Over the past few nights, you might have noticed the moon.  It’s been waxing or getting larger all week and tomorrow it will be officially full.  A full moon is not really special, after all we get one every month.  Now and again, we are treated to two full moons in a month.  On August 2nd, the moon was full, and it will be again tomorrow.  

Some would call that a Blue Moon.  Some might say that’s incorrect.

There are in fact two definitions for a blue moon. According to the more recent definition, a blue moon is the second full moon in a calendar month. For a blue moon to occur the first of the full moons must appear at or near the beginning of the month so that the second will fall within the same month; the average span between two moons is 29.5 days. The full Moon on August 31 will be this type of blue moon: the second full moon in a calendar month.

The older definition, which is recorded in early issues of the Maine Farmer's Almanac, states that the blue moon is the third full moon in a season that has four full moons. Why would you want to identify the third full moon in a season of four full moons? The answer is complex, and has to do with the Christian ecclesiastical calendar.

How rare is it?   Over the next 20 years there will be about 15 blue moons, with an almost equal number of both types of blue moons occurring.  I thought it was interesting to note that there was no blue moon of any kind last year, and there will be no blue moon in 2014 or 2017. 

So which definition is "correct"?   Experts agree that the second, more complex definition is correct, but with two decades of popular usage behind it, the second-full-Moon-in-a-month has earned widespread acceptance. 


Chief Meteorologist

Cindy Day