For thousands of years, stargazers around the world wondered why there were a handful of lights in the sky that did not follow the motions of all the other lights. Ptolemy in the 2nd century AD came up with the most accurate explanation involving these planets (Greek for wanderers) along with the sun and moon orbiting around the earth, with something called an epicycle to explain why some of the lights sometimes seemed to go backwards in the sky relative to the fixed stars. When the Christian church became the state religion of the Roman empire, this idea, called geocentrism, became widely known and locked into dogma. It wasn’t until the wars of the Reformation that the idea that the Earth along with all the other planets instead orbited around the sun. This idea, written about by a Polish Catholic cleric called Copernicus in Latin, proposed that the planets all followed circular orbits. Copernicus wisely chose to not have his work published until shortly before his death, as questioning dogma was could get you a visit from the inquisition.
But this heliocentric model was actually a less accurate model for the motions of the planets than the existing Ptolemaic geocentric model. It took a protestant monk named Kepler to come up with the solution to this confusion, determining that the planets did orbit the sun, but on elliptical orbits, not circular orbits. And this is what is actually the case. This episode of Cosmos tells Kepler’s story. Enjoy!