http://en.wikipedia.org/wiki/Blue_moon

Quote - Wiki - A blue moon can refer to the third full moon in a season with four full moons.

Quote - Miss Nancy - a blue moon is when there are two full moons in the same month

Both are correct - it amounts to 13 full moons in a year. For this to happen, it is necessary to have four in a season and two in a single month.

However, there are lots of months with two full moons that are not blue moons because there are still only 12 in that year. This is a simple consequence that a lot of months are 31 days.

So - a blue moon is not defined by two full moons in a month but four in a season or quarter.

In any case, it is rare.

Furthermore, what I was referring to as atmospheric events is described in the wiki article:

Quote - The most literal meaning of blue moon is when the moon (not necessarily a full moon) appears to a casual observer to be unusually bluish, which is a rare event. The effect can be caused by smoke or dust particles in the atmosphere, as has happened after forest fires in Sweden and Canada in 1950 and 1951,[10] and after the eruption of Krakatoa in 1883, which caused the moon to appear blue for nearly two years. Other less potent volcanos have also turned the moon blue. People saw blue moons in 1983 after the eruption of the El Chichon volcano in Mexico, and there are reports of blue moons caused by Mt. St. Helens in 1980 and Mount Pinatubo in 1991.

 

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