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From nytimes.com:  An Ancient Computer Surprises Scientists

They said their findings showed that the inscriptions related to lunar-solar motions and the gears were a mechanical representation of the irregularities of the Moon’s orbital course across the sky, as theorized by the astronomer Hipparchos. They established the date of the mechanism at 150-100 B.C.

The Roman ship carrying the artifacts sank off the island of Antikythera around 65 B.C. Some evidence suggests that the ship had sailed from Rhodes. The researchers speculated that Hipparchos, who lived on Rhodes, might have had a hand in designing the device.

The mechanism, presumably used in preparing calendars for seasons of planting and harvesting and fixing religious festivals, had at least 30, possibly 37, hand-cut bronze gear-wheels, the researchers reported. An ingenious pin-and-slot device connecting two gear-wheels induced variations in the representation of lunar motions according to the Hipparchos model of the Moon’s elliptical orbit around Earth.

The functions of the mechanism were determined by the numbers of teeth in the gears. The 53-tooth count of certain gears, the researchers said, was “powerful confirmation of our proposed model of Hipparchos’ lunar theory.”

The detailed imaging revealed more than twice as many inscriptions as had been recognized from earlier examinations. Some of these appeared to relate to planetary as well as lunar motions. Perhaps, the researchers said, the mechanism also had gearings to predict the positions of known planets.

A photo of this device can be seen here