Last week we discussed how Mars periodically exhibits retrograde motion, where the planet moves east to west relative to the background stars instead of its more typical west to east motion. Understanding why Mars undergoes this motion has been a challenge for hundreds of years. One of the first people to present a mathematical theory for the motions of the Sun, Moon, and planets was Claudius Ptolemy, who was thought to have lived from 85 – 165 AD. Ptolemy’s motion of the planets was a sophisticated mathematical model to fit observational data, which included an explanation for retrograde motion. He suggested that the planets moved in relatively small circles, called epicycles, and these epicycles then traveled in a circular orbit around the Earth. This “small circular motion on top of a bigger circular motion” physically produces an occasional ‘backwards loop’ that would be seen as retrograde motion. The analogy for this motion is the old Spirograph drawing game of the past. So Ptolemy thought the retrograde motion of Mars was actual motion, but he provided no explanation for why it was moving like this; only that it was. His theory of planetary motion, for Mars and all the other planets, was not superseded for over a millennium until Nicolaus Copernicus presented his Sun-centered theory in 1543. While Copernicus was not a revolutionist, his theory was revolutionary and is considered one of the great products of the Renaissance.
Next week we will continue this astronomical tale of how the motion of Mars arguably sparked the origin of modern astronomy and science.
Follow your curiosity to the Fred G. Dale Planetarium at Wayne State College.