from The American Heritage® Dictionary of the English Language, 5th Edition.

  • noun The third full moon in a three-month calendrical season that has four full moons.
  • noun The second of two full moons occurring in the same month.
  • noun Informal A relatively long period of time.

from the GNU version of the Collaborative International Dictionary of English.

  • The second full moon occurring in the same month; -- derived from the expression once in a blue moon.
  • very rarely; -- from the observation that the moon rarely has a bluish tint.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

  • noun The second of two full moons that occur in the same calendar month.
  • noun rare The moon tinted towards blue as it appears in the sky, caused by dust or smoke in the atmosphere.
  • noun archaic Something absurd.
  • noun obsolete The third full moon in a quarter that contains four rather than the usual three full moons.
  • noun idiomatic (by extension) A long time.
  • noun A flavor of sweet ice cream that is popular in the Midwestern U.S. and is a component of Superman ice cream.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

  • noun a long time


from The American Heritage® Dictionary of the English Language, 4th Edition

[Sense 2, probably from the rare occurrence whereby the moon appears blue from high amounts of dust in the atmosphere, as from a volcanic eruption.]

from Wiktionary, Creative Commons Attribution/Share-Alike License

Mistake in Sky & Telescope magazine in 1946, retracted nearly 60 years later.


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  • From a glossary provided by the American Meteorological Society:

    blue moon: 1. Those rare occasions when the moon takes on a markedly blue cast.

    Usually, when the moon (or sun) is seen low in the sky or even high in a polluted one, it is yellowish or reddish. This is a consequence of the greater extinction of short wavelength radiation by small particles and molecules. Yet, there are particle sizes for which extinction is greater for long wavelengths, and the moon seen through a sky populated by such particles is bluish. It is rare that a large population of the particles of the appropriate size (and only that size) are produced, so the blue moon is very rare: 1883 (Krakatoa), 1927 (a late monsoon in India), 1951 (forest fires in Alberta, Canada). Because of the rarity of blue moons, the phrase, “once in a blue moon,�? has been used metaphorically since the midnineteenth century to denote a real, but rare event.

    2. In recent times, often used incorrectly to mean the occurrence of a second full moon within a calendar month.

    This strange new use of the term, popularized by a board game in the mid-1980s, arose from a mistake made by the author of a magazine article written in the 1940s. Since two full moons in one month occur fairly regularly, it should not be considered a rare event.

    I love the dry tone of "this strange new use" and the failure to identify the board game, presumably Trivial Pursuit.

    February 25, 2007

  • Yikes! I listed a monster!"

    February 25, 2007

  • Jeez, oroboros, I'd a thunk someone as word-loving as yourself would have an attention span that was good for two paragraphs, at least.

    February 25, 2007

  • Blue Moon ice cream is the name of a number of bright blue colored flavors of ice cream. All of the varieties of Blue Moon are available almost exclusively in the mid-west United States.

    Blue Moon is found mainly in Michigan, Minnesota, and Wisconsin; found less frequently in Indiana, Iowa, Illinois and Ohio; and has reportedly been available (either in the past or currently) in Missouri and even western Pennsylvania. A possibly similar, possibly identical flavor has also been sold in Italy under the name "Puffo", which is Italian for "Smurf"; it was probably given this name because its blue color resembled the Smurfs.

    The varieties of Blue Moon vary in both color and flavor. Many aficionados of each variety of Blue Moon claim that their variety is the "real one," the "original," etc. Some dairies that make Blue Moon keep their ingredients a secret, adding to the mystique.

    A number of different sources on the internet claim to know what flavor Blue Moon is. Since these sources disagree with each other, it is unclear which, if any, are correct.


    January 27, 2008

  • *earworm alert*

    Ba ba bomp ba ba bomp ba dangy dang dang ba dingy dong ding Blue Moooooon....

    January 27, 2008

  • "You saw me standing aloooooonne...."

    January 28, 2008

  • Without a song in my heaaaaaaart, without a love of my ooooooooown!

    September 12, 2008

  • Finally, more than two years later, we are in a position to reveal the correct definition of the phrase blue moon.

    A BLUE MOON is the third full moon in a season with four full moons.

    According to our good buddies at Sky and Telescope, from which the diagram above is taken:

    When is the Moon 'blue,' in a calendrical sense? According to the Maine almanac, a Blue Moon occurs when a season has four full Moons, rather than the usual three. This type of Blue Moon is found only in February, May, August, and November, one month before the next equinox or solstice. According to modern folklore, a Blue Moon is the second full Moon in a calendar month. This type of Blue Moon can occur in any month but February, which is always shorter than the time between successive full Moons.

    For full details, see Sinnott, Olson & Fienberg

    March 15, 2009