Calendars with Olympiad Display and Eclipse Prediction on the Antikythera Mechanism Essay

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Calendars with Olympiad display and eclipse prediction on the Antikythera Mechanism
Nature, July 31, 2008
On the Antikythera Mechanism established a highly complex ancient Greek geared mechanism with front and back output dials (1-7). The upper back dial is a 19-year calendar, based on the Metonic cycle, arranged as a five-turn spiral (1, 6, and 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 predictions (6). 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 source (9), have hitherto been dismissed as implausible (10,11). 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 thought (8), 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 modification (6). 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 gear work.
This extraordinary astronomical mechanism from about 100 BC employed bronze gears to make calculations based on cycles of the Solar System (1,6) (Supplementary Notes 1). Recovered in 1901 by Greek sponge-divers, its corroded remains are now split into 82 fragments--7 larger fragments (A-G) and 75 smaller fragments (1-75) (6). Data, gathered in 2005'[degrees]', included still photography, digital surface imaging (12) and, crucially for this study, microfocus X-ray computed tomography (CT) (6,13) (Figs 1-3)--details are in Supplementary Notes 2 (and at www.antikythera-mechanism.gr).
The main upper back dial is now established as a Metonic calendar (1, 6, and 1) (Figs 1 and 2, Supplementary Box 1). The calendar dial bears inscriptions, only viewable using X-ray CT. We have now identified all 12 months of this calendar (Fig. 2, Supplementary Notes 3), providing conclusive evidence of the regulation of a Greek civil calendar by a Metonic cycle, and clues to the instrument's origin. Whereas the Babylonian calendar followed a Metonic cycle from about 500 BC, it has commonly been assumed that the intercalary months of the numerous lunisolar calendars of the Greek cities were determined arbitrarily-Metonic and Callippic cycles (Supplementary Box 1) only being used by astronomers (14). The month names on the Metonic spiral, however, belong to a regional calendar unassociated with technical astronomy, suggesting that it may have been common for Greek civil calendars to follow the Metonic cycle by about 100 BC.
The inscriptions show that not only the names and order of the months were regulated, but also which years had 13 months, which month was repeated in these years, and which months had 29 or 30 days. The rules are…