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A blue moon is an additional full moon that appears in a subdivision of a year: either the third of four full moons in a season, or a second full moon in a month of the common calendar. (wikipedia)


Blue moon of the December 2009 lunar eclipse

The phrase has nothing to do with the actual color of the moon, although a literal "blue moon" (the moon appearing with a tinge of blue) may occur in certain atmospheric conditions: e.g., when there are volcanic eruptions or when exceptionally large fires leave particles in the atmosphere.

Origin of the term

The suggestion has been made that the term "blue moon" for "intercalary month" arose by folk etymology, the "blue" replacing the no-longer-understood belewe, 'to betray'.

The original meaning would then have been "betrayer moon", referring to a full moon that would "normally" (in years without an intercalary month) be the full moon of spring, while in an intercalary year, it was "traitorous" in the sense that people would have had to continue fasting for another month in accordance with the season of Lent. (wikipedia)


Blue moon of August 31, 2012, viewed from Slobozia, Romania.

Blue Moon Dates

These dates are based on the more well-known definition of a Blue Moon (the second full moon in a calendar month).

1999 January & March
2001 October & November
2004 July
2007 May
2009 December
2012 August
2015 July
2018 January & March
2020 October

http://www.moonconnection.com/blue_moon.phtml

Coatesville, PA July 2015 Cassie Gail Studio

it wasn't really blue over pennsylvania last 31 july 2015

"The most recent blue moon that was truly blue in hue was in Edinburgh, Scotland, in September 1950. Astronomer Robert Wilson of the Royal Observatory observed the event and concluded that the moon was blue in color because the satellite’s light was traveling through a patch of clouds that had particles of smoke and ash from forest fires burning in Alberta, Canada. Those particles crossed the Atlantic Ocean and hovered over Scotland during the lunar event, creating a rare spectacle, according to the National Oceanic and Atmospheric Administration.”

Read More: Grab the Orange Slice, it’s a Rare Blue Moon Tonight | http://1049thehawk.com/grab-the-orange-slice-its-a-rare-blue-moon-tonight/?trackback=tsmclip

once in a blue moon - very rarely

synonyms: hardly ever, almost never, scarcely ever, rarely, very seldom

formula (whatever this means):

once in a blue moon = 1.16699016 × 10-8 hertz