Retrograde motion is a fascinating celestial phenomenon that has puzzled astronomers and stargazers for thousands of years. Imagine you were stargazing across multiple nights and traced the planets as they danced across the sky, but suddenly, one of the planets stops, reverses direction, and then after a while, resumes its original motion with the stars. This phenomenon is called retrograde motion.
When the Greeks first developed their geocentric models of the solar system, retrograde motion posed a big problem. If the Earth was truly at the center of the universe, the planets would always move eastward with the stars. However, the fact that the planets stopped, reversed course to move westward, then stopped and resumed their eastward motion had the Greeks questioning the accuracy of their models. To this point, the Greeks proposed a complex system where all the planets other than Earth follow circular orbits within circular orbits. This became known as the Ptolemaic system, formulated by the astronomer and mathematician Ptolemly around 150 CE. Although we know now that the Ptolemaic model was not correct in its explanation of planetary motion, Ptolemy’s tables derived from his model were so effective at predicting positions in the night sky that they were used for over 1,500 years!
It wasn’t until the 16th century that Copernicus proposed a heliocentric model to try and account for the slight errors that resulted from using Ptolemaic’s model. Copernicus’ model was much simpler and stated that every planet orbited the sun in a perfect circle. Although this model accurately described the retrograde motion as an optical illusion, it still had inaccuracies as Copernicus did not realize that planetary orbits are actually eccentric.
Overall, retrograde motion was once a baffling phenomenon to astronomers that had a pivotal effect in reshaping the general consensus that the Earth is not truly at the center of the universe and instead orbits the Sun as all the other planets do in our Solar System!
Jack Abrams Blog 