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Understanding the Metonic cycle

Unlike the Gregorian calendar that we all know and use, the Druidcraft calendar reconciles the cycles of the moon with the cycles of the sun, based on the 19-year cycle recorded by and named after Meton of Athans. Meton, like the Babylonians before him noticed that 19 solar years is almost the same as 255 synodic months (lunations of the moon). This 19-year cycle of 255 months is known as the great year.

This means that rather than having months all of different lengths (months of 28/29, 30 and 31 days), the Druidcraft Calendar follows the phases of the moon, starting each new month on the new moon and alternating between months of 29 and 30 days.

The reason we alternate between 29 and 30 days is because the actual time between one new moon and the next (the lunar synodic month or lunation period – the time it takes the moon to orbit the earth in relation to the sun) is 29.53 days on average. So, by adding a 30th day every other month, our months stay in sync with the lunar cycle.

This results in a lunar year of 12 months totalling 354 days, which is 11 days shorter than the solar year. To reconcile this, a 13th leap month is introduced approximately every two and a half years to bring the months back in line with the solar seasons. In the Metonic system, this means that a 13th month is included in the calendar 7 times in a 19-year period. In the 3rd, 6th, 8th, 11th, 14th, 17th and 19th years.

When there are 4 full moons in an astronomical season or between an equinox and a solstice, the 3rd full moon is known as a Blue Moon. When there are 4 new moons in between an equinox and solstice, this is when we add the 13th month. In most years there are only 3 new moons in each astronomical season. When we add a leap month, it is always 30 days long.

Of the 255 months in the 19-year Metonic cycle if half of them are 30 days long, and half are 29 days long, that is 117.5 months of each length. First, we round up the 0.5 make that 117 short months and 118 long months. Now we apply a rule that the 7 extra leap New Moons are all 30 days long, and any month following a leap moon is also 30 days long. Finally, we apply the rule that the 7th and 16th years must always start with a 30-day month. This results in a pattern of 110 short months and 125 long months in each 19-year period. This keeps the lunar and solar cycles in sync, gaining only one day every 219 years.

In summary, in order to follow this pattern, use the following rules:

  1. All Leap Moons are 30 days long
  2. The month after a Leap Moon should also be 30 days long.
  3. The first month in the 7th and 16th year should be 30 days long.
  4. At all other times months should alternate between 29 and 30 days in length.

No particular set of month names has been selected for the Druidcraft Calendar so that you can use whatever moon names are most appropriate for your practice. Lists of month names from around the world can be found in appendix A. For the purpose of this guide I have chosen to use names based on the astronomical season in which the new moons occur. In most years there are 12 months, and all the leap moons are skipped. During a leap year, the 13th month can be included in whichever season it happens to fall.

Winter Solstice
First Winter Moon
Second Winter Moon
Leap Winter Moon
Last Winter Moon
Spring Equinox
First Spring Moon
Second Spring Moon
Leap Spring Moon
Last Spring Moon
Summer Solstice
First Summer Moon
Second Summer Moon
Leap Summer Moon
Last Summer Moon
Autumn Equinox
First Autumn Moon
Second Autumn Moon
Leap Autumn Moon
Last Autumn Moon

Leap moons and blue moons can occur during any season, but they are much more likely to occur during the summer or spring. This is because the Earth’s orbit around the Sun is not even.

This orbit results in the seasons being slightly different lengths

Winter: 88.99 days
Spring: 92.75 days
Summer: 93.65 days
Autumn: 89.85 days

The average time between the appearance of a new moon and then three more is 88.59 days, which is 0.4 days shorter than winter which is the shortest season, and 5.6 days shorter than summer, the longest season. This means that in an average Metonic cycle the 13th moons are included in the following seasons

year 3 – Summer
year 6 – Spring
year 8 – Summer or Autumn
year 11 – Summer
year 14 – Spring
year 17 – Spring
year 19 – Summer

Roughly every 19.52 metonic cycles or great years (371 solar years) there will be only 2 winter New Moons. This anomaly results in a leap month both in the autumn preceding and the spring following the 2 moon winter. The consequence of this is an 8th extra leap month in the metonic cycle disrupting the pattern of which years the 13th New Moon will appear in. This can be easily resolved as there are warning signs that this 2 moon winter is approaching.

Approximately 2 cycles before the 2 moon winter will occur there will be no leap moon in the 8th year. Instead it will appear in the 9th year. When this happens, the current cycle must be ended early, and a new cycle is started. The current cycle is continued adding another leap moon in year 11. The following year this cycle is ended, and a new cycle is started. Year 12 becomes year 1 of the next cycle. This new cycle will follow the standard pattern. After that there will be up to 4 cycles that do not fit the standard pattern. The first cycle, and sometimes the second will have an 8th leap moon in the 16th year. Following this there will be 2 more cycles where the leap moon moves from the 17th to the 16th year. Once these 3 or 4 cycles have concluded, the standard Metonic sequence is resumed for 15 or 16 cycles until the next time the leap moon occurs in the 9th year.

The last time the leap moon moved to the 9th year was in 2015. 2018 marked the early ending of that cycle and the start of the next. When our current cycle ends in 2036, the next 4 cycles will not fit the standard metonic pattern as described above.

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