The Antikythera Mechanism

14 mins read

 An ancient Greek machine rewriting the history of technology

A seemingly unassuming lump of corroded bronze has confounded investigators for more than a century, ever since it proved to contain precision gearwheels that simply should not have existed in the ancient Greek world. A new study, using cutting-edge techniques, has now revealed what this machine could do, and how it did it, as Tony Freeth explains.

A digital reconstruction of the Antikythera Mechanism, a gold rectangular object with dials and gears.
The discovery of the Antikythera Mechanism revealed that the ancient Greeks had achieved a level of technological sophistication previously undreamed of. But while the remains of the machine clearly demonstrated its ingenuity, understanding exactly what it did and how it did it has challenged generations of scholars. Now fresh evidence has provided a new understanding of the Mechanism’s workings. Here, we see a digital reconstruction of the front (on the left) and back (right) faces of the new reconstruction, with an exploded diagram of its intricate gearing. [Image: Tony Freeth 2020]

In spring 1900, a party of sponge divers took shelter from a violent Mediterranean storm. When the storm subsided, they dived for sponges in the local waters near the tiny island of Antikythera, between Crete and mainland Greece. By chance, they found a wreck full of ancient Greek treasures, triggering the first major underwater archaeology operation in history. Overseen by a gunboat from the Greek navy to deter looters, by early 1901 the divers had begun to recover a wonderful array of ancient Greek goods – beautiful bronze sculptures, superb glassware, jewellery, amphorae, furniture fittings, and tableware.

They also found an undistinguished lump, the size of a large dictionary, which was probably recovered because it looked green, suggesting bronze. It was not considered to be anything remarkable at the time. Now, though, it is recognised as by far the most important object of high technology ever recovered from the ancient world: an ancient Greek astronomical calculating machine, known as the Antikythera Mechanism.

Images of the front and back covers and fragments of the Mechanism  creating using Polynomial Texture Mapping.
Surviving fragments of the Antikythera Mechanism that are particularly relevant for this study. These are imaged using Polynomial Texture Mapping (PTM – a digital imaging technique) with specular enhancement. The letters refer to the official fragment designations, so the front and back of Fragment A are shown to the left. [Image: Hewlett-Packard 2005]

Months after it was recovered, the object split apart, revealing tiny gearwheels inside, around the size of coins. It was an astonishing discovery: no one had even thought that such precision gearwheels could exist in ancient Greece. Today, only a third of the original Mechanism survives, split into 82 fragments – designated by letters A-G and numbers 1-75. It is a fiendish 3D jigsaw puzzle, all jumbled together, with incomplete and severely corroded components. Over the years, various scholars have sought to use these fragmentary elements to deduce the purpose of the machine. The latest to tackle this challenge are a multidisciplinary team of scientists, of which I am part: the University College London (UCL) Antikythera Research Team. The team was created when imaging specialist Lindsay MacDonald and materials scientist Adam Wojcik invited me to join UCL. We widened our expertise by teaming up with Myrto Georgakopoulou, an archaeometallurgist, plus two PhD students, horologist David Higgon and physicist Aris Dacanalis. Both of our students made essential contributions to our research. We have used new ideas and a close examination of all the data to challenge previous research and to create the first model that satisfies all the evidence.

An astronomical calculating machine

From the beginning, the Mechanism generated controversy, with fierce arguments about whether it was an astrolabe for tracking the stars or a navigation device. Both proved to be wrong, but uncovering the machine’s secrets would be a long and difficult detective story, peppered with major mistakes as well as surprising progress.

A PMT image of a fragment of the Mechanism with an inscription, the relevant numbers are highlighted in red.
Fragment 19, imaged using PTM with specular enhancement. Highlighted are the numbers 76, 19, and 223, which represent the Moon–Sun cycles identified by Rehm. [Image: Hewlett-Packard 2005]

The first real enlightenment came from a German philologist, Albert Rehm, in the period from 1905. Buried in his unpublished research notes are some extraordinary ideas. Rehm read inscriptions on the Mechanism concerning the risings and settings of the stars as viewed from Earth, and he found key astronomical cycles, too – 19-year and 76-year cycles of the Moon and a 223-month eclipse cycle. Rehm also made the radical suggestion that the device was an astronomical calculating machine. He had the groundbreaking idea that it contained epicyclic gearing – that is, gears mounted on other gears – a level of sophistication seemingly incredible for ancient Greece. In addition, Rehm proposed that all five planets known in the ancient world (Mercury, Venus, Mars, Jupiter, and Saturn) were displayed in a ring system on the front of the Mechanism. He simply did not have enough evidence to make coherent sense of his intuitions, and Rehm’s understanding of the internal mechanical structure was entirely wrong. More than a century later, though, his astonishing ideas are at the core of the new model of the machine created by the UCL Antikythera Research Team.

