Blue moons only happen once in a while, but there's a difference between a calendrical blue moon and a zodiacal one, ...
Blue moons occur every 2.7 years on average. Friday is your last chance to see one until July 31, 2015. This Friday's full moon is known as a "blue moon," the second full moon in the calendar month.
A rare Blue Moon will appear on May 31 as the second full moon of the month, the only Blue Moon this year, highlighting how ...
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