The Antikythera Mechanism: How Bronze Gears Tracked the Sky

A tooth cut into a bronze wheel was one of the clues that a corroded object recovered off Antikythera had once moved. The pieces came from an ancient shipwreck near the Greek island. Among the statues and other cargo brought ashore was evidence that someone had built a portable model of the sky more than two thousand years ago.

Table of Contents

What survived the wreck

The National Archaeological Museum in Athens holds the fragments. The Greek cultural authorities date the device to roughly 150–100 BCE and record its recovery in 1901. Their inventory counts 82 pieces containing 30 surviving toothed wheels. That last number describes the surviving material. A reconstruction of the complete machine may require gears for which no physical wheel remains.

Originally the bronze works fitted inside a wooden frame. Inscriptions on the plates gave guidance about the displays. With much of the mechanism missing, an isolated gear offers only a ratio between movements. An inscription can supply the name of the cycle that ratio was meant to represent. Reconstructing the device has depended on reading both kinds of evidence together.

Looking through the bronze

In 2006 an international research team published results from high-resolution surface imaging and X-ray tomography. The scans let researchers follow mechanical features hidden inside the fragments and examine lettering that had been difficult to read. Those results helped explain displays for the Moon’s movement and recurring eclipse cycles.

The distinction between the cycles matters. A lunisolar calendar reconciles lunar months with solar years. An eclipse cycle helps identify occasions when an eclipse may recur. The operator could follow different spans of astronomical time through a linked set of dials. That ability to represent numerical relationships through the physical motion of gears explains the description “analog computer.”

Timeline

Second century BCE — The Greek museum catalog places manufacture roughly between 150 and 100 BCE.
1901 — The wreck off Antikythera yields the remains now held in Athens.
2006 — X-ray tomography and surface imaging reveal previously obscured inscriptions and mechanical features.
2021 — UCL researchers publish a model proposing how the front could have represented the known planets.

The missing face

The front of the device presents a harder problem. Inscriptions associate it with the motions of the five planets known to ancient Greek observers. Much of the gearing that might have produced a planetary display is missing. Researchers can propose wheels and tooth counts that fit the space and the surviving descriptions, then test whether the arrangement works.

In 2021 a UCL-led group published such a reconstruction. Its model shows how the planetary display could have fitted together. A successful model narrows the engineering problem while leaving room for a future find to change the design. The evidence establishes more about the intended astronomical display than about every wheel the maker used to drive it.

Who built the Antikythera mechanism?

The fragments preserve the work of a Greek-speaking maker with access to sophisticated astronomy and metalworking. They have yielded no secure identification of an individual inventor. Its surviving instructions give us something more intimate than a name in one respect. Someone expected another person to operate the device and follow the sky with it.

FAQ

What did the Antikythera mechanism do?

Its indicators related the motions of the Sun and Moon to calendar and eclipse cycles. One display also followed the cycle of Greek athletic festivals, connecting the instrument to civic life.

Why call it a computer?

Geared motion performed calculations. Turning an input caused indicators to change according to ratios chosen by the maker.

Can we reconstruct the whole machine?

Models can reproduce many functions from surviving wheels and inscriptions. The original arrangement of the missing planetary gearing is still under investigation.

Research Sources

The following resources were consulted to verify and update this article.

Greek Ministry of Culture, The Antikythera Mechanism — Museum inventory, dating, recovered gears and details of housing and inscriptions.
Freeth and colleagues, Nature (2006) — Original imaging study of gear trains, lunar motion and eclipse prediction.
University College London, reconstruction of the planetary display — Authors’ account of the 2021 front-display model and the limits imposed by missing parts.
American Society of Mechanical Engineers, Antikythera Mechanism — Engineering account of the gears, calendar, eclipse functions and dates of the Panhellenic Games.

Related stories

Explore this topic