Clockwork moon

Jo Marchant brings us up to date on the reconstruction of the Antikythera Mechanism, a second- or first-century BCE gearwork model of the heavens salvaged from a shipwreck more than 1,900 years later. Michael Wright has used computer-assisted tomography on the badly-corroded assembly of bronze to reveal a pin-and-slot model of a nine-year cycle in the Moon’s movements:

One of the wheels connected to the main drive wheel moves around once every nine years. Fixed on to it is a pair of small wheels, one of which sits almost—but not exactly—on top of the other. The bottom wheel has a pin sticking up from it, which engages with a slot in the wheel above. As the bottom wheel turns, this pin pushes the top wheel round. But because the two wheels aren’t centred in the same place, the pin moves back and forth within the upper slot. As a result, the movement of the upper wheel speeds up and slows down, depending on whether the pin is a little farther in towards the centre or a little farther out towards the tips of the teeth….

The researchers realized that the ratios of the gear-wheels involved produce a motion that closely mimics the varying motion of the Moon around Earth, as described by Hipparchus. When the Moon is close to us it seems to move faster. And the closest part of the Moon’s orbit itself makes a full rotation around the Earth about every nine years. Hipparchus was the first to describe this motion mathematically, working on the idea that the Moon’s orbit, although circular, was centred on a point offset from the centre of Earth that described a nine-year circle. In the Antikythera Mechanism, this theory is beautifully translated into mechanical form. “It’s an unbelievably sophisticated idea,” says Tony Freeth, a mathematician who worked out most of the mechanics for Edmunds’ team. “I don’t know how they thought of it.”

Follow links in Marchant’s piece to more technical material, nifty illustrations of the reconstructed device, and Freeth et al.’s paper.