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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?

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Duplication of english.stackexchange.com/questions/8877/… ? –  oosterwal Sep 29 '11 at 18:36
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No; mine is much better, and hence not a duplicate. B-) –  Daniel Sep 29 '11 at 22:50
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Seriously, though, my question goes further, almost asking a different question, and the answer to that question doesn't satisfactorily answer mine. –  Daniel Sep 29 '11 at 22:56
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+1 for "much better." –  oosterwal Sep 30 '11 at 1:52

3 Answers 3

up vote 9 down vote accepted

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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It seems the term blue moon has been around for over 400 years, but originally it just meant something ridiculous (akin to the moon is made of green cheese/pigs might fly/etc.). The fact that unusual atmospheric conditions may impart a blue tinge to moonlight is apparently not relevant. Over time, the meaning morphed from ridiculous to very rare.

In 1819 The Maine Farmers' Almanac came up with the definition cited by OP, and from then on, it listed "blue moon" dates every year. But the dates they list often don't even meet their own definition, which is somehow bound up with the Christian ecclesiastical calendar.

Insofar as it's a meaningful concept in respect of actual lunar cycles (which I don't really accept), we normally define blue moon today as any second full moon occurring within a single calendar month (i.e. - the first full moon must have occurred in the first few days of that month).

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@drɱ65: Apparently it doesn't make much difference which definition you use. We get a blue moon about once every 2.5 years on average. So there have probably been a few more since the one in Nov 2001 linked to there, but obviously we didn't see them! –  FumbleFingers Sep 29 '11 at 22:49

Blue moon refers to two full moons within the same calendar month. These are rare because the ion cycle of approximately 29.5 days usually occurs once within a 30 or 31 day month. The blue moon can appear during any season; usually about once every three years.

Obviously, it has nothing to do with the color, although dependent upon atmospheric condition, any moon can take on a slight blue tinge.

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I know what it means - I didn't know why it's called blue. –  Daniel Sep 25 at 3:28

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