Scientists have uncovered the workings of an ancient computer called the Antikythera Mechanism. Built at the end of the second century B.C.E, the device was used to calculate and display moon phases and a luni-solar calendar. Its exact workings have been something of a mystery since it was first found in 1901 at the site of a Roman shipwreck. Now, researchers from the UK, Greece, and US report that high-resolution imaging have revealed the function of the gears and the partial inscriptions on the body of the machine. They report their findings in this week’s issue of the scientific journal Nature. 
From the New York Times:
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…
Historians of technology think the instrument is technically more complex than any known device for at least a millennium afterward.
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.
Link to NYT article, Link to abstract at Nature (Thanks, Mike Liebhold!)