The Antikythera mechanism ( AN-tih-kih-THEER-) is an Ancient Greek hand-powered orrery, described as the oldest example of an analogue computer used to predict astronomical positions and eclipses decades in advance. It could also be used to track the four-year cycle of athletic games which was similar to an Olympiad, the cycle of the ancient Olympic Games.This artefact was among wreckage retrieved from a shipwreck off the coast of the Greek island Antikythera in 1901. On 17 May 1902, it was identified as containing a gear by archaeologist Valerios Stais. The device, housed in the remains of a 34 cm 18 cm 9 cm (13.4 in 7.1 in 3.5 in) wooden box, was found as one lump, later separated into three main fragments which are now divided into 82 separate fragments after conservation efforts. Four of these fragments contain gears, while inscriptions are found on many others. The largest gear is approximately 13 centimetres (5.1 in) in diameter and originally had 223 teeth.In 2008, a team led by Mike Edmunds and Tony Freeth at Cardiff University used modern computer x-ray tomography and high resolution surface scanning to image inside fragments of the crust-encased mechanism and read the faintest inscriptions that once covered the outer casing of the machine. This suggests that it had 37 meshing bronze gears enabling it to follow the movements of the Moon and the Sun through the zodiac, to predict eclipses and to model the irregular orbit of the Moon, where the Moon's velocity is higher in its perigee than in its apogee. This motion was studied in the 2nd century BC by astronomer Hipparchus of Rhodes, and it is speculated that he may have been consulted in the machine's construction. There is speculation that a portion of the mechanism is missing and it also calculated the positions of the five classical planets.

The instrument is believed to have been designed and constructed by Greek scientists and has been variously dated to about 87 BC, or between 150 and 100 BC, or to 205 BC. In any case, it must have been constructed before the shipwreck, which has been dated by multiple lines of evidence to approximately 7060 BC. In 2022 researchers proposed that the initial calibration date of the machine (not its actual date of construction) could have been 23 December 178 BC. Other experts disagree proposing a date in the summer of 204 BC as a more likely calibration date. Machines with similar complexity did not appear again until the astronomical clocks of Richard of Wallingford and Giovanni de' Dondi in the fourteenth century.All known fragments of the Antikythera mechanism are now kept at the National Archaeological Museum, Athens, along with a number of artistic reconstructions and replicas to demonstrate how it may have looked and worked.

Valerios Stais (Greek: Βαλέριος Στάης; b. Kythira 1857 – d. Athens 1923) was a Greek archaeologist. He was born in Kythera. He studied medicine and later archaeology. He became the director of the National Archaeological Museum of Athens in 1887 and held that post until his death. During that period he organized or participated in excavations in Epidaurus, Argolis, Attica, Dimini, Antikythera and elsewhere. He also wrote a lot of archaeological studies, published in various papers and mainly in Archeologiki Efimeris (Αρχαιολογική Εφημερίς "Archaeological Newspaper"), and many books.

Valerios Stais also became the first to recognise the Antikythera mechanism from the lumps of archaeological material which was retrieved from a wreck found near the coast of Antikythera in 1900.