Coligny calendar
It is written in Latin inscriptional capitals and numerals, but terms are in the Gaulish language.
Based on the style of lettering and the accompanying statue, the bronze plaque probably dates to the end of the second century, although the copying errors indicate the calendar itself is much older.
Eight small fragments of a similar calendar were found at the double-shrine of Villards-d'Héria.
The names of the twelve lunar year months are reconstructed as Samonios, Dumannios, Rivros, Anagantios, Ogronios, Cutios, Giamonios, Simivisonnios, Equos, Elembivios, Edrinios, and Cantlos.
A synodic month has 29.53 days, so the calendar overcomes any slight slippage or temporary imbalance by the month of MID EQVOS having either 29 or 30 days as required to keep the calendar in sync with the lunar phase.
[b] The Coligny calendar is designed to keep perfectly in sync with the lunar phase,[c] with a tolerance of less than 24 hours.
At the end of the 19-year Metonic cycle, the calendar has overrun the 62-month lunar point by 0.312 days.
If the plaque was part of a 30-year calendar, it overruns the lunar phase by 0.151 days.
The calendar is based on the Metonic cycle, a period of 19 years after which the sun and moon complete their phase within about two hours, 0.087 days, of each other.
Eventually the calendar would require a 30-day lunar month to be skipped once every 6,536 years.
Pliny stated that the Celts treated 30 years as an ‘age’,[e] and this period is seen many times in stories.
If the calendar was a 30-year cycle, the luni-solar swing would be 46 days, meaning that the timing of festivals would often be inappropriate to their seasonal nature.
But the lunar/solar difference is larger at 1.4172 days, requiring a 30-day month to be skipped every 198 years.
The quarter moon with its D-shape is the only moment in the lunar phase that is easily identifiable by eye.
Notations, which will govern activities, usually focus in the first, brighter, coicise when the moon has ‘considerable power and influence’.
A full reconstruction of the calendar by McKay (2020)[15] includes the latest information about the intercalary notations and the triple marks.
Olmsted (2001)[9] offers a previous reconstruction, which usefully aligns the notations with photographic images.
RIG III (1986)[16] presented an earlier in-depth description of terms with a reconstruction.
The notations are usually visually aligned on the D or N. Terms are often shortened, and the spelling is non-standard and often varies.
Next are occasional triple-marks of the form ƚıı ıƚı or ııƚ, in that order before major movements and overwriting.
[19] These follow the same offset pattern as the PRINNI notations, and likely divide the daytime into three periods.
Finally, Day 1 has its 'day' terms overwritten by a single N, without changing the rest of its notations.
[i] The use of odd numbers as inauspicious is also seen with most months of 29 days being ANMAT ‘not good’.
It is symptomatic of Celtic cultures, as the Romans held the reverse view, that odd numbers were auspicious.
The equivalent sequence starts on Days 1-2-3 of GIAMONIOS in Year 3 and follows the ANM months, so mirroring one intercalary period to the other.
The result turns a complex pattern of triple marks into visual chaos.
Four of these IVOS runs break the year into four-quarters, just as the four main Celtic festivals do in historic times, only here they are centered on Day 1 every three lunar months, rather than Day 1 of every three solar months as today.
TIOCOBRIXTIO is an exceptional term which only occurs on three days in the year – SIM 7, AED 8, and CAN 15.
The notation IVOS is also sequentially dragged upwards a month in the post-intercalary year.
Thirdly, the days 7-8-9 and 7a-8a-9a which have been dragged from the following month are again merged with the copied notations.
