Ancient Greek artefact was an 'astronomical computer'

By Steve Connor, Science Editor

Published: 30 November 2006

An astronomical instrument built by the Ancient Greeks in the 2nd century BC has turned out to be a complex computer for calculating the relative position of the sun, the moon and the planets.

Scientists studied the internal workings of the machine by using a sophisticated medical scanner. They concluded it was at least 1,000 years ahead of its time.

The Antikythera Mechanism was rescued from a Roman shipwreck at the turn of the last century but its precise function was little understood because it was broken into 82 pieces.

Made of bronze and wood, the device was evidently an instrument of some sort because it used a complicated set of gears to move a series of concentric wheels and pointers that appeared to predict movements of astronomical objects. But scientists were surprised to find it was in fact a sophisticated analogue computer that acted as a long-term calendar for predicting lunar and solar eclipses and planetary movements.

An international team of scientists drawn from many different disciplines took part in the study. Their picture of how the device worked and what it was intended to do has astonished classical scholars.

Professor Mike Edmunds of Cardiff University, a leading member of the research team, said: "This device is just extraordinary, the only thing of its kind. The design is beautiful, the astronomy is exactly right. The way the mechanics are designed just makes your jaw drop.

"Whoever has done this had done it extremely well. It does raise the question of what else were they making at the time. In terms of historic and scarcity value, I have to regard this mechanism as being more valuable than the Mona Lisa."

The scientists, including researchers from the National Archaeological Museum of Athens, found the gearing mechanism of the device acted as a long-term calendar, enabling its operators to track the moon and the sun through the zodiac, predict eclipses and even calculate the irregular orbit of the moon.

"Calendars were important to ancient societies for timing agricultural activity and fixing religious festivals," the scientists write in a study published in the journal Nature.

"Eclipses and planetary motions were often interpreted as omens, while the calm regularity of the astronomical cycles must have been philosophically attractive in an uncertain and violent world," they say.

Greek sponge-divers discovered the Roman shipwreck off the island of Antikythera in 1900. A year later, archaeologists recovered the device which had been submerged for about 2,000 years.

The shipwreck was dated to about 65BC but the instrument was thought to have been made earlier, between 100BC and 150BC, possibly by the great Greek astronomer Hipparchos, who, at that time, lived on the island of Rhodes.

Astronomers believe Hipparchos was probably involved because he was the first to track the irregularities in the orbit of the moon, which the device seems to be designed to predict.

There are three dials on the device. The front dial displays the position of the sun and the moon in the zodiac and a corresponding calendar of 365 days, which could be adjusted for leap years. The back dials track the long-term lunar cycle, including the Metonic cycle of 19 years, when the same phase of the moon returns on the same date of the year.

The dials also track the Callippic cycle of 76 years, when the moon returns to the same position in the sky relative to the zodiac and its monthly lunar phase.

Francois Charette, an astronomer at Ludwig-Maximilian University in Munich, said finding such a complicated computer in Ancient Greece was like finding the plans for a steam engine in Renaissance Italy.