The Celtic Lunar Calendar

This is a notional calendar that I have devised but it is based upon the knowledge that we have of how the Celts in the north-west of Europe kept track of the lunar cycles to organise their year and their monthly festivals.

The ancient Celts used primarily the cycles of the moon to organise their time, festivals and harvesting etc, because this was the most obvious thing marking out the passage of time. The Celts, like many before them, had studied the characteristics of the lunar cycle over hundreds, perhaps thousands, of years, and they knew these characteristics very well. They knew of the 19-year cycle in which the dates of the lunar phases fall on the same dates. They also knew of the 18.6-year Saros cycle, which affects the occurrence of eclipses and the major and minor lunar standstills. It is thought that many of the stone circles in Scotland were designed to keep track of these events.

It is not, therefore, surprising that they used these inconstant, but predictable lunar events to mark the passage of time. Of course, the tropical year was still important, particularly for the purposes of planting and harvesting etc, which is why their calendars were designed to be luni-solar , i.e. adjusted to ensure that they kept synchronisation with the tropical year (because a lunar year of 12 months lasts only about 354 days - 11 short of a tropical year). This means that an extra, intercalary lunar month had to be inserted into the calendar every now and then. Different calendars use different rules to determine when these additional months are inserted.

The 19-year Cycle

The system used in this calendar is that the month of Samhain, which normally begins the year, begins in late October or early to mid-November in the solar calendar. This will, of course, drift as the lunar year is only 354 days long, so an extra month, Eadrán, is inserted at the end of the year every 2-3 years to keep everything in synchronisation. Having studied the cycles, it was worked out that the extra, or intercalary, months should be added in years 2, 5, 7, 10, 13, 16 and 18 of each 19-year cycle. There is also an extra day in years 4, 9, 14 and 19 of the calendar, although the extra day in year 19 is omitted in some cycles. Further small adjustments are made over periods of hundreds of years to keep it all synchronised, but the basic cycle looks like the following:

One way of looking at the pattern is that, if, for each leap year, you work out the number of years since the previous leap year, the pattern is 3-3-2-3-3-3-2 in every 19-year cycle. As regards the leap days, these happen every five years within the cycle, however the final year of the cycle is the odd one. It has a leap day in all cycles except those evenly divisble by three but not divisble by 45. So, in the current cycle (164), year 19 will have a leap day. In the next cycle (165) it will only have 354 days. The next time that a year 19 in a cycle divisible evenly by three will have a leap day is in cycle number 180.

Months

The months in the calendar are as follows:

Dates

The tables below show the starting dates of the months in the calendar over the next three years:

Celtic Year 12 begins 7th November 2010.

Dates in the calendar are shown as a series of four numbers, separated by full stops, denoting the cycle, year, month and day. Optionally, the month name may be added at the end as well, e.g., today's date is .

Other dates in the Celtic calendar can be worked out here.