The Antikythera Mechanism: How the Ancient Greek Computer Worked

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This Mysterious 2,000-Year-Old Mechanism Is the World’s First Computer

Image credit: DailyTelegraph

The Antikythera Mechanism is a hand-cranked bronze gearwork device made by Greek craftsmen roughly 2,100 years ago. Turning its handle moved pointers that showed the positions of the Sun and Moon, the phase of the Moon, the dates of eclipses and the timing of the great athletic games. It was found in 1901 in a Roman-era shipwreck off the Greek island of Antikythera and is often called the world's first analogue computer, because nothing of comparable mechanical complexity is known again for more than a thousand years.

It survives today as 82 corroded fragments in the National Archaeological Museum in Athens. Over a century of study, and especially the imaging work done since 2005, has turned this lump of green bronze into one of the best-understood scientific instruments of the ancient world. Here is what it is, how it worked and what is still being debated.

How a shipwreck gave up a computer

In 1900 a crew of sponge divers from the island of Symi, led by Captain Dimitrios Kontos, sheltered from a storm near Antikythera, between Crete and the Peloponnese. Diving at a depth of about 45 metres, they found the remains of a large ship and its cargo. With help from the Greek navy, the wreck was explored during 1900 and 1901 in one of the first major underwater archaeology projects.

The ship had sunk around 70 to 60 BC. It carried bronze and marble statues, glassware, pottery, coins and jewellery, probably luxury goods on their way to buyers in the Roman world. Among them was a wooden box of corroded bronze that attracted little attention at first. In 1902 archaeologist Valerios Stais noticed a gear wheel embedded in one of the pieces, and the mystery began.

What the mechanism could do

The device was roughly the size of a shoebox, with dials on the front and back. Inside, at least 30 interlocking bronze gears survive, and researchers believe there were originally more. The largest wheel is about 13 centimetres across and had 223 teeth. That number is no accident: 223 lunar months make up the Saros cycle, which ancient astronomers used to predict eclipses.

The front: a model of the sky

The front dial showed the zodiac and a 365-day Egyptian calendar. Pointers marked the positions of the Sun and Moon, and a small rotating ball, half silver and half black, displayed the Moon's phase. Many researchers think the front also showed the five planets visible to the naked eye: Mercury, Venus, Mars, Jupiter and Saturn.

The back: calendars and eclipses

  • Metonic dial: a spiral covering 235 months, or about 19 years, after which the phases of the Moon fall on the same calendar dates again.
  • Saros dial: a spiral of 223 months with symbols marking predicted solar and lunar eclipses, including the expected time of day.
  • Games dial: a small four-year dial showing which of the Panhellenic games, including the Olympics, would be held in a given year.

The cleverest trick

The Moon does not move across the sky at a constant speed; it speeds up and slows down as its distance from Earth changes. The mechanism's makers reproduced this using two slightly offset gears linked by a pin sliding in a slot. The result is a pointer that varies its speed just as the Moon appears to. It is a strikingly elegant piece of engineering for the 2nd century BC.

A century of detective work

The German philologist Albert Rehm was among the first to argue that the device was an astronomical calculator. From 1951 the British physicist and historian of science Derek de Solla Price studied it for two decades. Working with Greek radiologist Charalampos Karakalos, he took X-ray and gamma-ray images of the fragments and published his landmark study, Gears from the Greeks, in 1974.

Michael Wright, a former curator at London's Science Museum, built the first working model that included planetary pointers in 2002. But the real breakthrough came in 2005 with the launch of the Antikythera Mechanism Research Project. Its core team included astronomer Mike Edmunds and mathematician Tony Freeth of Cardiff University, John Seiradakis of the Aristotle University of Thessaloniki, Xenophon Moussas and Yanis Bitsakis of the University of Athens, and palaeographer Agamemnon Tselikas of the National Bank of Greece Cultural Foundation, working with the National Archaeological Museum under the Hellenic Ministry of Culture.

Two technology firms provided the tools. Hewlett-Packard brought a dome-shaped imaging rig that revealed faint inscriptions on the surfaces, while X-Tek Systems built a powerful X-ray CT scanner that showed the hidden gears and thousands of tiny letters inside the fragments. The project's findings appeared in the journal Nature in 2006 and 2008, and a full edition of the inscriptions followed in 2016.

Who made it, and when?

Nobody knows the maker's name. Popular stories link the mechanism to Archimedes or to the astronomer Hipparchus of Rhodes, but there is no direct evidence for either. The calendar month names on the back dial, decoded in 2008, are of Corinthian type, which led the team to suggest an origin in a Corinthian colony in the western Greek world, possibly Syracuse. Later work noted that the names also match calendars used in Epirus in north-western Greece, so the question remains open.

Dating is also debated. Estimates for when it was built range from about 205 BC to around 87 BC, and some researchers argue that its dials were set to start from a date in the late 3rd or early 2nd century BC. Either way, it is older than the shipwreck that preserved it.

The 2008 study also renamed the small four-year dial. It had been called the Olympiad dial, but because it tracks a cycle of several games, not only the Olympics, it is now usually described as the Games dial.

New discoveries since 2014

In 2014 a new programme of dives began at the wreck under the title "Return to Antikythera", using modern diving technology to recover more of the ship's cargo. The underwater survey continued in several seasons, with a five-year campaign running until 2019 and new work starting in 2020.

In March 2021 a team at University College London led by Tony Freeth published a new reconstruction of the front display in Scientific Reports. Using inscriptions that list cycles of 462 years for Venus and 442 years for Saturn, they designed a compact gear system that could show the Sun, Moon and all five planets. The authors described it as the first model consistent with all the surviving physical evidence, while noting that they still needed to show it could be made with ancient techniques.

(The video originally embedded here is no longer available on YouTube.)

Was it really a computer?

Calling the mechanism a computer is fair if we use the word carefully. It was an analogue computing device: it turned the motion of a handle into calculated astronomical positions through fixed gear ratios. It could not be programmed, stored no data and did not use numbers the way a digital computer does.

A common myth is that the mechanism proves some lost super-technology. It does not. Rather, it shows that Hellenistic Greek astronomers had collected centuries of observations, many built on Babylonian records, and that their craftsmen could translate that knowledge into precise bronze gears. Similar geared devices are mentioned by Cicero and other ancient writers, so this was probably not the only one ever made, just the only one to survive.

Seen that way, the mechanism is less a mystery than a window into a scientific culture whose achievements were mostly lost. It is one of the treasures of the National Archaeological Museum in Athens, where visitors can see the original fragments alongside modern reconstructions.

Sources: Wikipedia: Antikythera mechanism; Antikythera Mechanism Research Project (official site); Freeth et al., Nature (2008): calendars with Olympiad display and eclipse prediction; UCL News (March 2021): Experts recreate a mechanical Cosmos for the world's first computer; Learning-Mind (earlier version of this story).

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