. A planet orbiting Earth would also have to move in a smaller orbit around the orbital path to address retrograde motion. Ptolemy called this an epicycle, and it would remain unquestioned for almost 12 centuries. Over time, planetary motion data became more precise, and the mathematics behind epicycles no longer matched observed data. The Copernican model eliminated the complexity and imprecision of epicycles and came very close to matching the observed data. When Johannes Kepler showed that planets move in ellipses, not circles, the Copernican model worked perfectly.