The remains of one such device was discovered at a shipwreck in 1901.
In 1900, a group of divers caught sight of a bronze hand on the ocean floor and notified authorities of a possible archaeological site off the coast of the island of Antikythera. The following year the government sent more divers out and found a trove of lost sculptures and art.
Live Science reports that among the artifacts found was a shoebox-size contraption covered with dials and filled with about 30 gears made of bronze. Although the little box was in many fragments, scientists were able to determine that by using a small hand crank on the side, the box could reveal the position of the sun, moon, and stars of any future time.
The box became known as the Antikythera mechanism.
It was not until the 21st century gave rise to 3D scanning that scientist were able to see the insides of the metal, which began to corrode as soon as it was removed from the ocean. Scans also allowed researchers to better decipher the ancient Greek writing on the side, which they found to be instructions for use.
The device could predict eclipses. The letters denoted whether an eclipse was lunar or solar:
Lunar eclipses, for instance, were denoted by the glyph for Σ, which was short for the moon goddess ΕΛΗΝΗ (Selene), while solar eclipses were denoted by H, which is short for the sun god ΗΛΙΟΣ (Helios)… What’s more, the Greek computer was surprisingly sophisticated. The Antikythera mechanism could not only predict the timing of eclipses but also reveal characteristics of those eclipses, such as the amount of obscuration, the angular diameter of the moon (which is the angle covered by the diameter of the full moon) and the position of the moon at the time of the eclipse, the study found.
There is a longer, more comprehensive special on the Antikytherian mechanism on Youtube, if you’d like to learn more.