The Antikythera mechanism: a Greek calculator of lunar cycles
Gear wheels, spiral dials for Meton and Saros and the irregular Moon of Hipparchus: how the Antikythera mechanism was built and what it calculated.
In 1900 a group of sponge divers, sheltering from a storm off the Greek island of Antikythera, discovered on the seabed the wreck of a ship laden with statues and precious objects. Among the finds recovered in the following months were some corroded fragments of bronze, which only later turned out to be the remains of the most complex scientific instrument to have reached us from antiquity.
A wreck and a riddle
The shipwreck dates from the first century BC, and the mechanism has been dated, with varying estimates, between the second and the first century BC. The fragments are kept in the National Archaeological Museum in Athens. In 1902 the archaeologist Valerios Stais noticed a gear wheel embedded in one of the pieces, but for decades the object remained a riddle. It was the historian of science Derek de Solla Price, between the 1950s and the 1970s, who proposed the first systematic reconstruction of it as an astronomical calculator.
Since 2005 an international research team has studied the fragments with X-ray tomography and advanced photographic techniques, revealing hidden gears and thousands of characters engraved in Greek. These inscriptions, a kind of user's manual, have made it possible to understand precisely the function of many dials and to confirm that the device was devoted above all to the cycles of the Sun and the Moon.
The faces of the mechanism
The device was enclosed in a wooden case and operated with a crank. On the front face a dial showed the zodiac and the Egyptian calendar of 365 days; pointers indicated the positions of the Sun and the Moon, and a small sphere, half light and half dark, showed the lunar phase as it turned. According to the reconstructions, the dial also indicated the positions of the planets, but this part has survived only in fragments.
On the back face were two large spiral dials. The upper one reproduced the Metonic cycle of 235 months, the lower one the Saros cycle of 223 months, with an indication of the months in which eclipses were expected. Smaller dials showed the Callippic cycle, the exeligmos and the four-year cycle of the great Panhellenic games, among them those of Olympia.
The irregular Moon
The most ingenious detail concerns the Moon's variable speed. Two gear wheels mounted on slightly offset axes, connected by a pin sliding in a slot, made the lunar pointer advance faster at some times and more slowly at others, reproducing the anomaly described by Hipparchus' theory. The whole assembly in turn rotated slowly, simulating the shift of the axis of the lunar orbit in about nine years.
Who built it
The maker remains unknown. Cicero, in De re publica, recalls a sphere built by Archimedes that reproduced the motions of the Sun, the Moon and the planets, and in De natura deorum he mentions a similar instrument made by Posidonius at Rhodes. Some scholars have suggested a link with these traditions or with the schools of Rhodes and Syracuse, but no attribution has been proved, and the mechanism remains an anonymous work.
The Antikythera mechanism shows that the Greeks not only knew the lunar cycles discovered by the Babylonians but could translate them into bronze gearing. Instruments of comparable complexity reappear in Europe only with the astronomical clocks of the late Middle Ages, testifying to a technical knowledge that for centuries was largely lost.