Scientific investigations

Fifty years after Rehm and his struggle with inadequate data, a British physicist, Derek de Solla Price, started a 20-year odyssey of research that culminated in a famous paper Gears from the Greeks (1974). He appreciated that to understand the Mechanism, there was a pressing need for new data to guide him through the fragmentary and confusing evidence.

Much of Price’s progress was based on X-rays of the Mechanism fragments, gathered and analysed by Charalambos and Emily Karakalos. These enabled the identification of 30 surviving gears: 27 in Fragment A and one in each of Fragments B, C, and D. Almost none of the gears were complete, so they needed to estimate the all-important number of teeth on each one – essential for understanding the workings of a geared calculating machine. From these X-rays, Price made a crucial discovery that the 19-year cycle of the Moon, identified by Rehm in the inscriptions on the Mechanism, could be calculated using its gearing.

Though Price made great progress, he also got much wrong, and only made unresolved suggestions about the planets. When Price died in 1983, the challenge was taken up by Michael Wright, a curator of Mechanical Engineering at London’s Science Museum, who had extensive experience of studying geared devices. While Price had discovered how some of the Sun–Moon system worked, it was Wright who set about reconstructing the gearing and a display for the planets.

Here, it is helpful to pause and consider how the ancient Greeks perceived the Cosmos. Their view was (almost) entirely Earth-centred and dominated by the mistaken belief that the Sun, Moon, and planets all moved around the Earth, against a background of ‘fixed stars’. When seen from Earth, the planets appear to move against the backdrop of the stars in perplexing ways. This is even reflected in the ancient Greek origin for the modern word ‘planet’: planetai, meaning ‘wandering’. Venus, for example, is sometimes ahead of the Sun and sometimes behind when viewed from Earth. Mostly it seems to move westwards through the sky, in the same direction as the Sun, but at times Venus will stand still against the stars at a stationary point, before looping backwards towards the east and reaching another stationary point, then resuming westwards motion once more. This synodic cycle – that is, its cycle relative to the Sun – is repeated again and again. Similar motions are shared by all the planets, creating a central problem for ancient astronomers. It was the failure to appreciate that the planets move around the sun that made the planetary motions seem so inexplicable.

In the 1st millennium BC, the Babylonians discovered what are known as ‘period relations’ for the planets, which equated a whole number of synodic cycles with a whole number of years. In the case of Venus, for example, they found the period relation that the planet goes through five synodic cycles in eight years. They could then use these period relations to predict the future positions of the planets in the sky. The ancient Greeks built on this by proposing geometrical theories for explaining planetary motions. These theories were ideal for mechanising the variable motions of the planets in a geared calculating machine. It was a revolutionary idea: thanks to the machine, the outcomes of ancient Greek astronomical theories could be calculated with the simple turn of a handle.

The UCL team looked at the pioneering work by Wright. He found evidence of bearings and other structures on the Main Drive Wheel. This four-spoked gear is prominent at the front of Fragment A. It is turned by the input handle and rotates once a year, thereby setting all the other gears in motion. Wright judged that there must have been an extensive epicyclic gearing system, mounted on the Main Drive Wheel. On the basis of this evidence, he proposed that one of the main purposes of the machine was to calculate the positions of the planets, which were displayed at the front of the machine. Inspired by astronomical clocks from the Middle Ages, Wright also introduced devices known as a ‘pin-and-slotted follower’ mechanisms to his reconstruction of the Antikythera Mechanism. When used alongside the gears, these devices could be used to mimic the backward loops of the planets. With great ingenuity, he managed to construct a planetarium for the Mechanism, which tracked the date, Sun, Moon, and five planets. He thought the outputs were shown as a system of pointers on the front of the machine to indicate their positions in the Zodiac. The publication of his results in 2002 was a landmark in Antikythera research, even though multiple challenges to his model would subsequently follow.

This is an extract of an article that appeared in CWA 108. Read on in the magazine (Click here to subscribe) or on our new website, The Past, which offers all of the magazine’s content digitally. At The Past you will be able to read each article in full as well as the content of our other magazines, Current ArchaeologyMinerva, and Military History Matters

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