What's A Blue Moon?

Once in a Blue Moon

The full moon that you saw on Nov. 21, 2010, looked like an ordinary full moon, but it was actually a bit unusual, in fact it was a “blue moon”.

Perhaps the most literal meaning of blue moon is when the moon (not necessarily a full moon) appears to a casual observer to be bluish in color, which is actually a rare event.  The effect may be caused by smoke or dust particles in the atmosphere, which may occur after very large forest fires for example.     Other causes of record include events such as: the eruption of Krakatoa in 1883, which caused the moon to appear blue for nearly two years.  Other less potent volcanoes have also turned the moon blue.   For example people saw blue moons after the eruption of Mt. St. Helens in 1980, and Mount Pinatubo in 1991.

There are generally two accepted definitions for a blue moon. The more recent definition (and the one I can keep track of) is when a full moon occurs twice in a given calendar month. The first of such full moons must appear at or near the beginning of the month so that the second full moon, the “blue one” will fall within the same month (the average span between two moons being 29.5 days).

The most resent and perhaps the most unusual “Calendar Blue Moon” occurred on New Year’s Eve, December 31, 2009; the next “New Year’s Eve” calendar occurrence of a Blue Moon will not be until 2028.  We will see a typical “Calendar Blue Moon” again on August 31, 2012 and again on July 31, 2015.   

The older definition / type (which is a little more complex to keep track of)  states that a blue moon is the third full moon in a season (such as summer or winter) that has four full moons instead of three.  The full moon which occurred on Nov. 21, 2010, was this type of blue moon; it was the third of four full moons between the fall equinox and the winter solstice.

Now that you know more about Blue Moon’s than the average “Joe”, perhaps the phrase "once in a blue moon" now has a more significant meaning.


Sources ...

http://en.wikipedia.org/wiki/Mt._St._Helens    

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

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

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

http://urbanlegends.about.com/od/errata/a/blue_moon.htm