Originally “blue moon”
referred to an absurdity according to
the Oxford English Dictionary which notes a reference to a proverb from 1528 “If
they say the moon is blue, we must believe that it is true”. However the moon can actually appear blue.
Although it happens only
rarely when small particles in the atmosphere scatter more red light than blue allowing
less red light to be seen from the ground the moon temporarily
appears blue. Although this phenomenon is
quite rare it has altered the meaning of the phrase “once in a blue moon” which
now commonly is understood to mean “every now and then”.
The term also often refers to the second full
moon in a given month. In an article published
in the Sky & Telescope magazine in 1946 James H. Pruett used the phrase to
refer to the second full moon to occur during a given month. While this was a misinterpretation of the original
meaning for “blue moon” after a couple of decades of popular use (or should
that be misuse?) we now also consider the 13th full moon in a year
to be a “blue moon”.
The next blue moon, according to the Sky & Telescope convention, will occur in June 2007.
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