The Thirteenth Moon
Taken from Space.com, an alternate definition of the term "Blue Moon".
On the almanac page for August 1937, the calendrical meaning for the term "blue moon" was given.
That explanation said that the moon "... usually comes full twelve times in a year, three times for each season."
Occasionally, however, there will come a year when there are 13 full moons during a year, not the usual 12. The almanac explanation continued:
"This was considered a very unfortunate circumstance, especially by the monks who had charge of the calendar of thirteen months for that year, and it upset the regular arrangement of church festivals. For this reason thirteen came to be considered an unlucky number."
And with that extra full moon, it also meant that one of the four seasons would contain four full moons instead of the usual three.
"There are seven Blue Moons in a Lunar Cycle of nineteen years," continued the almanac, ending on the comment that, "In olden times the almanac makers had much difficulty calculating the occurrence of the Blue Moon and this uncertainty gave rise to the expression 'Once in a Blue Moon.'"
On the almanac page for August 1937, the calendrical meaning for the term "blue moon" was given.
That explanation said that the moon "... usually comes full twelve times in a year, three times for each season."
Occasionally, however, there will come a year when there are 13 full moons during a year, not the usual 12. The almanac explanation continued:
"This was considered a very unfortunate circumstance, especially by the monks who had charge of the calendar of thirteen months for that year, and it upset the regular arrangement of church festivals. For this reason thirteen came to be considered an unlucky number."
And with that extra full moon, it also meant that one of the four seasons would contain four full moons instead of the usual three.
"There are seven Blue Moons in a Lunar Cycle of nineteen years," continued the almanac, ending on the comment that, "In olden times the almanac makers had much difficulty calculating the occurrence of the Blue Moon and this uncertainty gave rise to the expression 'Once in a Blue Moon.'"