Saturday, November 31, 2020, was the last “blue moon.”  But what the heck is a blue moon?  Well... that all depends on your definition, and you have three to pick from, not counting the popular brand of beer or The Marcel’s 1961 hit song.

The most current definition of a blue moon is that of having two full moons in the same month.  This definition has lent us the popular phrase “once in a blue moon,” meaning something that doesn’t happen very often at all.  However, the conditions that create this meaning of a blue moon are truly not that rare.  First, you need a month with 31 days (there are seven to pick from), though it is possible (but less likely) in any month but February, and then you need for there to be a full moon at the first of the month so 29 and ½ days later, in the same month, you can have another full moon, which happens about every 2 ½ years, or more often than the presidential elections. (Rice)

The second, older definition is a bit more convoluted, and it is where our current definition of blue moon stems from.  The older definition for “blue moon” deals with the Christian ecclesiastical calendar.  In that calendar, each full moon has a name.  Easter, for instance, is determined by the Paschal Moon.  There is, though, an exception.  Some years have 13 full moons instead of 12, which means one season would have four full moons instead of three.  It was the third full moon out of four that became known as a blue moon, simply because it didn’t officially have a name. (Brunner)  As well, it couldn’t be the fourth full moon in that season that was called a blue moon because then the names of other full moons, “such as the Moon Before Yule and the Moon After Yule [would not] fall at the proper times relative to the solstices and equinoxes.” (Olson)

How that definition for a blue moon became the current definition is attributed to several editorial mistakes. The third full moon in a season being a blue moon was the definition that was listed in the Maine Farmers’ Almanac from 1932 to 1957.  In a July 1943 article on blue moons in Sky & Telescope magazine, Laurence J. Lafleur wrongly interpreted the old Farmers’ Almanac, confusing a tropical year for a calendar year, though he never mentioned any specific dates, nor did he mention that a blue moon had anything to do with two full moons in one month.  We leave that mistake for an amateur astronomer named James Hugh Pruett.  In 1946, once again in Sky & Telescope, Pruett confused both the Farmers’ Almanac and Lafleur and came up with the current definition for a blue moon.  (Olson) 

This definition was then used by Deborah Byrd, who relied on Pruett’s definition for the January 31, 1980, edition of StarDate, a popular radio program, and was further used by Margot McLoon-Basta and Alice Sigel in their popular Kids’ World Almanac of Records and Facts, which was published in 1985.  From that point on, this has been the definition that we use, even appearing as the answer in the board game Trivial Pursuit. (The next Blue Moon is August 31, 2012)

And if all this isn’t confusing enough, there are times when the moon really can turn blue, which is perhaps the rarest of them all.  A moon doesn’t have to be full, though, to appear blue, nor does the date have anything to do with it.  There just needs to be enough ash particles high enough in the atmosphere, and if those ash particles are the right size, then “they can block reds and yellow from getting to our eyes, giving us tints of blue — and sometimes green — moons.”  (Newcomb)

There were blue moons, for instance, many years after the Indonesian volcano Krakatoa exploded in 1883 with the force of a 100-megaton nuclear bomb.  As well, though far less dramatic, there were blue moons following the eruptions of Mt. St. Helens in 1980, El Chichon in 1982, and Mount Pinatubo in 1991. It is even possible for a large forest fire to cause a blue moon.  (Blue Moon)

So what does all this mean?  Really, not a darned thing.  Fairies won’t dance, wishes won’t be granted, and children conceived under the light of the blue moon won’t be smarter, cuter, or less inclined to believe astrological nonsense.  However, it could be as good a reason as any – if you need a reason at all – to sit out on your porch and drink a Blue Moon and listen to the Marcels.  And if that’s the case, then you’d better have an extra beer or three, because (barring any volcanoes) if you missed the blue moon on Halloween, then you’ll have to wait until August 22, 2021, for the next seasonal blue moon, and August 31, 2023, for the next monthly blue moon, and to see the next full moon on Halloween (which will be a monthly blue moon) you'll have to wait until 2039.  (A Rare Halloween Blue Moon)