The Reason Why We Say 'Once in a Blue Moon'

You ever wonder about the origins of the phrase "once in a blue moon?" You might hear the familiar phrase sprinkled among your daily conversations with family and friends.

It is usually a neutral phrase that implies something may rarely happen, something absurd, or can be used along the same lines as, "when hell freezes over." But where did the phrase start? It's not like there is a blue moon. Or is there?

Blue Moons have appeared throughout history, but it is not what you think

Let's get this out of the way first. Believe it or not, the moon can look "blue." Though you have probably seen a white moon or maybe even a yellow or red moon, a blue moon is indeed very rare. A blue moon may appear after a volcanic eruption. Large dust particles from the ash from the volcano wider than .7 microns will diffract red light creating a blue-green moon. This happened, in 1883 after the eruption of Krakatao.

This phenomenon has also appeared after other major volcanic events, such as Mt. St Helens, El Chicon, and Mount Pinatubo, as well as a few Canadian forest fires in the 1950s. Yet, the bluish tint still does not have anything to do with the phrase.

However, the Blue Moon you are thinking of really has nothing to do with the color blue

So, what does a blue moon actually mean? The phrase "once in a blue moon really has nothing to do with color. Let us explain. Traditionally, there are 12 fully lit, or full moons, each year. Each season there should be three full moons. However, occasionally there is an outlier. There can be four full moons in a season.

This moon is traditionally known as a "blue" moon. So, how often is once in blue moon? 

These celestial events happen on average once every 2.7 years. Occasionally two full moons will fall within the same month. This is also known as a full blue moon. In 2020, on Halloween, there was a blue moon. How appropriate. 

There are multiple definitions of a Blue Moon

You can find the first recorded use of the phrase, or something similar, in an anti-clerical pamphlet published by William Roy and Jeremy Barlowe around 1528 with the phrase "if they say the moon is [blue], it must be true." The expression would be used in situations to imply a fool or gullible people will believe just about anything.

Nevertheless, the phrase that we love to use in our everyday talks did not really appear until much later. When the idiom appeared in the 19th century, in the publication of Real Life in London by Pierce Egan, once in a blue moon meant "close to impossible" or "rarely." You can see this insert from the story, “How’s Harry and Ben? – haven’t seen you this blue moon.” 

You are probably more familiar with the idea that a blue moon is the second full moon in a calendar month. This idea actually started in 1946, when, in his article Sky and Telescope, James Hugh Pruett misinterpreted the concept of the fourth full moon in a season being a blue moon, from the Maine Farmer’s Almanac, stating instead that the second full moon of the month was called a blue moon. This misinterpretation gained steam after it was referenced on a popular radio program StarDate, in 1980, and in a question in the Trivial Pursuit game in 1986.

Some do argue that it is a blend of these two events coming together. The phrase came into existence and was perpetuated by the blue moon phenomenon. When the first astrological blue moon occurred, more people have been inclined to use the expression.

Blue moons can happen twice in a year, but this is extremely rare

Yes, it is possible. This event happens every 19 years. In 1999 there were two blue moons, one in January and March. Interestingly, when there are two blue moons, they usually occur in these two months. February's unique 28-day long length, impact the occurrence of this event. The average span between full moons is 29.5 days. If a full moon falls at the end of January, it is very possible for the next full moon to skip February entirely and fall at the beginning of March.

What do you think came first the actual blue moon, or the phrase? Celestial events are always fascinating.