July 31, 2008

Antikythera mechanism and the timing of the Olympiads

Complex clock combines calendars:
The Antikythera Mechanism, a clockwork device made in Greece around 150–100 BC, astounded the world two years ago when scientists deduced how this machine was used to make complex astronomical time-reckonings. Now they say that the instrument, discovered in 1901 in a Mediterranean shipwreck, did much more than that.


Researchers have been trying to decode the mechanism's inscriptions and functions for several years. Their latest findings reveal that it links the technical calendars used by astronomers to the everyday calendars that regulated ancient Greek society — most strikingly, the calendar that set the timing of the Olympic Games.

“The mechanism is full of surprises,” says Alexander Jones of the Institute for the Study of the Ancient World in New York, who is one of the decoding team. “The latest revelations establish its cultural origin for the first time.”


In 2006, Freeth was part of a team that used this and other techniques to figure out much of the mechanism's function, showing it to be an instrument of unparalleled sophistication in antiquity, more or less unrivalled until the clockwork mechanisms of the later Middle Ages3.

Now they say that the device was even more sophisticated than that — it unites abstruse astronomical determinations of time with the calendar of civic society. Another ancient Greek calendar cycle, called the Metonic cycle, was established to cope with the incommensurability of the lunar cycle and the solar year — the period of Earth's rotation around the Sun, as determined, say, by the time between successive summer solstices. One Metonic period is equal to 235 lunar months, which is almost exactly 19 solar years. The Metonic cycle, thought previously to be used only by astronomers, is represented on a dial on the Antikythera Mechanism. But this dial now turns out to be inscribed with the names of months in a regional calendar used in Corinthian colonies in northwest Greece — providing evidence that the device was used for mundane reckonings, and giving a surprising clue to its origin.


But Freeth and his team now think that the instrument may have come from Syracuse in Sicily, the Corinthian colony where Archimedes devised a planetarium in the third century BC. “Archimedes died at the siege of Syracuse in 212 BC, so we are confident that he did not make the mechanism,” says Freeth. “But it is possible that it came from a heritage of instrument-making that originated with him in Syracuse. It is an attractive idea, but purely speculative at present.”
Nature 454, 614-617 (31 July 2008) | doi:10.1038/nature07130

Calendars with Olympiad display and eclipse prediction on the Antikythera Mechanism

Tony Freeth1,2, Alexander Jones3, John M. Steele4 & Yanis Bitsakis1,5

Previous research on the Antikythera Mechanism established a highly complex ancient Greek geared mechanism with front and back output dials1, 2, 3, 4, 5, 6, 7. The upper back dial is a 19-year calendar, based on the Metonic cycle, arranged as a five-turn spiral1, 6, 8. The lower back dial is a Saros eclipse-prediction dial, arranged as a four-turn spiral of 223 lunar months, with glyphs indicating eclipse predictions6. Here we add surprising findings concerning these back dials. Though no month names on the Metonic calendar were previously known, we have now identified all 12 months, which are unexpectedly of Corinthian origin. The Corinthian colonies of northwestern Greece or Syracuse in Sicily are leading contenders—the latter suggesting a heritage going back to Archimedes. Calendars with excluded days to regulate month lengths, described in a first century bc source9, have hitherto been dismissed as implausible10, 11. We demonstrate their existence in the Antikythera calendar, and in the process establish why the Metonic dial has five turns. The upper subsidiary dial is not a 76-year Callippic dial as previously thought8, but follows the four-year cycle of the Olympiad and its associated Panhellenic Games. Newly identified index letters in each glyph on the Saros dial show that a previous reconstruction needs modification6. We explore models for generating the unusual glyph distribution, and show how the eclipse times appear to be contradictory. We explain the four turns of the Saros dial in terms of the full moon cycle and the Exeligmos dial as indicating a necessary correction to the predicted eclipse times. The new results on the Metonic calendar, Olympiad dial and eclipse prediction link the cycles of human institutions with the celestial cycles embedded in the Mechanism's gearwork.


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