The Antikythera Mechanism: A Computer from Antiquity

In the spring of 1900, two boats of sponge divers from the Greek island of Symi ran into a storm between Crete and Kythera and sheltered at the tiny island of Antikythera. When the weather cleared, one of the divers went down near the shore and found the wreck of an ancient cargo ship, scattered with bronze and marble statues, glassware and gold jewelry. It had sunk sometime between about 75 and 50 BC. Over the next year, in the first underwater archaeological salvage of an ancient shipwreck, the divers raised its cargo under the supervision of Greek government archaeologists. Among the finds, probably brought up in 1901, were some small, corroded scraps of bronze that nobody examined closely. They sat in the National Archaeological Museum in Athens for nearly a year. Then, in 1902, Spyridon Stais, the former education minister who had overseen the salvage, noticed toothed gear wheels and inscribed Greek letters on two of them. Those fragments are now known as the Antikythera Mechanism. When Nature published a new reconstruction of it in 2006, the journal described it as the world's oldest known analog computer.

Why does the Antikythera Mechanism matter?

Gears were not unknown in the ancient world, but nothing else from antiquity comes close to this. The mechanism is technically more complex than any known device from at least the following thousand years. Simpler geared calendars appear in Islamic instruments from around AD 1000, but nothing as intricate is known until the astronomical clocks of 14th-century Europe. Derek de Solla Price, the historian of science whose study of the fragments in the late 1950s made the first real breakthroughs, told reporters how far ahead of its age it seemed. It was, he said, like finding a jet plane in the tomb of King Tut, or "like opening a pyramid and finding an atomic bomb."

How did it work?

At its core, the mechanism was an astronomical calculator. It was box-shaped, with bronze dial faces about 32 centimeters tall and 17 centimeters wide on the front and back, and probably wooden sides. A knob or crank on one side turned a train of bronze gears inside. Thirty gears survive, 27 of them in the largest fragment. By turning the drive to a chosen date, the user could read off:

• The positions of the Sun and the Moon against the zodiac.
• The phase of the Moon.
• When solar and lunar eclipses were due, from a spiral dial based on the Saros, a Babylonian eclipse cycle of 223 lunar months.
• A 19-year calendar, and the four-year cycle of the Olympiad and the other Panhellenic games.

Its most remarkable trick concerns the Moon, which moves faster across the sky when it is closer to Earth. That irregularity comes from the Moon's elliptical orbit, and in the second century BC the astronomer Hipparchos developed a theory to account for it. The mechanism builds that theory into bronze: two gears linked by a pin running in a slot speed the Moon's pointer up and slow it down over each month.

Where did it come from?

Its origin is still debated. When researchers read all 12 month names on its calendar dial in 2008, they turned out to be Corinthian, which points to one of Corinth's colonies in northwestern Greece or to Syracuse in Sicily, the home of Archimedes. The National Archaeological Museum in Athens links its technology to the successors of Archimedes and to the school of the philosopher Posidonius on Rhodes. It was made toward the end of the second century BC, decades before the ship that carried it went down. Why nothing like it survives is unknown. Its refinement suggests it was not the only one ever made, but the record is thin, and this one lay on the seabed for two thousand years before anyone saw it. Today, surface imaging and X-ray tomography let researchers read inscriptions hidden inside the corroded fragments: the 2006 study alone doubled the number of deciphered inscriptions, and each new study adds detail to the reconstruction. The Antikythera Mechanism does not prove that the ancient Greeks had lost technologies we still lack. What it shows is more specific, and still astonishing: more than two thousand years ago, Greek instrument-makers could turn the best astronomical theories of their time into a working machine of bronze gears.