The Antikythera Mechanism: An Ancient Greek Computer in a Shipwreck

Picture a wooden case full of bronze gears, with dials on the front and back, that could show where the Sun, Moon and planets were in the sky and warn you about upcoming eclipses. Now picture it built more than two thousand years ago. That's the Antikythera Mechanism, and the story of how we figured out what it does is almost as good as the object itself.

A shipwreck full of treasure

It starts with a wreck off the small Greek island of Antikythera, south of the Peloponnese. According to the National Archaeological Museum in Athens, the ship was a freighter with an estimated capacity of 300 tons. It sank sometime between 75 and 50 BC, most likely on its way to Italy. (A museum exhibition page gives roughly 60 to 50 BC for the wreck, so treat the exact decade as fuzzy.)

The ship was recovered in two phases. The first, in 1900 and 1901, was supervised by the Greek Archaeological Service with sponge divers from the island of Syme and help from the Greek Royal Navy. The museum calls it the first major underwater archaeological expedition and the beginning of underwater archaeology in Greece. A second round came in 1976, with support from Jacques-Yves Cousteau's research ship Calypso, and the museum notes that underwater research at the site continues.

The cargo was spectacular on its own: the bronze statues known as the Youth of Antikythera and the Philosopher, marble statues of gods and heroes, bronze beds, gold jewelry, glassware, transport amphorae and lamps. And among all that, a corroded lump of bronze and wood that turned out to be the most surprising thing on board.

What it actually is

Here's what survives, in plain terms:

• The museum describes the Mechanism as gear wheels, axles, dials and pointers made of bronze, housed in a wooden case with a metal plate on the front and back.
• It's now in 82 fragments, and only about a third of the original survives, including 30 corroded bronze gearwheels, according to a 2021 study in Scientific Reports.
• The biggest piece, Fragment A, holds 27 of those 30 gears.

The museum dates the Mechanism to the 2nd century BC. The team behind a landmark 2006 study in Nature put its construction around the end of that century. Either way, it was made before the ship went down.

The breakthrough: looking inside

The big leap came in 2005, when the fragments were scanned with microfocus X-ray computed tomography (think a very fine CT scan) plus a surface-imaging technique called Polynomial Texture Mapping. It revealed fine mechanical details and thousands of tiny text characters buried inside the fragments, unread for more than 2,000 years.

The 2006 Nature paper, by a joint British and Greek team, used that data to work out how the gears functioned and doubled the number of deciphered inscriptions. Among their findings:

• The Mechanism predicted lunar and solar eclipses using Babylonian arithmetic-progression cycles.
• Its gearing included a mechanical version of the theory Hipparchos developed in the 2nd century BC to explain the Moon's irregular speed across the sky.
• The authors described it as technically more complex than any known device for at least a millennium afterwards.

The back: calendars, eclipses and the Olympics

A follow-up paper in Nature in 2008 dug into the two big dials on the back:

• The upper back dial is a 19-year calendar based on the Metonic cycle, laid out as a five-turn spiral.
• The lower back dial is a Saros eclipse-prediction dial, a four-turn spiral of 223 lunar months, with little symbols marking predicted eclipses.

Then came the fun surprises. The team identified all 12 month names on the calendar, and they turned out to be Corinthian. That points to a Corinthian connection, and the authors named the Corinthian colonies of northwestern Greece or Syracuse in Sicily as leading contenders. Syracuse, the authors note, would suggest a heritage going back to Archimedes.

And one small dial, previously thought to track a 76-year cycle, actually follows the four-year cycle of the Olympiad and its associated Panhellenic Games. Yes: this ancient astronomical computer had a sports calendar.

The front: a whole cosmos

The front was the harder puzzle, because so much of it is missing. Inscriptions revealed by the scans describe the motions of the Sun, the Moon and all five planets known in antiquity. One inscription on the back cover works a bit like a user manual, describing a display where the planets were marked by "little spheres" and the Sun by a "little golden sphere."

In 2021, a team at University College London published a new model of this front display in Scientific Reports. A few highlights:

• Numbers found in a front-cover inscription, 462 for Venus and 442 for Saturn, point to very accurate cycles for those planets.
• Those cycles break down into small prime factors, which matters because the gears had to fit in tight spaces. The team worked on the basis that the gears needed fewer than 100 teeth.
• Their model shows the cosmos as nested rings with nine outputs: Moon, the lunar nodes, Mercury, Venus, Sun, Mars, Jupiter, Saturn and the date.

The authors are refreshingly upfront about the limits. They say they cannot claim their model is a replica of the original, only that it matches the surviving evidence. Some features, like a pointer for the Moon's nodes, are labeled as hypothetical, with no direct physical evidence.

How you might have used it

Imagine you're the owner. Turn the mechanism, and the dates advance; the pointers and rings move to show the Sun, Moon and planets across the zodiac. Glance at the back to check the calendar month or whether an eclipse is due, and maybe which games are coming up this year. That's a simplification of a very complicated object, but it captures the spirit: a model of the sky you could hold.

The museum lists its likely uses as an instrument of scientific research, a way to predict eclipses and a calendar, and probably an aid for astrology and navigation. It also links the technology to the successors of Archimedes and the school of Poseidonios on Rhodes.

What we still don't know

Plenty, which is part of the appeal:

• Who built it, and who owned it.
• Exactly what the lost front looked like.
• Whether it was one of a kind or one survivor of a tradition of devices that didn't make it to us.

New readings keep changing the picture, from the Venus and Saturn numbers spotted in 2016 to the 2021 model, so expect the story to keep evolving.

Where to see it

The Mechanism is displayed at the National Archaeological Museum in Athens, among the Antikythera shipwreck finds in Room 38 of the metalwork collection, under inventory number X 15087. The museum's own description calls it the most significant scientific instrument of antiquity.