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The word “planet“ comes from the Greek word “planetos“ meaning wandering star (thus, unlike the Earth, in Ptolemy's model, the Sun and Moon were planets). Because of retrograde, for centuries unexplainable movements, the heavenly bodies in Ptolemy's model does not move by ellipses but rather by curves called epicycles. This animation shows simplified Ptolemy's model of the Universe with Moon as a first, and Sun as a fourth planet orbiting around Earth. All planets except Moon and Sun are moving on epicycles whit their centres positioned on circles called deferent. Last sphere, after Saturn, is a sphere of Stars. In this particular figure, orbital periods are adjusted so they always draw the same path. The background sound is Simphonies of planets recorded by NASA Voyager.
