Blue moon is called a full moon that occurs in unusual time. Most years have twelve full moons, most times a month. However each calendar year contains twelve lunar cycles and eleven days yet. These results are summed and every two or three years have an extra moon, a phenomenon that happens every 2.72 years. Different interpretations place the extra moon at different times, but this extra full moon in each case called "blue moon."

The term has a metaphorical meaning, emphasizing the rarity of a phenomenon.  In the English expression «once in a blue moon», which means something that happens very rarely. At the other end there is a very literal sense as a moon may actually be blue, because of atmospheric disturbances. This is a very rare phenomenon in which the moon (not necessarily a full moon) appears in the eyes of the observer bluish due to the presence of smoke or dust particles in the atmosphere. This occurred after forest fires in Sweden and Canada in 1950 and, notably, after the eruption of the volcano Krakatoa in 1883, which entailed the moon to appear blue for a period of two years.


Between the years 2005 and 2015 are expected the following blue moons:

A. Considering blue moon on the third full moon in a season four full moon:

• August 19, 2005
• May 19, 2008
• November 21, 2010
• August 21, 2013