Jumping ahead in time, the systems of three astronomers were prominent in Kepler’s day (around the turn of the 17th century). They were: Claudius Ptolemy, who developed the mathematics for an earth-centered planetary model in the second century AD, Nicolaus Copernicus, who is famous for introducing (in modern times) the idea that the sun is the center of the planetary system, and Tycho Brahe, a well-to-do Danish nobleman who understood the importance of advancing the observational techniques behind astronomy, if the science was to truly progress.
These three astronomers each created systems to understand the motion of the planets, which, at first glance, appear very different. Let’s have a look:
As a very brief overview, using only the sun, earth, and Mars, Ptolemy has the sun, moon, and planets orbit the earth, while Copernicus has everything move around the sun, and Tycho Brahe has a combined motion in which the planets move around the sun, which itself revolved around the earth. Let’s look at each model, one at a time, in more detail, and then arrive at Kepler’s surprising conclusion about them.