Antikythera Mechanism planet indicator unraveled

Antikythera Mechanism

New reconstruction of the Antikythera Mechanism (Image: 2020 Tony Freeth)

The Antikythera Mechanism, discovered in an ancient shipwreck in 1901, is considered to be the most complex device in all of antiquity – and a kind of ancient celestial computer. But how the planet display on its front works has not yet been determined. Researchers have now succeeded in reconstructing the representation and mechanics of this part of the device. It opens up new insights into the cosmic worldview and the technology of the ancient Greeks.

The Antikythera Mechanism probably dates back to the first century BC, as suggested by features of the inscriptions and the age of the shipwreck in which this ancient device was found. Only around a third of the roughly shoebox-sized instrument is still preserved today, in the form of a handful of heavily encrusted larger fragments and around 70 small parts. Using X-ray analyzes, archaeologists have made around 30 gears and numerous inscriptions and scales on and in the fragments visible in recent years.

As a result, a group of researchers led by Michael Wright from the London Science Museum succeeded in 1989 in deciphering the functionality and display of the reverse side of the Antikythera mechanism. Accordingly, the two superimposed circular scales on the back indicated, among other things, solar and lunar eclipses, served as a calendar and also indicated the four-year period between the Olympic Games. But the front of the Antikythera Mechanism posed more mystery. From the reconstructed inscriptions it emerged that the movements of the moon, sun and the five planets known in antiquity were displayed from a geocentric point of view. It remained unclear which features of the orbital movements the planetary display recorded and how the whole thing was implemented mechanically.

Inscriptions on the planets and their placement in the mechanism. (Image: 2020 Tony Freeth)

Of epicycles and synodic periods

Because the Greeks assumed a geocentric view of the world, the planets did not follow simple circular orbits in their celestial models, but instead drew loops and U-turns in the sky from their point of view. However, the ancient astronomers recognized that these cycles could be approximately reproduced by combining two circular movements – so-called epicycles. A smaller circular path moves around a point rotating on a larger circular path. “It was a remarkable finding,” explains Tony Freeth of University College London.

Freeth and his team have now unraveled how this knowledge was reflected in the Antikythera mechanism and what that means for the mechanics of the front display. This was made possible by indications from some inscriptions inside the instrument in combination with mathematical-astronomical calculations and a mechanical reconstruction. From an inscription it emerged that, contrary to previous assumptions, the positions of the planets were not indicated by pointers, but by rings in the front panel of the device. In another inscription on the inside of the front cover, the researchers came across planet names in combination with numbers – for example for Venus 442, for Saturn 462. A comparison with astronomical data showed that these numbers mark sections in the synodic periods of the planets – the intervals , in which certain positions or encounters of the heavenly bodies are repeated.

The ancient cosmos in one system

From this the scientists conclude that the Greek creators of this instrument must have known the synodic periods for the five planets known at the time, Mercury, Venus, Mars, Jupiter and Saturn. This in itself is astonishing: “Classical astronomy in the first millennium BC originated in Babylon, but nothing in this astronomy suggests it as the Greeks did to the highly accurate 462-year cycle for Venus and the 442-year cycle came for Saturn, ”says Freeth’s colleague Aris Dacanalis. With the help of these numbers and the knowledge of the astronomical constellations likely represented, Freeth and his team were now able to calculate mathematically how and with how many gears and teeth the Antikythera mechanism reproduced and displayed these intervals.

The result is a complex system of seven gears with locks and connecting struts, which enabled a variable and both direct and indirect meshing of the gear train – comparable to a complex manual transmission. Powered by this mechanism, the front of the Antikythera Mechanism indicated the phases of the moon and the position of the sun, as well as the orbits and synodic phases of the planets. “Ours is the first model that matches all physical evidence with the descriptions of the inscriptions on the mechanism,” says Freeth. “Earlier research unraveled the ingenuity of the dials on the back, we are now showing the fullness of the cosmos shown on the front.”

Source: University College London, Article: Scientific Reports, doi: 10.1038 / s41598-021-84310-w

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