Once in a Blue Moon
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Farmer's Almanac
An older definition for the Blue Moon is recorded in an issue 150 years ago
of The Farmer's Almanac from Maine, USA. According to this definition, the
Blue Moon is the third full moon in a season that has four full moons. Why
would one want to identify the third full moon in a season of four full
moons? Some years have an extra full moon—thirteen instead of twelve.
Since the identity of the moons was important in the ecclesiastical calendar
(the Paschal Moon, for example, was crucial for determining the date of
Easter), a year with a thirteenth moon upset the Christian calendar, since
there were names for only twelve moons. By identifying the thirteenth moon
as a Blue Moon, the church calendar was able to be consistent.
The very idea of a "calendar month" is a relatively recent
concept, as months were originally measured by the period between lunations,
creating a Lunar Calendar of some 13 months per year (the word
"month" comes from "moon"). A cultural revolution took
place in the Classical Period, when the ancient rule of the lunar goddesses
was replaced by the rule of the solar gods. This also saw the development of
the solar year of 12 months, based on the seasons and connected with the 12
signs of the Zodiac. The true origins of the phrase 'Blue Moon' are really
much, much older. In the ancient Lunar Calendars, time was measured
according to moon cycles, change of seasons and the period between equinoxes
and solstices.