**The ‘Antikythera Mechanism’ can now be admired in the National Archaeological Museum in Athens.** Wikimedia

An ancient Greek computer

After decades of research, a seemingly innocuous find retrieved from a Roman shipwreck was revealed as a sensational scientific discovery, proving that the ancient Greeks were capable of making mechanical models of the cosmos with hitherto unimagined precision. The ‘Antikythera Mechanism’ is an analogue ‘calendar computer’.

Thomas Weibel

Thomas Weibel is a journalist and Professor of Media Engineering at the Fachhochschule Graubünden and the Hochschule der Künste in Berne.

In May 1902 at the National Museum of Archaeology in Athens, Greek Minister for Education Spyridon Stais picked up a previously overlooked lump of badly corroded bronze out of sheer curiosity. Then, an accident happened: a piece of the lump broke off – revealing an amazingly well preserved gear wheel equipped with 1.5 millimetre-long teeth, akin to those of a modern day timepiece. It turns out that the fragment had been salvaged from the wreck of a Roman ship, which had sunk off the coast of Antikythera, an island off the southern tip of the Greek mainland, circa 70 BCE. The wreck, discovered by sponge divers around 1900, had been carrying spoils of war from the eastern Aegean and Asia Minor: marble and bronze statuary, pottery and glassware, jewellery and coins that were found to have been minted in the period between 70 and 62 BCE.

**Recovering artifacts from the Roman wreck in the winter of 1900/1901.** Wikimedia

Scientists were naturally shocked to discover a high-precision gear wheel dating back to antiquity. So, what on earth was it? It was not until the 1950s that British physicist and historian of science Derek de Solla Price came up with the answer. “Price realised that the original device had been flat and rectangular, must have been about the size of a modern mantel clock and had a knob or crank handle on the side for winding,” says London-based mathematician Tony Freeth. He continues: “A complex gearing system housed inside the outer casing moved a number of pointers around circular dials on both the front and back of the device. This enabled the exact positions of the sun and moon to be displayed. Precisely, for any given day and, in some cases, time of day.” The names of celestial bodies are engraved on the Mechanism, leading many researchers to speculate that it was designed to show the position of the five planets known in ancient times – Mercury, Venus, Mars, Jupiter and Saturn – even though no parts of the related gears have survived.

**Reconstructions of the ancient Antikythera Mechanism have been attempted several times to date, including in Switzerland.** Wikimedia

We know from the literature of the time that the ancient Greeks constructed clockwork mechanisms of this kind. In the 1st century BCE Poseidonius, one of the most learned men of the age, settled as a teacher on the island of Rhodes. It was there that lawyer and later Roman consul Cicero saw a device, “cuius singulae conversiones idem efficiunt in sole et in luna et in quinque stellis errantibus quod efficitur in caelo singulis diebus et noctibus” (“which at each revolution reproduces the same motions of the sun, the moon and the five planets that take place in the heavens every day and night”), as he wrote in his work “De natura deorum” in 45 BCE. No one ever imagined that this description was meant to be taken literally. If you want to understand how a mechanical gear system works, the first thing you should do is examine the gear wheels: their position, size (diameter) and, above all, number of teeth. But even following the first x-ray studies of the Mechanism, this proved no easy task. The images weren’t particularly sharp and the teeth themselves had only survived in fragmentary form. For example, in one particular gear wheel radiologists believed they could count 128 teeth. 128 is a power of 2 and has no significance in astronomy. Price, on the other hand, suggested that the actual number should be 127 teeth. “127 is a primary number,” explains Freeth. “It refers to the orbit of the moon. If you observe the moon night after night, you will see that it moves across the sky and passes through all the signs of the zodiac once every 27.3 days. As far back as the 5th century BCE, the ancient Babylonians already knew that the moon passes through the zodiac 254 times in a period of almost exactly 19 years.” 254 is two times 127 – and so Price had found this precise Babylonian lunar calendar built into the ancient mechanism with its 30 or so gear wheels.

**Cicero discovered a device on Rhodes that had many surprising parallels with the Antikythera Mechanism.** Wikimedia

After further time-consuming examinations of all 82 surviving fragments, including the use of high-resolution computed tomography to scan through the metal, we now know that the Antikythera Mechanism was a complex mechanical calendar fashioned from bronze – a gear-driven analogue computer whose plates were inscribed with scales and texts describing the individual functions. One face represented a sun calendar, complete with date display. One of the pointers contained a rotating black and silver ball that indicated the phase of the moon. A fixed ring dial displayed the 12 signs of the zodiac and another ring marked off the 365 days of the year in accordance with the Egyptian calendar, i.e. 12 months of 30 days plus 5 additional days. This dial was designed to be moveable to accommodate the extra ‘leap day’ or ‘intercalary day’ required to be added every four years. There were two more displays on the other side of the Mechanism: the top dial in the form of a spiral showed a lunar calendar that tracked the 19-year Metonic cycle, named after Greek astronomer Meton of Athens. Below it was another large spiral dial used to predict solar and lunar eclipses. And finally, there was a smaller subsidiary dial inside the Metonic dial showing the four-year cycle of ancient Olympic Games, indicating when – and where – the Panhellenic games were to be held.

Reconstruction of the Antikythera Mechanism. Ludwig Oechslin ochs and junior

Ludwig Oechslin, a historian of technology and science and also a master watchmaker, was Director of the International Museum of Horology in La-Chaux-de-Fonds from 2001 to 2014. In 2006, a new analysis of the Antikythera Mechanism in “Nature” magazine grabbed Oechslin’s attention, and he decided to construct his very own replica of the device. He manufactured the gear wheels using a computer controlled (CNC) milling machine to create his own model of the ancient mechanism. “We know from the literature that gear trains of this kind existed in antiquity,” says Oechslin, “but none had ever been found. Based on the latest findings, we can say without a shadow of a doubt that the Antikythera Mechanism is an analogue calendar computer and thus a model of the cosmos of enormous precision.”

TV documentary about the Antikythera Mechanism. YouTube / BBC

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