A ten-year project to decipher inscriptions on the ancient Greek “Antikythera mechanism” has revealed new functions, including the first hint that the device was used to make astrological predictions. The writings also lend support to the idea that the gadget, often called the world's first computer because of its ability to model complex astronomical cycles, originated from the island of Rhodes.

Until now, scholars have focused on decoding the sophisticated array of gearwheels inside the 2000-year-old artifact.

The new publication tackles instead the lettering squeezed onto every available surface. “It’s like discovering a whole new manuscript,” says Mike Edmunds, emeritus professor of astrophysics at Cardiff University, U.K., who edited the special issue of Almagest in which the results are published.

The mechanism was found on an ancient shipwreck by sponge divers in 1901. The ship sank close to the island of Antikythera in the first century BC, loaded with Greek treasure. Among gorgeous statues and jewelry retrieved by the divers was a collection of battered bronze pieces, with traces of gearwheels, dials and pointers that stunned scholars.

The squashed, crumbling fragments are so corroded that barely any metal remains, and it has taken a century of study—most recently by an international collaboration known as the Antikythera Mechanism Research Project (AMRP)—to reconstruct the mystery device. Scholars now agree that it was a mechanical calculator, used to replicate in miniature the motions of the heavens.

It originally took the form of gearwheels in a wooden case, similar to a mantelpiece clock, with a handle that the user turned to move forwards and backwards in time. Instead of hours and minutes, pointers on the large front dial traced the movements of the sun, moon and planets through the sky. Two spiral dials on the back functioned as a calendar and predicted eclipses.

The spaces around the dials were filled with engraved text. AMRP researchers have now completed their efforts to read around 3,400 characters on the surviving surfaces.

Lead author Alexander Jones, a classicist at the Institute for the Study of the Ancient World in New York, estimates that the original mechanism probably held up to 20,000 characters.

The letters are tiny—some less than a millimeter tall—and often hidden beneath the surface of the corroded fragments. Jones and his colleagues used CT scans to reveal new sections of text and update previous readings. “We’ve made a big jump in terms of the quality of the inscriptions and their intelligibility,” says Jones. He and the AMRP will officially announce their results at the Aikaterini Laskaridis Foundation in Athens on June 9.

The new readings are “very valuable,” says Michael Wright, a London-based scholar and former curator of mechanical engineering at the London Science Museum who has spent decades studying the Antikythera mechanism independently. “We’ve got the most reliable readings yet of each piece of inscription.”

Scholars already knew that the front dial features two concentric scales, showing months of the year and signs of the zodiac, so that the position of the sun pointer gives the date as well as its position in the sky. Text fixed above and below this dial describes the risings and settings of star constellations at various dates throughout the year. Jones and colleagues now show that this star calendar, or “parapegma,” was more extensive than thought, listing at least 42 events, and that it also included solar events such as solstices and equinoxes.

The researchers used these new dates to estimate more accurately the likely location of the astronomer who compiled the parapegma. They match a latitude of around 35 degrees. That rules out Egypt or northern Greece, but is perfect for the Aegean island of Rhodes, where Jones believes the mechanism was most likely made, possibly for a buyer in northwest Greece. He has also identified handwriting from at least two different people, suggesting the device was made in the context of a workshop or family business, rather than by a lone mechanic.

The researchers have also gleaned new details from the text on the back face of the mechanism, which describes upcoming eclipses. They were surprised to find references to the color and size of each eclipse, as well as the expected winds during each event.

There is no basis in astronomy for such predictions, says Jones. An eclipse's characteristics have no astronomical significance, nor is there a way to accurately predict an eclipse's color. But it was widely believed in the Greek world that such characteristics could predict the weather, as well as “large-scale astrology”—the fortunes of countries and peoples. The Greeks inherited this belief from the Babylonians, whose priest astronomers obsessively watched the skies for bad omens.

The Antikythera text appears to go one step further: rather than telling fortunes from observed colors and winds, it predicts these signs before they happen. This fits a broader trend in ancient Greek astronomy “to replace astronomy with calculating and predicting,” says Jones. “Theory is now replacing observation.”

The link with astrology was unexpected because the device’s other functions are purely astronomical—apart from the calendar, which uses colloquial month names and displays the timings of athletics events including the Olympic games. The researchers conclude: “The Antikythera mechanism simulated a Hellenistic cosmology in which astronomy, meteorology and astral divination were intertwined.”

Text on bronze plates found at the front and back of the mechanism (these were once thought to be hinged doors, but Wright and the AMRP team agree there is no evidence of hinges) provides further information about its functions. The front plate ends a long-running argument among Antikythera researchers by confirming that the planets were modeled using mathematical cycles accurate to within one degree in 500 years, something that would have required complex chains of gearwheels. This was suggested by Wright, but other scholars have been skeptical, suggesting simpler schemes.

Meanwhile a back plate previously described as a “user manual” in fact contains no instructions. Instead, says Jones, it’s more like a picture caption or exhibit label. “It doesn’t help the operator to run the thing but it describes what the viewer is seeing.” The text assumes knowledge of astronomical cycles, suggesting the intended audience was well educated.

But beyond that, “we still don't know what it was for,” says Edmunds. He sees the mechanism as “a statement, saying ‘this is what we know about the universe.’ But whether then you want to put it on a rich man’s mantelpiece, in a school or academy, or in a temple, we just don’t know.”