You can enjoy the full Moon this evening, August 1 (10:27 PM CDT) and again on August 31.  

You’ve probably heard the phrase “once in a blue moon” used to describe a rare occurrence or event.  It has become popular to refer to the second full Moon in a month as a blue moon because it does occur relatively infrequently.  However, at once every 2.5 years or so, it is certainly not as rare as some astronomical events such as a transit of Venus for example.  As it turns out, this usage is perhaps based on a misinterpretation that was published in Sky & Telescope magazine in 1946.  You can read about it here:

http://www.skyandtelescope.com/observing/objects/moon/3304131.html

So what does a blue moon actually refer to?  Well, it could refer to a Moon that literally appears blue due to atmospheric phenomena such as particulates being present.  This is indeed a rare occurrence.  The article above goes on to provide another interpretation used by the Maine Farmers’ Almanac that defines a blue Moon as the third full Moon in a season that contains four full Moons.  Since a season is traditionally about three months, seasons that contain four full Moons would be worth noting.  This interpretation is complicated by different definitions of “seasons.”  Seasons can be defined by the solstices and equinoxes, or, as in the Maine Farmers Almanac, they can be defined by historical calendar systems. 

So here’s the kicker, the August 31 full Moon in 2012 is technically not a blue moon – the second full Moon in a month is not a formal definition of a blue moon and the next full Moon occurs after the September equinox meaning only three occurred in our summer season.  Unless of course, the August 31 full Moon actually appears blue.