Rare Event

I awoke yesterday morning to find a beautiful full moon slowly cruising toward the horizon across Lake Esau.  But this was no ordinary full moon cutting its glittering wake across the windswept surface of the lake.  It was a blue moon.

Blue Moon over Lake Esau, Aug. 21, 2013.

A blue moon is generally defined as the second full moon in a solar calendar month.  This occurs once every two or three years, and gives TV weather folk something different to talk about.  Blue moons are a surprisingly complicated topic, because people disagree about what moons are to be considered blue! The conflict arises from the fact that the solar and Christian calendars are not identical.  Check out the Wikipedia discussion!

The blue moon of August 31, 2012, as seen from
Slobozia, Romania.  (Wikimedia image)

The term blue moon seems to result from the calculations needed to set the date of Easter.  When two full moons occurred in a critical month, and thereby messed up the procedure for figuring out when Easter would fall, the first of these full moons was called a betrayer (or belewe) moon.  In time this quisling became the blue moon.  The term "once in a blue moon" recognizes its relative rarity — roughly seven times every 19 years. •••