The Antikythera Mechanism

Summary

Greek sponge divers pulled a corroded lump of bronze from a shipwreck in 1901. It sat in a museum for decades, split apart, and was largely ignored. It is a geared analogue computer, built more than two thousand years ago, which models the motions of the Sun, the Moon and the planets and predicts eclipses.

Nothing else like it survives from antiquity. Nothing of comparable mechanical sophistication appears again in the record for well over a thousand years.

What is documented

The object. It now exists as eighty-two fragments. Only about a third of the original machine survives. Thirty corroded bronze gearwheels remain, twenty-seven of them in a single fragment.

How it was read. The fragments are opaque, deformed and encrusted. What decoded them was microfocus X-ray computed tomography, applied in 2005, which imaged the gearing inside the corrosion and, unexpectedly, revealed thousands of tiny text characters buried within the fragments and unread for two thousand years. The machine came with instructions, and the instructions survived inside it.

The Saros dial. A spiral on the back plate, running four full turns, divided into 223 cells. Two hundred and twenty-three is not a decorative number. It is the Saros cycle: the interval, known to Babylonian astronomers, after which the pattern of solar and lunar eclipses repeats. A 223-tooth gear at the rear of the machine drives a pointer around that spiral.

Some of the cells carry inscriptions describing the character of the eclipse expected in that month. Twenty-two such eclipse records survive.

The Metonic dial. A second spiral, encoding the relation that 235 lunar months is very nearly exactly nineteen solar years. This is the reconciliation between the lunar calendar people lived by and the solar year that governs the seasons, and the machine performs it mechanically, every time the handle is turned.

The other dials. A subsidiary dial tracked the four-year Olympiad cycle and the schedule of the Panhellenic games. The Callippic cycle, a longer calendrical period, was also represented.

The calendar ring. In 2024, researchers at the University of Glasgow applied statistical methods developed for gravitational-wave detection to the pattern of holes surviving in the calendar ring, and concluded that it almost certainly carried 354 holes: a lunar year, not the 365-hole Egyptian solar calendar that some earlier reconstructions had assumed.

The front, and the 2021 model. The rear of the machine was substantially resolved by 2006. The front was not. In March 2021, the UCL Antikythera Research Team, led by Tony Freeth, published a reconstruction in Scientific Reports of the front display: a model of the cosmos, showing the Sun, the Moon and the five planets known to antiquity, driven by epicyclic gearing. Freeth stated that theirs is the first model that conforms to all the physical evidence and also matches the descriptions inscribed on the mechanism itself.

What it is not. It is not a navigational instrument, a clock, or an orrery in the modern decorative sense. It is a calculator. You set a date and it tells you the state of the sky, including whether an eclipse is due.

Leading explanations

The nature of the object is not in dispute. What remains open is narrower and more interesting.

Who made it. Unknown, and possibly unknowable. Freeth has noted that the indirect and subtle way the machine computes the Moon's variable motion, using pin-and-slot epicyclic gearing, is an extraordinary conception, and has suggested it points toward Archimedes or his tradition. This is an argument from the quality of the design, not from a signature, and the registry records it as such.

The front display, in detail. The UCL epicyclic reconstruction is the most complete proposal advanced and it fits the evidence and the inscriptions. It is not the only reconstruction, and the physical evidence needed to decide definitively between competing schemes may simply not have survived. This is an honest limit, not a mystery.

Why nothing like it survives. Two possibilities, and they are not exclusive. Bronze was valuable and was recycled, so the survival of any such object at all required something like a shipwreck. And a tradition capable of producing this cannot plausibly have produced only one, which implies that others existed and were melted down.

Current status

Later explained. For most of the twentieth century this was a genuinely anomalous object: something no one could account for, from a period no one believed capable of it. It was resolved not by a theory but by an instrument, when X-ray tomography let researchers read a machine that was too corroded to open and too fragile to take apart.

The registry keeps it as the best example in the archive of an anomaly that dissolved into something more impressive than the mystery it replaced.

Last reviewed: July 2026. Records are provisional. Where the evidence changes, the entry changes. Found an error? Tell us.