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 235 synodic months (lunations of the moon). This 19-year cycle of 235 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 13^{th}
month is included in the calendar 7 times in a 19-year period. In the
3^{rd},
6^{th},
8^{th},
11^{th},
14^{th},
17^{th}
and 19^{th}
years.

When
there are 4 full moons in an astronomical season or between an
equinox and a solstice, the 3^{rd}
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 13^{th}
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 235 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 7^{th} and 16^{th} 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:

- All Leap Moons are 30 days long
- The month after a Leap Moon should also be 30 days long.
- The first month in the 7th and 16th year should be 30 days long.
- 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 13^{th} 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 8^{th}
extra leap month in the metonic cycle disrupting the pattern of which
years the 13^{th}
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 8^{th}
year. Instead it will appear in the 9^{th}
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 8^{th}
leap moon in the 16^{th}
year. Following this there will be 2 more cycles where the leap moon
moves from the 17^{th}
to the 16^{th}
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 9^{th}
year.

The
last time the leap moon moved to the 9^{th}
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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Next: The sun and moon