The Antikythera Mechanism: An Ancient Greek Computer

In 1901 divers working a Roman-era shipwreck off the Greek island of Antikythera brought up statues, pottery and a shapeless lump of corroded bronze and wood. Nobody paid much attention to the lump until a year later, when someone noticed a gear wheel inside it.

More than a century of study has shown that it is a hand-powered machine for modelling the sky, able to predict the positions of the Sun and Moon and eclipses decades ahead. It is often called the oldest known analogue computer, and nothing of comparable complexity survives from the next fifteen hundred years.

A wreck full of treasure

Sponge divers led by Captain Dimitrios Kontos found the Antikythera wreck in early 1900, and recovery work over the following year raised a remarkable cargo of bronze and marble sculpture. The mechanism came up in 1901, probably in July, and on 17 May 1902 it was noticed that one of the fragments contained a gear.

The ship itself has been dated to roughly 70 to 60 BC. That gives a latest possible date for the device, but not its age: estimates for when it was built range from about 205 BC to about 87 BC, and researchers still argue over the date its calendar was set to start from.

Gears inside a lump of bronze

The mechanism was housed in a wooden case about 34 by 18 by 9 centimetres, a little larger than a shoebox lid. Over the centuries it broke into three main pieces, and those have since come apart into 82 fragments, four of which contain gears. The largest gear is about 13 centimetres across with 223 teeth.

For decades the inside could only be guessed at. The physicist and historian Derek de Solla Price studied it with X-rays and published a reconstruction in 1974, but the real breakthrough came in 2005, when a team used X-ray computed tomography to see through the corrosion. It found 35 meshing bronze gears, 30 of them still visible, and hundreds of hidden letters engraved on the surfaces.

What it calculated

Turned by a handle, the gears drove pointers on dials on the front and back. The front showed the Sun and Moon moving through the zodiac, and the Moon's phase. One clever train of gears even reproduced the Moon's uneven speed across the sky, faster at some points of its orbit than others.

The back carried dials for long astronomical cycles. One followed the Metonic cycle of 19 years, which keeps a lunar calendar in step with the seasons; another followed the Saros cycle of about 18 years, used to predict eclipses. A smaller dial tracked the four-year cycle of the great athletic games of Greece, including the Olympics. Inscriptions deciphered in 2016 include numbers tied to the cycles of Venus and Saturn, which suggests the device also showed the planets, although exactly how is still debated.

Reconstructing the whole machine

Because so much of the front is missing, every reconstruction involves some educated guessing. In March 2021 a team at University College London led by Tony Freeth published a new proposal for the entire mechanism, fitting the surviving gears and inscriptions into a design that would show the planets known to the ancient Greeks.

Whether every detail of that design matches the lost original cannot be known without more fragments. What the evidence does show is that the people who built it understood the cycles of the sky precisely and knew how to translate them into gear ratios, a level of mechanical skill nobody expected from the ancient world when the lump was first pulled from the sea.

Why it stands alone

Machines of comparable complexity do not appear again in the surviving record for around 1,500 years, until the astronomical clocks of fourteenth-century Europe. That gap is the strangest part of the story, and it has invited wild explanations, from lost civilisations to time travellers.

The more likely explanation is simpler and sadder. Bronze was valuable and was routinely melted down and reused, so a device like this would rarely survive unless it sank, as this one did. Ancient writers mention mechanical models of the heavens, which suggests the Antikythera mechanism was part of a tradition rather than a one-off miracle. As with the builders of the pyramids, the evidence points to skilled people, not mysteries.

Seeing it today

The fragments are kept at the National Archaeological Museum in Athens. Much of what we know about them comes from imaging rather than handling: X-ray tomography and surface imaging let researchers read inscriptions and count teeth on gears without touching the fragile bronze.

Those techniques are part of a wider change in how archaeology works, with new tools revealing details that earlier generations could not see; our look at AI in archaeology follows the same trend into the present.