Once in a Blue Moon
August 31st will see the second of two full moons in the same month (the other was on the 1st in North America or on the 2nd in Europe, Africa, Asia and Australia). You've probably heard the second of those full moons referred to as a "blue moon" — but you might be surprised at the origin of the phrase.
"According to old folklore," some people say, the second full Moon in a calendar month is called a "blue Moon." They go on to explain that this is the origin of the expression "once in a blue Moon." But it isn't true! The term "blue Moon" has been around a long time, well over 400 years, but its calendrical meaning has become widespread only in the last 25 years.
A Variety of Meanings
The concept that a blue Moon was absurd (the first meaning) led eventually to a second meaning, that of "never." The statement "I'll marry you, m'lady, when the Moon is blue!" would not have been taken as a betrothal in the 18th century.
But there are also historical examples of the Moon actually turning blue. That's the third meaning — the Moon appearing blue in the sky. When the Indonesian volcano Krakatoa exploded in 1883, its dust turned sunsets green and the Moon blue all around the world for the best part of two years. In 1927, the Indian monsoons were late arriving and the extra-long dry season blew up enough dust for a blue Moon. And Moons in northeastern North America turned blue in 1951 when huge forest fires in western Canada threw smoke particles up into the sky.
So, by the mid-19th century, it was clear that visibly blue Moons, though rare, did happen from time to time — whence the phrase "once in a blue Moon." It meant then exactly what it means today, a fairly infrequent event, not quite regular enough to pinpoint. That's meaning number four, and today it is still the main one.
But meaning is a slippery substance, and I know of a half dozen songs that use "blue Moon" as a symbol of sadness and loneliness. The poor crooner's Moon often turns to gold when he gets his love at the end of the song. That's meaning number five: check your old Elvis Presley or Bill Monroe records for more information.
And did I mention a slinky blue liquid in a cocktail glass, one that requires curaçao, gin, and perhaps a twist of lemon? That's number six.
The Second Full Moon in a Month
I searched high and low for an earlier example of this usage, or any other name for two full Moons in a single calendar month. But the search was in vain — this meaning seemed to have no history. I did find information on the other meanings of "blue Moon," but not this one, number seven.
Then in December 1990, with another "blue Moon" coming on, I started getting more calls and decided to write about it in the local newspaper. I searched harder this time, exhausting all the usual sources: specialized dictionaries, indexes of proverbial sayings, and regional collections of folklore. A brand-new edition of the huge Oxford English Dictionary had recently come out, but even it omitted this particular meaning. "Blue Moon" seemed to be a truly modern piece of folklore, masquerading as something old.
Used in this way, the term was certainly very, very local before they included it in their book. It seemed never to have been written down before. Of course, authors sometimes "invent" information to protect themselves against plagiarists. Well, if that were the case they'd already lost, because the new "blue Moon" almost immediately entered the folklore of the modern world. It became as living a meaning as any of its predecessors.
Our new blue Moon has something of the modern times in it, a technical aspect that most of the earlier meanings lacked. Perhaps that's why it caught on so quickly. It appeals to our modern sensibilities, including our desire to have plausible origins. But any folklorist will tell you that plausibility is the mantle that folklore wears to sneak through history's lines. "Old folklore" it is not, but real folklore it is. Given its present popularity, it may last a long time.