Blue Moon Confusion: Was it or Wasn't it?

At the beginning of my astronomy course, I work with my students on a unit involving astronomical misconceptions.  One perception covered is that of the blue moon. When two full moons occur within the time span of one month, the second full moon is termed the blue moon.  

So you can imagine my surprise when I saw a NASA release claiming that the Aug. 20 full moon was a blue moon.  

“Impossible,” I thought!  Full moons occur with a frequency of approximately 29.5 days, so a blue moon can only occur near the very end of a month if the first full moon happened near the very beginning.  

The evolution of the blue moon began with The Maine Farmer’s Almanac.  Between 1932 and 1957 the Almanac maintained a seasonal scheme for determining when a blue moon occurred. Their blue moon formula followed the solstices and the equinoxes which occur around the 21st of March, June, September, and December. Moon names in the Almanac were synchronized to an ecclesiastical calendar which was mostly set to the yearly cycle of the sun.  

Simply said, a full moon with a particular name had to occur at the same time of the year.  When there were four full moons in a season, an event that occurred about every 2.6 years, the calendrical sequence was thrown out of order.  

To correct for this and keep the moon names concurrent with the year, the third full moon of a four full moon cycle became known as the blue moon.  

Our modern definition of a blue moon, two full moons occurring within a calendar month, stems from the misunderstanding of The Maine Farmer’s Almanac’s rule which was first published in a 1946 article in Sky and Telescope magazine.  The new two full moons in a month idea caught on and by the 1980s this definition rose to supremacy and was accepted by the astronomical community worldwide. 

Message to NASA: Stick to spaceflight.