Planetary Gear Assembly

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 This "Planetary Gear Assembly" or epicyclic gear train is a system consisting of one or more outer gears, or planet gears, revolving about a central, or sun gear. Used by the Greeks, the device was named after its earliest application, modeling of the movements of the planets in the heavens. At around 500 BCE, the Greeks invented the idea of epicycles, of circles traveling on the circular orbits and with this theory were able to approximate the moon's elliptical path through the heavens.