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Blue moon

There are four uses of the term blue moon:

• something that does not exist, similar to a moon made of green cheese.
• When the moon appears blue from earth, caused by dust in the atmosphere. Often coincides with events such as volcanic eruptions, monsoons and forest fires.
• The third of four full moons in one season (this had implications for the date of some church festivities, which had to take place at the first or last full moon of a season; still duly noted in almanacs from Maine, USA in the 19th century).
• The second full moon within one calendar month (originated in Sky & Telescope magazine in 1946, as a result of a misunderstanding of the definition in the Maine Farmers' Almanac).

The term has been found in proverb "If they say the moon is blue, We must believe that it is true." dating as far back as 1528 according to Oxford English Dictionary and it is also a common metaphor for an impossible, or rare, event — once in a blue moon. The literary and idiomatic usages do not refer to the recently invented definitions.

Blue moons in 2004 through 2010:

• Based on the meaning behind the old sayings — none
• Due to atmospheric conditions — unpredictable
• 31 July 2004 — Second full moon in July
• August 2005 — Third full moon in a season of four full moons
• June 2007 — Second full moon in June
• May 2008 — Third full moon in a season of four full moons
• December 2009 — Second full moon in December
• November 2010 — Third full moon in a season of four full moons