DEAR SUN SPOTS: What is a blue moon? — J.G., Leeds

ANSWER: According to Wikipedia, there are several explanations, but the most widely accepted is actually due to a mistaken interpretation of the Farmers’ Almanac.

In explaining the term blue moon, Wikipedia begins with astronomy:

“Most years have twelve full moons that occur approximately monthly. In addition to those twelve full lunar cycles, each solar calendar year contains roughly eleven days more than the lunar year of 12 lunations. The extra days accumulate, so every two or three years (seven times in the 19-year Metonic cycle), there is an extra full moon. Lunisolar calendars have rules about when to insert such an … ‘leap’ month, and what name it is given …. The term blue moon comes from folklore. Different traditions and conventions place the extra blue full moon at different times in the year.”

Also from Wikipedia are these explanations of the naming of a blue moon:

* “In calculating the dates for Lent and Easter, the clergy identify the Lent moon. It is thought that historically when the moon’s timing was too early, they named an earlier moon as a “betrayer moon” (belewe moon), thus the Lent moon came at its expected time.”

* “Folklore gave each moon a name according to its time of year. A moon that came too early had no folk name, and was called a blue moon, retaining the correct seasonal timings for future moons.”

* Another explanation is when a moon, not even necessarily full, is shaded blue by particles in the air from pollution, volcanic eruptions, etc.

Sun Spots thought that a blue moon was the second full moon in a single month, and it turns out she was right, sort of, since this definition is now widely accepted even by the group whose original explanation was misinterpreted.

The mistake occurred when a scientist misinterpreted the Farmers’ Almanac definition in a March 1946 Sky and Telescope article. The Farmers’ Almanac had originally adopted an ancient explanation that defined the blue moon as an extra full moon that occurred in a season (spring, summer, fall, winter). One season was normally three full moons. If a season had four full moons, then the third full moon was named a blue moon.

The Sky and Telescope writer said it was the second full moon in a single month. Although this differs from the Farmers’ Almanac original definition, according to farmesalmanac.com, that definition has become widely accepted and is now adopted by the Farmers’ Almanac as well as being commonly believed.

As for the next blue moon, Wikipedia and the Farmers’ Almanac differ on when the next blue moon will be. Wikipedia says that according to the Farmers’ Almanac the next blue moon will be Aug. 21, 2013.

But when Sun Spots went to the Farmers’ Almanac moon calendar (all of the moons have different names, such as strawberry moon, harvest moon, etc.; see http://tinyurl.com/chh686u for a list), it said the next one would be Aug. 31, 2012.

This goes to show you that no one source should ever be completely relied on!

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