Internal Mounting Plate
The Antikythera mechanism calculated the positions of planets based on a geocentric model, as if they orbited the Earth, rather than the sun. Each planet was assumed to revolve around a point in space, the epicenter, which in turn revolved around the Earth, an orbiting orbit.
This model has since been disproven, and we now know that the movements of the planets depend not on us, but something much bigger, an enormous sun that pulls us just as it pulls them. Despite this major gap in the Greeks’ understanding, however, the mechanism manages to predict the planet’s positions quite accurately. The heliocentric model, shown above, represents the planet’s position (P) as the sum of two rotating vectors—the planet’s position relative to the epicenter (EP) and the epicenter’s position relative to the planet (ET). When the sun is placed at the center instead of the earth, we find that these vectors have real meanings: the first is the planet’s average revolution around the sun, while the second is the negative of the Earth’s orbit. Even with all the wrong assumptions, the Greeks were able to find the correct answers.