Once In A Blue Moon.....

This week we had a blue moon!  That is when there is 2 full moons in a 30 day period!  Blue moons don't happen often.  Each calendar year contains roughly 11 days more than the number of days in 12 lunar cycles. The extra days accumulate, so every two or three years (7 times in the 19-year Metonic cycle), there is an extra full moon. The extra moon necessarily falls in one of the four seasons, giving that season four full moons instead of the usual three, and, hence, a blue moon.