The New Celtic Lunar Calendar

The New Celtic lunar calendar is a notional calendar that I have devised for my my own interest and enjoyment. It is based partly upon the Gaulish "Coligny" calendar, so called because a bronze tablet was discovered near Coligny, France in 1897 that details the design of a lunisolar calendar, i.e. one that attempts to keep in synchronisation with the lunar and solar cycles (see elsewhere on this site for more details). The Celtic calendar is my attempt to create a lunisolar calendar that keeps in synchronisation with these cycles as closely as possible whilst being reasonably easy to use, calculate which years are leap years and so on.

Mean Months and Years

The system used to achieve this synchronicity is based on the 19-year Metonic cycle, whereby the Moon’s phases occur on or aorund the same dates at 19-year intervals. To follow this cycle exactly, though, would result in the months drifting gradually later in the year over a long period of time. This is actually occurring in the Hebrew calendar, whereby the month of Nisan, which contains Passover, is drifting later such that Passover will eventually occur in the summer if it is not corrected. The aim, therefore, is to achieve a mean lunar month as close to the actual mean synodic month and a mean year as close to the actual mean solar year as possible. A pure 19-year cycle with 7 long years (i.e. those with an intercalary month) and 4 abundant years (i.e. those with an additional day) per cycle would give a mean lunar month of 29.53191 days, i.e. 6,940 days divided by 235 lunar months, which is 0.00132 more than the actual mean synodic month of approximately 29.53059 days. If a cycle only has three abundant years, i.e. a total of 6,939 days, then the average would be 29.52776, which is 0.00293 days too short. Over long periods of time the calendar would drift so that it no longer properly corresponded to the correct phases of the moon. With respect to the solar year, a cycle of 6,940 days over 19 years would give an average year of 365.26316 days, which is 0.02078 days too long, whereas a 6,939-day cycle gives a mean year of 365.21053 days, 0.03185 days too short.

Cycles

To achieve greater accuracy, therefore, it is first necessary to use a "grand cycle" which contains multiple 19-year cycles, and then add 3 or 4 days as necessary in each cycle to obtain mean months and years that more closely match reality. This, however, is difficult to achieve using cycles that are exact multiples of 19 years. The answer is to "truncate" one of the cycles from 19 years to 11 years. If this is done in every 18th cycle we get a mean lunar month of 29.53062 days, which is only 0.00003 days longer than the actual mean lunar month, and a year of 365.24252 days, which is only 0.00014 days longer than the actual mean solar year at the present time.

Structure of the Calendar

The precise structure of the calendar is as follows:

  1. A "grand cycle comprises seventeen 19-year cycles and one 11-year cycle, i.e. 18 cycles in all;
  2. If these 18 cycles are arranged in groups of three, in the first fifteen cycles the first cycle of each group of three has 6,940 days, the second has 6,939 and the third has 6,940;
  3. In the last group of the there are 6,940, 6,939 and 4,017 days respectively.
  4. Each of the 19-year cycles has 8 or 9 regular years of 354 days, 3 or 4 abundant years of 355 days and 7 long years of 384 days;
  5. The 11-year cycle usually has 4 regular years of 354 days, 3 abundant years of 355 days and 4 long years of 384 days;
  6. For long-term adjustment, a day is subtracted from the 11-year cycle in every 12th "grand cycle", i.e. roughly once every 4000 years.

The following table shows how each cycle is structured. The first, second and third cycles of each group of three within each grand cycle follow this pattern, and the last column shows the structure of the last cycle of 11 years. Each column shows whether an extra day or an extra month is added to the regular year of 354 days.

Year
1st cycle
2nd cycle
3rd cycle
18th cycle
1
reg
reg
reg
reg
2
+month
+month
+month
+month
3
+day
reg
+day
+day
4
reg
reg
reg
reg
5
+month
+month
+month
+month
6
reg
+day
reg
reg
7
+month
+month
+month
+month
8
+day
reg
+day
+day
9
reg
reg
reg
reg
10
+month
+month
+month
+month
11
reg
+day
reg
+day
12
reg
reg
reg
 
13
+month
+month
+month
 
14
+day
reg
+day
 
15
+month
+month
+month
 
16
reg
+day
reg
 
17
reg
reg
reg
 
18
+month
+month
+month
 
19
+day
reg
+day
 

In every 12th grand cycle, the extra day that is normally added in year 11 is omitted. This adjustment alters the mean lunar month to 29.5306 days, just 0.00001 days of a difference, and a mean solar year of 365.24229, just 0.00008 of a difference. This produces an extremely accuarate calendar with regard to the mean lunar month. However, over time the cycles will occur slightly earlier, such that by the year 10,000 CE they will be occurring approximately two days earlier in the Gregorian calendar than at the beginning of the calendar in the year 1,207 BCE, approximately 11,000 years earlier. Another small adjustment will therefore have to made at some point, but as it stand the celndar is good for several millennia without adjustment and will correspond more than adequately to the phases of the moon for quite some time.

