The Antikythera mechanism is an ancient artifact believed to be an early clockwork mechanism. It was discovered in a shipwreck off the Greek island of Antikythera, between Kythera and Crete, and has been dated to about 87 BC.

The wreck was discovered in 1900 at a depth of about 40 m (140 ft), and many statues and other works were retrieved from it by sponge divers. On May 17, 1902, archaeologist Spyridon Stais noticed that one of the pieces of rock had a gear wheel embedded in it.

The mechanism is the oldest known surviving geared mechanism, made from bronze in a wooden frame, and has puzzled and intrigued historians of science and technology since its discovery. The most commonly accepted theory of its function is that it was an analog computer designed to model the movements of heavenly objects. Recent working reconstructions of the device support this analysis. The device is all the more impressive for its use of a differential gear, which was previously believed to have been invented in the 16th century.

Derek J. de Solla Price, a science historian at Yale University, published an article on the mechanism in Scientific American in June 1959 while the device was still only partially inspected [1]. In 1973 or 1974, he published an analysis based on gamma ray imaging by Greek archaeologists. He claimed that the device had been built by a Greek astronomer, Geminus of Rhodes. His conclusion was not accepted by experts at the time, who believed that the ancient Greeks had the theoretical knowledge but not the necessary practical skills.

A partial reconstruction was built by Australian computer scientist Allan George Bromley (1947–2002) of the University of Sydney and Sydney clockmaker Frank Percival. This project led Bromley to review Price's X-ray analysis and to make new, more accurate X-ray images that were studied by Bromley's student, Bernard Gardner, in 1993.

Later, a British orrery maker named John Gleave constructed a working replica of the mechanism. According to his reconstruction, the front dial shows the annual progress of the sun and moon through the zodiac against the Egyptian calendar. The upper rear dial displays a four-year period and has associated dials showing the Metonic cycle of 235 synodic months, which equals 19 solar years. The lower rear dial plots the cycle of a single synodic month, with a secondary dial showing the lunar year of 12 synodic months.

Another reconstruction was made in 2002 by Michael Wright, mechanical engineering curator for the Science Museum in London, working with Allan Bromley. He analyzed the mechanism using linear tomography, which can create images of a narrow focal plane and thus visualized the gears in great detail.

In Wright's reconstruction, the device not only models the motions of the sun and moon, but those of every celestial body known to the Ancient Greeks: Mercury, Venus, Mars, Jupiter, and Saturn.

This new reconstruction gives credence to ancient mentions of such devices. Cicero, writing in the first century BC, mentions an instrument "recently constructed by our friend Poseidonius, which at each revolution reproduces the same motions of the sun, the moon and the five planets." Such devices are mentioned elsewhere as well. It also adds support to the idea that there was an ancient Greek tradition of complex mechanical technology which was transmitted via the Arab world, where similar devices are found later, and could have yielded to or integrated with European clockmaking and ancient cranes. Some scientists believe that not only was the device used to track celestial bodies, but to calculate their positions for events or births.

The original mechanism is displayed in the Bronze collection of the National Archaeological Museum in Athens, accompanied by a replica. Another replica is on display at the American Computer Museum in Bozeman, Montana.

The Antikythera mechanism, not described in any surviving source, shows that our knowledge of ancient technology is incomplete. In 1996, the Italian physicist Lucio Russo (professor at Università di Roma "Tor Vergata") published an essay putting new light on the issue. The essay has been translated and published in English in 2004 under the title "The Forgotten Revolution: How Science Was Born in 300 BC and Why it Had to Be Reborn".