Antikythera Mechanism: The Ancient Greek Astronomical Computer

The Antikythera Mechanism is a hand-cranked bronze device, made in the Greek world around 150-100 BC, that used dozens of interlocking gear wheels to model the sky. Recovered from a shipwreck off the Greek island of Antikythera in 1900-1901 and recognised as a geared instrument in 1902, it could show the positions of the Sun and Moon, the phase of the Moon, calendar cycles and coming eclipses. It is the oldest known geared machine, and nothing of comparable complexity survives from the following thousand years.

The shipwreck and the discovery

In the spring of 1900, Greek sponge divers led by Captain Dimitrios Kontos sheltered from a storm near Antikythera, a small island between Crete and the Peloponnese. Diving at a depth of around 45-50 metres, they found the remains of a large ancient ship covered in bronze and marble statues. With the backing of the Greek government, a salvage operation ran from late 1900 into 1901. It was one of the first major underwater archaeological recoveries ever undertaken, and it was dangerous: one diver died and others were injured by decompression sickness.

The ship carried a rich cargo of luxury goods, including bronze and marble statues, glassware, pottery and coins. Coins and pottery suggest it sank around 70-60 BC, probably on the way from the eastern Aegean towards Italy. The finds went to the National Archaeological Museum in Athens, where the most famous of the statues, the bronze Antikythera Youth, was reassembled.

Among the cargo were several shapeless, corroded lumps of bronze and wood. On 17 May 1902 the archaeologist Valerios Stais noticed that one of them contained a gear wheel. That small observation started more than a century of research. Divers returned to the wreck with Jacques Cousteau in 1976, and new underwater excavations since 2014 have recovered further objects, but no further pieces of the mechanism have been securely identified.

What the device looked like

The surviving remains consist of 82 fragments, the largest known as Fragment A. Together they preserve about 30 bronze gear wheels, the biggest around 13 centimetres across with 223 hand-cut triangular teeth. The original device was housed in a wooden case roughly the size of a large shoebox, about 34 by 18 by 9 centimetres, with dials on the front and back and bronze plates covered in Greek inscriptions.

A user turned a knob or crank on the side. Turning it moved a date pointer and, through the gear trains, all the other pointers in step. In effect, the user chose a date, past or future, and the device displayed what the heavens would look like or what astronomical events were due.

How scientists decoded it

Early studies were limited by the corrosion. The first sustained work was by the British historian of science Derek de Solla Price, who studied the fragments from the 1950s and in the early 1970s used gamma-ray and X-ray images made with the Greek radiographer Charalambos Karakalos. His book Gears from the Greeks (1974) proposed a first reconstruction of the gear trains and showed that the device was a sophisticated calculating machine, not a simple astrolabe.

Michael Wright, a curator at the Science Museum in London, built on this from the 1990s using linear X-ray tomography. He showed that the front of the device displayed the Moon’s changing speed and its phase, and he argued it had also shown the planets.

The decisive advance came with the Antikythera Mechanism Research Project, a collaboration of British, Greek and other researchers including Tony Freeth, Mike Edmunds, Yanis Bitsakis and Xenophon Moussas. In 2005 they used high-resolution X-ray computed tomography and a surface-imaging technique called reflectance transformation imaging. These revealed hidden gears and thousands of previously unread characters of inscription. Results published in Nature in 2006 and 2008 explained the back dials, including an eclipse predictor and a dial for the Panhellenic games. A further reconstruction by a team at University College London, published in 2021, proposed how the front could have shown all five planets known in antiquity.

What it could calculate

The back of the device carried two large spiral dials. The upper spiral tracked the 19-year Metonic cycle of 235 lunar months, which brings the lunar and solar calendars back into line and was used to organise festival calendars. A smaller subsidiary dial showed the four-year cycle of the Panhellenic games, including the Olympic Games. The lower spiral showed the 223-month Saros cycle used to predict eclipses, with glyphs marking which months would have a solar or lunar eclipse and at what hour.

