etymology - Why do we call some full moons "blue" when they're not?


I've heard the phrase "once in a blue moon" used to mean "once in a great while". Looking it up on Wikipedia revealed that "blue moon" originally meant the third full moon in a season with four full moons, but that a popular mistake caused it to mean the second full moon in a calendar month (which is a definition I'd heard before).


But that didn't dispel all my curiosity: I've seen a "blue moon", and it's not any bluer than any normal full moon. So why is it called "blue"? Wikipedia becomes vague here:



The most literal meaning of blue moon is when the moon (not necessarily a full moon) appears to a casual observer to be unusually bluish, which is a rare event. The effect can be caused by smoke or dust particles in the atmosphere, as has happened after forest fires in Sweden and Canada in 1950 and 1951, and after the eruption of Krakatoa in 1883, which caused the moon to appear blue for nearly two years. Other less potent volcanos have also turned the moon blue.
...The key to a blue moon is having lots of particles slightly wider than the wavelength of red light (0.7 micrometre)--and no other sizes present. This is rare, but volcanoes sometimes produce such clouds, as do forest fires.



What does the color of the moon after forest fires or volcanoes have to do with its name when it happens to be the third full moon in a season with four full moons?



Answer



This Sky & Telescope article attributes it to the Christian ecclesiastical calendar:



The ecclesiastical vernal (spring) equinox always falls on March 21st, regardless of the position of the Sun. Lent begins on Ash Wednesday, 46 days before Easter, and must contain the Lenten Moon, considered to be the last full Moon of winter. The first full Moon of spring is called the Egg Moon (or Easter Moon, or Paschal Moon) and must fall within the week before Easter.


At last we have the "Maine rule" for Blue Moons: Seasonal Moon names are assigned near the spring equinox in accordance with the ecclesiastical rules for determining the dates of Easter and Lent. The beginnings of summer, fall, and winter are determined by the dynamical mean Sun. When a season contains four full Moons, the third is called a Blue Moon.



This article traces the etymology of "blue moon:"



"Blue moon" appears to have been a colloquial expression long before it developed its calendrical senses. According to the Oxford English Dictionary, the first reference to a blue moon comes from a proverb recorded in 1528:


If they say the moon is blue,
We must believe that it is true.

Saying the moon was blue was equivalent to saying the moon was made of green (or cream) cheese; it indicated an obvious absurdity. In the 19th century, the phrase until a blue moon developed, meaning "never." The phrase, once in a blue moon today has come to mean "every now and then" or "rarely"—whether it gained that meaning through association with the lunar event remains uncertain.



Neither source mentions possible atmospheric phenomena that can absorb red light and turn the moon blue.


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