A 2000-year-old Greek analogue computer recovered from a Roman-era shipwreck. Calculated astronomical positions to a precision unmatched until the 14th century.
From Wikipedia
The Antikythera mechanism is an ancient Greek hand-powered orrery. It is the oldest known example of an analogue computer. It could be used to predict astronomical positions and eclipses decades in advance. It could also be used to track the four-year cycle of athletic games similar to an olympiad, the cycle of the ancient Olympic Games.