Structure of the Months

The months in the calendar are as follows:

#
Moon Pronunciation Meaning Days
1
Samhain Sah-win/Sa-vin (Summer’s End) 30
2
Dumhainn Doo-in/Doo-vin (Darkening) 29
3
Riùr Ree-oor/Roor (Cold & Ice) 30
4
Naghaid Nah-hid (Staying Home & Storing) 29
5
Uarain Oor-un (Cold’s End) 30
6
Cuithe Kee-huh (Wind) 29 {30}*
7
Geamhain Gyah-win/Gya-vin (Winter’s End) 30
8
Siùfainn Shoo-fin (Brightening) 29
9
Eacha Ech-uh (Horse) 30
10
Eilmì Ell-mee (Claim) 29
11
Aodhrain Air-un/Urr-un (Arbitration) 30
12
Cadal Cad-ull (Song) 29
13
{Eadràn} Ed-rahn (Intercalary) {30}**
* Has 30 days in years 3, 8, 14 & 19 in every first and third cycle in each group of three, years 6, 11 and 16 of the second cycle out of every three, and years 3, 8 and 11 in every eighteenth cycle.
** A leap month is added in years 2, 5, 7, 10, 13, 15 and 18 of every cycle, apart from every 18th cycle, when the month is added in years 2, 5, 7 and 10.

Epoch and New Year Dates

The epoch of the calendar is 5th November -1042 in the proleptic Gregorian calendar (i.e. the date as it would be if the Gregorian calendar were continued back from the date of its first adoption in 1582 CE and using astronomical year numbering, which includes a Year 0). This date has been chosen because it is close to the mid-point between the autumnal equinox and the winter solstice, or the mid-point of the astrological star sign of Scorpio, which is when the ancient Celtic festival of Samhain takes place, marking the beginning of winter. The pattern of leap years within each cycle is intended to ensure that the start dates occur roughly within the date range 22nd October to 20th November, and also to keep the winter and summer solstices in the months of Dumhainn and Siùfainn in most years. Sometimes dates may drift outside this range, and in the long term the order of the leap years within each cycle may change to keep the start dates within this time frame, as far as possible. The following table shows the new year dates for the years in the previous, current and next cycles.

Dates can be shown in the form of the Celtic Count (see below) or in the traditional "date month-name year" format. Using this format the date at the time of writing (17th December 2012 CE) is 165·11·2·4, or 4th Dumhainn 3055 N.C.C.

Year
Cycle 163
Cycle 164
Cycle 165
1
5 Nov, 1964
6 Nov, 1983
5 Nov, 2002
2
25 Oct, 1965
25 Oct, 1984
25 Oct, 2003
3
13 Nov, 1966
13 Nov, 1985
12 Nov, 2004
4
3 Nov, 1967
2 Nov, 1986
2 Nov, 2005
5
22 Oct, 1968
22 Oct, 1987
22 Oct, 2006
6
10 Nov, 1969
9 Nov, 1988
10 Nov, 2007
7
30 Oct, 1970
30 Oct, 1989
29 Oct, 2008
8
18 Nov, 1971
18 Nov, 1990
17 Nov, 2009
9
7 Nov, 1972
7 Nov, 1991
7 Nov, 2010
10
27 Oct, 1973
26 Oct, 1992
27 Oct, 2011
11
15 Nov, 1974
14 Nov, 1993
14 Nov, 2012
12
4 Nov, 1975
4 Nov, 1994
3 Nov, 2013
13
23 Oct, 1976
24 Oct, 1995
23 Oct, 2014
14
11 Nov, 1977
11 Nov, 1996
11 Nov, 2015
15
1 Nov, 1978
31 Oct, 1997
31 Oct, 2016
16
20 Nov, 1979
19 Nov, 1998
19 Nov, 2017
17
8 Nov, 1980
9 Nov, 1999
8 Nov, 2018
18
28 Oct, 1981
28 Oct, 2000
28 Oct, 2019
19
16 Nov, 1982
16 Nov, 2001
15 Nov, 2020

The evening before new year in the calendar is called Oìdhche Shamhna (pronounced eech-uh how-nuh), meaning "Night of Samhain", and new year’s day is called A’ Bhliadhna Ùr (uh vlee-uh-nuh oor), "The New Year".

Dates and the Celtic Count

Dates are written in the normal fashion, i.e. “date month, year”. If a numeric format is preferred, however, then the Celtic Count is used. The Celtic Count is a series of four numbers, separated by full stops (or more correctly, dots), denoting the cycle, year, month and day. Knowledge of the current Celtic Count date allows for the number of days there are in the current year or month and the number of months there are in the current year, according to the rules of the calendar as related above.

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