Plato’s Universe is a simulation of the geocentric model of the universe that was popularized by Plato in the Timaeus in 360 BC. The model was later formalized with mathematical rigor by Ptolemy in ~100 AD. Unlike what most would suspect of an earth centered model of the universe, Ptolemy’s model was unreasonably accurate in it’s predictions. The model used epicycles to describe the movement of the planets around the earth, and was able to make predict the positions the planets would take in the sky.

Plato’s Model

Plato reasonably assumed that the earth was unmovable. Plato less reasonably assumed that the planets must orbit in concentric circles, because circles are simple and beautiful. With these assumptions Plato created a model of the universe with earth at the center and seven celestial bodies orbiting it in concentric circles: the moon, mercury, venus, the sun, mars, jupiter and saturn, in that order. Plato thought that all the stars and constellations were part a single sphere that rotated at a constant speed.

Ptolemy's Model

Ptolemy noticed that Plato's model of the universe was not able to make predictions, especially in regard to retrograde motion of the planets, so he used astronomical data to adjust it and make it more accurate. Ptolemy held onto Plato’s assumptions that the earth is immovable and that the celestial bodies orbit in circles. However, he also allowed the planets to orbit around a central point that was itself orbiting earth. Ptolemy called these epicycles and they allowed him to fine-tune his model to be able to make accurate predictions. The orbit of the sun was also tilted in order to better explain the seasons.

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VERY COOL!  I was looking to build something like this for my astronomy class, but I may just use your sim instead.  Nicely done.