After a century of study, scientists have unlocked the secrets of a mysterious 2,100-year-old device known as the Antikythera mechanism, showing it to be a complex and uncannily accurate astronomical computer.
The bronze-and-iron mechanism, recovered in more than 80 highly corroded fragments from a sunken Roman ship in 1901, could predict the positions of the sun and planets, show the location of the moon and even forecast eclipses.
The international team of scientists reported today that the 1st century BC Greek device, the earliest known example of an arrangement of gear wheels, shows a technological sophistication that was not seen again until clockwork mechanisms were introduced in the 14th century.
The results "imply that Greek technology was much more advanced in this area than was previously thought," said the team's leader, physicist Mike G. Edmunds of Cardiff University in Wales. "If they could do this, what else could they do?"
An even bigger question, says science historian Francois Charette of Ludwig Maximilians University of Munich in Germany, is why the technology disappeared for more than 1,400 years before reappearing in a less advanced form.
"Much of the mind-boggling technological sophistication available in some parts of the Hellenistic and Greco-Roman world was simply not transmitted further," he wrote in an editorial accompanying the team's report in the journal Nature.
The device was found by Greek sponge divers in 120 feet of water off the coast of the Greek island of Antikythera, midway between the southern tip of Greece and Crete. The site is on a major trade route between the island of Rhodes and Rome.
Coins on the ship suggest it sank shortly after 85 BC. Other artifacts from the ship indicate it carried a cargo of luxury goods, including statues and silver coins, that probably originated in Rhodes and was bound for Rome's elite.
The new study suggests that the Antikythera mechanism predates the sinking by 15 to 20 years.
The late British science historian Derek de Solla Price was the first to use modern technology to study the device. Over the course of two decades beginning in the 1950s, he used X-ray and gamma-ray images to reveal gears inside the corroded pieces.
He concluded correctly that it was an astronomical calculator. But the device as he reconstructed it was unduly complicated, and there were many gaps in his explanation of how it worked.
Edmunds said his interest in the device was piqued several years ago by a student who pointed it out to him. He began studying the published literature, eventually producing a review article. But truly understanding it, he concluded, would require a more detailed knowledge of its encrusted gears.
Edmunds' team brought a 7.5-ton X-ray tomography machine -- similar to that used to perform CT scans on human patients -- into the National Archeological Museum of Athens to examine the fragments in greater detail than was possible before.
They were able to image the bronze gears more clearly, and deciphered twice as many inscriptions on the casing.
They concluded that the device contained 37 gears, about 30 of which still survive. It was originally housed in a wooden case slightly smaller than a shoebox.
Two dials on the front show the zodiac and a calendar of the days of the year that can be adjusted for leap years. Metal pointers show the positions in the zodiac of the sun, moon and five planets known in antiquity. Two spiral dials on the back show the cycles of the moon and predict eclipses.
The complicated meshing of the gears is a physical representation of the so-called Callippic and saros astronomical cycles. In the Callippic cycle, for example, the sun, moon and Earth return to the same relative orientations four times in 76 years minus one day.
The saros cycle predicts that, following a solar or lunar eclipse, a similar eclipse will occur 223 lunar months later.
By turning the gears with a hand crank, the user could select a specific day in the past or future and observe the positions of the heavenly objects on that day.
The device was most likely used to indicate the beginning and end of growing seasons, to mark religious holidays and to time other significant events during the year.
"The design is beautiful; the astronomy is exactly right," Edmunds said. "The way the mechanics are designed just makes your jaw drop. Whoever has done this has done it extremely well."
The device performs subtractions, multiplications and divisions to carry out its functions. That leads to the question "of whether they could have easily designed actual calculators for other purposes," Edmunds said.
The identity of the maker will probably remain a mystery -- no signature is apparent on the device.
But the astronomical information incorporated in the gears clearly is based on the calculations of the Greek astronomer Hipparchus, who worked on Rhodes from about 140 BC to about 120 BC.
Subsequently, the philosopher Posidonius established an astronomy school incorporating Hipparchus' ideas.
Cicero, the Roman lawyer and consul, later wrote that Posidonius had made an instrument "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."
Historians have generally dismissed Cicero's words as extravagant or even mythic, but the new discoveries suggest he may have been correct.