Once in a blue moon is a common English expression and it refers to a rare event. But why a blue moon? What’s a blue moon? There is no unique answer, but here are a few things it could be.

Blue Moons – depend on the month
Although the expression “once in a blue moon” is over 150 years old, the blue Moons you’re most likely to hear about today are a recent concept. And it isn’t as exciting as it sounds, because, unlike the lovely color in the header image, they aren’t blue. How disappointing! If a blue Moon isn’t blue, then what is it? Sad to say, it’s probably just the second full Moon in a calendar month.

This interpretation began as a simple misunderstanding, but has taken on a life of its own, and is now a common definition. Surprisingly, the original error occurred in Sky & Telescope magazine in the 1940s when a writer misinterpreted the Maine Farmers’ Almanac. The Sky & Telescope definition was picked up over the following decades by a children’s almanac and the game Trivial Pursuit. In the eighties, it went into general usage from a popular astronomy radio program.

Blue Moons – depend on the season
And what did the almanac say?

The Maine Farmers’ Almanac was first published in 1819 and had kept the tradition of counting the year from winter solstice to winter solstice. (This is known as a tropical year.) It also followed the old practice of naming the full Moons, as well as following the rules regarding Easter. I won’t try to explain these, but it means that the last full Moon of winter is in Lent (the Lenten Moon) and the first full Moon of spring (Egg Moon) must occur in the week before Easter.

Usually, a tropical year has three full Moons in each of the four seasons, and each one has a name. But sometimes there are thirteen full Moons in the year, leaving one season with four of them. What to do? You can’t just give the third full Moon the next name on the list without having the rest of the full Moons fall out of their correct seasonal places. For example, you can see that if there are four full Moons in winter, then the third one can’t be called the Lenten Moon.

So the Maine Farmers Almanac definition of a blue Moon is the third full Moon in a season that has four.

The Moon is blue – an old expression
I was surprised to find out that the idea of a blue Moon can be traced back five centuries, but it meant something rather different then. It meant something silly and impossible. After all, the Moon is fairly obviously not blue.

Philip Hiscock of Memorial University in Newfoundland found that in the sixteenth century, Cardinal Wolsey, adviser to Henry VIII, says contemptuously of some of his enemies, “they would have you believe the moon is blue.” And a younger contemporary, William Barlow, berates the clergy in the ironic lines:
Yf they saye the mone is belewe,
We must beleve that it is true.
By the early eighteenth century a blue Moon didn’t mean something ridiculous. It meant never. And over another century or so it evolved into meaning a long time. Hiscock found an 1821 example of London street slang, “I haven’t seen you this blue moon.” But within fifty years the sense had changed to mean a rare occurrence.

But wait! The Moon can be blue!
In the sixteenth century there is no record of any sighting of the Moon actually being blue. But that was not the case by the end of the nineteenth century. The 1883 Krakatoa eruption produced blue Moons over a wide area.

We now know that if there are a large number of particles of a certain size in the upper atmosphere, they act like a color filter. Volcanic ash, smoke from forest fires, ice crystals or fine sand can, very occasionally, produce particles of the right size. This creates blue Moons and also blue Suns.

Some sources say that the existence of genuine blue Moons is how the expression came to be associated with astronomy. However the blue Moons created by Krakatoa are the earliest examples I could find, and the expression “once in a blue moon” predates the eruption. Yet it's possible that as the meaning evolved, people began to associate these atmospheric effects with it.

Alas, this doesn't explain why the third full Moon in a season of four is blue. I have read the suggestion that the extra Moon was originally dubbed in Old English a “betrayer” Moon, the word being similar to the word for “blue”. When the first word became obsolete, people read it as blue. This is a good story, but I haven’t seen any evidence for it other than the similarity in the words.

How often is once in a blue Moon?
A chart of blue Moons as defined by Sky & Telescope and as defined by the Maine Farmers' Almanac shows that from 1999 -2022 there are ten, according to Sky & Telescope and eight according to the Maine Farmers' Almanac. So either way they occur every few years.

We can’t say how often genuinely blue Moons appear because it’s unpredictable. We can say that they are in fact very rare events.

References:
(1) Helen Sawyer Hogg, “Our of Old Books (Blue Sun)” Journal of the Royal astronomical Society of Canada, Vol. 44, p.241
(2) https://science1.nasa.gov/science-news/science-at-nasa/2004/07jul_bluemoon/
(3) Natalie Wolchover, “Blue Moon: The Strange Evolution of a Phrase” https://www.livescience.com/34143-blue-moon-phrase.html