The Antikythera Mechanism, discovered more than 100 years ago in a Roman shipwreck, was used by ancient Greeks to display astronomical cycles.
Using advanced imaging techniques, an Anglo-Greek team probed the remaining fragments of the complex geared device.
The results, published in the journal Nature, show it could have been used to predict solar and lunar eclipses.
The elaborate arrangement of bronze gears may also have displayed planetary information.
"This is as important for technology as the Acropolis is for architecture," said Professor John Seiradakis of the Aristotle University of Thessaloniki in Greece, and one of the team. "It is a unique device."
However, not all experts agree with the team's interpretation of the mechanism.
The remains of the device were first discovered in 1902 when archaeologist Valerios Stais noticed a heavily corroded gear wheel amongst artefacts recovered by sponge divers from a sunken Roman cargo ship.
A further 81 fragments have since been found containing a total of 30 hand-cut bronze gears. The largest fragment has 27 cogs.
Researchers believe these would have been housed in a rectangular wooden frame with two doors, covered in instructions for its use. The complete calculator would have been driven by a hand crank.
Although its origins are uncertain, the new studies of the inscriptions suggest it would have been constructed around 100-150 BC, long before such devices appear in other parts of the world.
Writing in Nature, the team says that the mechanism was "technically more complex than any known device for at least a millennium afterwards".
Although much of it is now lost, particularly from the front, what remains has given a century's worth of researchers a tantalising glimpse into the world of ancient Greek astronomy.
One of the most comprehensive studies was done by British science historian Derek Solla Price, who advanced the theory that the device was used to calculate and display celestial information.
There's more to it.
Though it is an intresting read.