An orrery or a planetarium is a construction which depicts the motion of the planets in the sky. Typical orreries place the sun at the centre, with the planets moving around it. These can be simpler to contruct, because the motions are more or less circular around the sun. However, if we want to navigate in the night sky, the heliocentric model (sun at centre) is not the right one to use for us on earth. For this purpose, one instead needs to place the earth at the centre (geocentric model). In this case, however, building a planetarium is more complex due to the epicyclic motions of the planets, from the earth's reference frame. This is exactly what the above construction does: by using truly circular gears it models the retrograde motions of the planets. This is done in a very economic way, by utilizing technology inspired by the Antikythera Mechanism.
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