The Antikythera Mechanism is a corroded lump of bronze gears recovered from a Roman-era shipwreck off the Greek island of Antikythera in 1901. For decades its purpose was unclear. Beginning in the 2000s, a research collaboration used surface imaging and high-resolution X-ray computed tomography to read the surviving fragments - 82 pieces now held at the National Archaeological Museum in Athens - and reconstruct what the device did. The work, published in Nature in 2006 and 2008, established that the mechanism was a hand-cranked astronomical calculator built around the 2nd century BC.
Turning a single input drive set dozens of interlocking bronze gears in motion, which moved pointers across dials on the front and back. The front showed the positions of the Sun and Moon through the zodiac, including a clever pin-and-slot gearing that reproduced the Moon’s varying speed. The back carried a spiral dial implementing the 19-year Metonic calendar that reconciles lunar months with solar years, and a four-turn spiral Saros dial that predicted lunar and solar eclipses using the 223-lunar-month Saros cycle. An auxiliary dial even tracked the four-year cycle of the ancient games, including the Olympiad.
Nothing of comparable mechanical complexity is known again for more than a thousand years - not until medieval astronomical clocks. The mechanism shows that ancient Greek craftsmen could embody astronomical theory in precision gearing, building a special-purpose analog computer that turned a mathematical model of the cosmos into a machine you could read off a dial.