The Coligny calendar is a bronze plaque with an inscribed calendar, made in Roman Gaul in the 2nd century CE. It lays out a five-year cycle of a lunisolar calendar, each year with twelve lunar months. An intercalary month is inserted before each 2.5 years. The lunar phase is tracked with exceptional precision, adjusted when necessary by a variable month, and the calendar uses the 19-year Metonic cycle to keep track of the solar year. It is the most important evidence for the reconstruction of an ancient Celtic calendar.

It was found in 1897 in France, in Coligny, Ain (46°23′N 5°21′E / 46.383°N 5.350°E / 46.383; 5.350, near Lyon), along with broken pieces of a bronze statue of a life-size naked male holding a spear, likely Roman Mars or Romano-Celtic Lugus.^[1] It was engraved on a bronze tablet, preserved in 73 fragments, that was originally 1.48 metres (4 ft 10 in) wide by 0.9 metres (2 ft 11 in) tall. 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. ^[2] It is now held at the Gallo-Roman Museum of Lyon-Fourvière.

Eight small fragments of a similar calendar were found at the double-shrine of Villards-d'Héria. It does not have the holes of a peg calendar^[3] that the Coligny calendar does, but otherwise has the same notations. It is now held in the Musée d'Archéologie du Jura at Lons-le-Saunier.

List of months

The names of the twelve lunar year months are reconstructed as Samonios, Dumannios, Rivros, Anagantios, Ogronios, Cutios, Giamonios, Simivisonnios, Equos, Elembivios, Edrinios, and Cantlos. The names occur in the form SAMONI (gen.), DUMANNI, RIVRI etc. in the internal notations of the calendar. The name of the first intercalary month is unknown being on a lost fragment, the second is reconstructed asantaran, antaran, antaran, or Antaran.

Mid Samonios refers to summer (Gaulish samo-,< *sṃHo-3)^[4]: 267 while Mid Giamonios refers to winter (Gaulish giamo-). These two months divide the calendar into summer and winter seasons of six months, each season led off by a festival of several days marked with IVOS. This indicates an early version of the same traditional seasons as seen in later Celtic contexts: “For two divisions were formerly on the year, viz., summer from Beltaine (the first of May), and winter from Samuin to Beltaine”.^[5]

It is not possible to align the Coligny lunar months accurately with modern solar months, but allowing for variation across the years it is likely that the month of MID SAMONIOS began around May–June.

The lunar month

The Coligny calendar as reconstructed consisted of 16 columns and 4 rows, with two intercalary months given half a column each, resulting in a table of the 62 months of the five-year cycle. The 5 years of the calendar plaque is part of a Metonic cycle of 19 years, although it could also be extended to a 30-year cycle. The full length of the calendar is still being debated.

Each lunar year has a 12 lunar months, six months of 30 days and five of 29 days, although not in 29/30 pairs, and one variable month of 29 or 30 days. 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.^[10] Its internal notations are organised according to the phase of the moon.

At the end of the 19-year Metonic cycle, the calendar has overrun the 62-month lunar point by 0.312 days. This would be fixed by reducing an EQUOS month from 30 days back to 29 once every 61 years.

If the plaque was part of a 30-year calendar, it overruns the lunar phase by 0.151 days. This requires a day to be removed (by turning a 30-day EQUOS into a 29-day) roughly once every 198 years. However, the internal months show a larger variation in accuracy for the lunar phase, nearly 48 hours (1.44 to −0.65 ), making the ability to track the lunar phase of 30-years notably less accurate.

The solar year

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.  This is created by 4 Coligny plaques, with the first year dropped.

All the days and their notations are luni-solar and move around within a space of 36 days.

The calendar itself must count in whole days, so 6940 days overruns the sun by 0.398396 and the moon by 0.311620 days.

The functioning of the calendar relies on the lunar months staying in sync with the lunar phase, so the calendar is already adjusting for any lunar difference through the use of the variable day in EQVOS. To keep in sync with the solar year it only needs to adjust for the difference of 0.087 days. Every 276 years this adds one day, but as all the notations are luni-solar and move around within 36 days, this extra solar day would be unnoticeable for many centuries. Eventually the calendar would require a 30-day lunar month to be skipped once every 6,536 years.^[d]

A full cycle of 19 or 30 years

The calendar can perform as a 30-year cycle, by extending the 19-year Metonic cycle to use six 5-year cycles, with a 30-day intercalary month dropped once every 30 years.^[11] But whether it was a 19-year or 30-year calendar is unknown.

Pliny stated that the Celts treated 30 years as an ‘age’,^[e] and this period is seen many times in stories. But it does not necessarily follow that the calendar was then based on a 30-year cycle. 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. For example, Lugnasad might find itself in the middle of the busy harvest.

In a 30-year calendar, the moon finishes only 0.1515 days earlier than the calendar, requiring a day to be removed from EQVOS every 199 years. But the lunar/solar difference is larger at 1.4172 days, requiring a 30-day month to be skipped every 198 years. This relatively fast slippage against the solar year would also add to the already large luni-solar swing, for a total of 75 days before a possible adjustment, further aggravating the solar discrepancy, and displacing seasonal festivals by up to two and a half months. This slippage and inaccuracy indicates that the Coligny calendar is more likely to be a practical 19-year calendar rather than a 30-year cycle.

The start of the lunar month

The calendar month is broken into two halves with the term ATENOVX^[f] between them. The first half-month has 15 days (called a cóicthiges ‘fifteen-days’ in Old Irish, coicís in modern Irish). ^[14] The second half-month has either 15 days, or 14 days with the term DIVERTOMV placed over the space for the 15th day. The notation patterns act as though this 'virtual' 15th day is present.

Pliny reported that the Celtic month began on the ‘6th day of the new moon’.^[12]

The mistletoe, however, is but rarely found upon the oak; and when found, is gathered with rites replete with religious awe. This is done more particularly on the sixth day of the moon, the day which is the beginning of their months and years, as also of their ages, which, with them, are but thirty years. This day they select because the moon, though not yet in the middle of her course, has already considerable power and influence; and they call her by a name which signifies, in their language, the all-healing.

Classical writers counted from the day of the first visible moon, so the 6th day would be the first quarter moon, Day 1, the start of the calendar's month. The quarter moon with its D-shape is the only moment in the lunar phase that is easily identifiable by eye. The internal notations of the calendar confirm Pliny's statement, with a focus on the middle triplet of days in each half-month, days 7-8-9 (the full moon) and days 7a-8a-9a (the dark invisible moon).

The first coicise tracks the gibbous moon, the phase in which the moon is more than half full, it's brightest half. The second coicise tracks the crescent moon, the darker half. Notations, which will govern activities, usually focus in the first, brighter, coicise when the moon has ‘considerable power and influence’. Every odd day in the darker coicise is marked as ‘inauspicious day’.

Full Reconstruction

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.

Sample month

MID SAMONIOS of year 2 is the only month out of 62 that has been preserved without any gaps.^[17]: 182 Currently, most of the patterns of the various notations are known, even if their significance may not be understood. Because of this, most days on the calendar can be reconstructed with confidence.^[g]