We may have figured out the date from which an ancient device often described as the first computer began its calculations. This device, called the Antikythera mechanism, was built sometime between the years 200 BC and 60 BC, and it was used to track time and predict the motions of celestial bodies.
A spiral shape inset in the back of the mechanism depicts a 223-month cycle called a Saros, which is based on the amount of time it takes for the sun, moon and Earth to return to the same relative positions and repeat a cycle of similarly timed solar and lunar eclipses.
at the Thessaloniki Directorate of Culture and Tourism in Greece and his colleagues used this to calculate a sort of starting date for the Antikythera mechanism, the âDay Zeroâ by which its calculations were calibrated.
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âAny measuring system, from a thermometer to the Antikythera mechanism, needs a calibration in order to [perform] its calculations correctly,â says Voulgaris. âOf course it wouldnât have been perfect â itâs not a digital computer, itâs gears â but it would have been very good at predicting solar and lunar eclipses.â
Based on our understanding of how the mechanism works and the inscriptions on it, the calibration date was expected to occur during a particular kind of solar eclipse in which the moon was located at the furthest spot in its orbit from Earth, a position called apogee. Voulgaris and his colleagues searched a NASA repository of eclipse calculations for unusually long-lasting eclipses, which occur when the moon is particularly far from Earth.
The longest such eclipse during the era when the Antikythera mechanism was built occurred in the small hours of 23 December 178 BC. When they looked further into this date, they found that several astronomical events of cultural significance occurred then, including the winter solstice, which is engraved at the top left of the front of the mechanism, which Voulgaris says is a hint at its importance to calibration.
âThis is a very specific and unique date,â says Voulgaris. âIn one day, there occurred too many astronomical events for it to be coincidence â this date was a new moon, the new moon was at apogee, there was a solar eclipse, the sun entered into the constellation Capricorn, it was the winter solstice.â
However, other prominent researchers in the field have found a different calibration date.
âThe eclipse predictions on the [deviceâs back] contain enough astronomical information to demonstrate conclusively that the 18-year series of lunar and solar eclipse predictions started in 204 BC,â says at New York University. There have been four independent calculations of this, he says.
âThe reason such a dating is possible is because the Saros period is not a highly accurate equation of lunar and solar periodicities, so every time you push forward by 223 lunar months⊠the quality of the prediction degrades.â
These other works relied on finding the one Saros period for which the mechanismâs astronomical predictions would have been most accurate. However, the date that they found is in the summer, which would leave the question of why the winter solstice engraving is so prominent on the Antikythera mechanismâs front plate unanswered, says Voulgaris.
It may take further analysis to be certain of the calibration date of this ancient computer. Its complex inner workings and the degradation of the device over time make it particularly difficult to study, but the complexity of those inner workings are also what make it so remarkable.
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