The front showed a zodiac scale and a movable calendar ring, traditionally read as the 365-day Egyptian calendar, although some recent studies argue that it had 354 holes for a lunar year. Pointers marked the positions of the Sun and Moon, and a small rotating ball, half silver and half dark, displayed the Moon’s phase.

One of the most admired features is a pin-and-slot mechanism. Two gears mounted slightly off-centre made the Moon pointer speed up and slow down, reproducing the Moon’s variable motion across the sky. This follows a lunar theory associated with the astronomer Hipparchus, whose work is described in Hipparchus (c. 150 BC): Star Catalogue and Precession Found. The inscriptions also name the planets and describe their stationary points, which is why most researchers think the planets were displayed, although their gears have not survived.

Who made it, and where

No name of a maker survives. The calendar on the Metonic dial uses month names found in Corinthian colonies, which points to north-western Greece or Syracuse in Sicily. Syracuse was the home of Archimedes of Syracuse, and the Roman writer Cicero describes a bronze sphere showing the motions of the Sun, Moon and planets that Archimedes had built and that was taken to Rome after the city fell in 212 BC. Cicero also mentions a similar device made by the philosopher Posidonius on Rhodes.

These references show that geared or mechanical models of the heavens were known in the Greek world, but none of them can be linked directly to the Antikythera device. Rhodes, an important centre of astronomy where Hipparchus worked, is another proposed origin. The date of manufacture is also debated: most estimates fall between about 150 and 100 BC, while one study of the eclipse dial argued that its cycle began in 205 BC.

Myths and facts

Myth: it was a modern object or an “out-of-place artefact”. The device was found in a sealed ancient wreck with other 1st-century BC objects, its inscriptions are in ancient Greek, and its astronomy matches known Hellenistic theory. There is nothing mysterious about its origin, only about the details of its making.

Myth: it was a navigation instrument. Although found on a ship, the mechanism does not help locate a ship’s position. It was an astronomical and calendrical display, probably a costly showpiece or teaching device.

Myth: it was a digital computer. It is called a computer because it calculated, but it was analogue: it represented astronomical cycles through the ratios of gear teeth, not through numbers stored as digits.

Fact: it was probably not unique in its own time. Its refinement suggests a tradition of instrument-making, now otherwise lost because bronze was usually melted down and reused.

Timeline

• c. 205-100 BC: The mechanism is made; most estimates favour c. 150-100 BC.
• c. 70-60 BC: The ship carrying it sinks off Antikythera.
• 1900, spring: Sponge divers under Captain Dimitrios Kontos discover the wreck.
• 1900-1901: Greek salvage operation recovers statues and other objects.
• 1902, 17 May: Valerios Stais notices a gear wheel in one of the corroded fragments.
• 1974: Derek de Solla Price publishes Gears from the Greeks.
• 2005: The Antikythera Mechanism Research Project carries out X-ray CT scans.
• 2006: Freeth and colleagues publish their decoding of the device in Nature.
• 2021: A UCL team publishes a reconstruction of the planetary display.

By the numbers

Legacy

The Antikythera Mechanism changed how historians see ancient technology. Before it was understood, it was widely assumed that the Greeks had theoretical science but little practical machinery. The device shows that Hellenistic craftsmen could translate mathematical astronomy into precise gearing, a skill not seen again in surviving objects until the astronomical clocks of medieval Europe and the Islamic world.

It also illustrates the scientific culture of the Hellenistic world, centred on places such as the Library of Alexandria and Rhodes, where Babylonian eclipse records and Greek geometry were combined. The mechanism is a museum icon in Athens, has inspired many working replicas, and remains an active research subject.

Why it matters

The Antikythera Mechanism is the earliest surviving evidence of complex gearing and of a machine built to calculate. It shows how advanced Greek astronomy was by the 2nd century BC and reminds us that much ancient technology has simply been lost. Every new scan has added detail, but the basic conclusion is secure: this was a portable model of the cosmos, made more than 2,000 years ago.