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

Curious about the origins of the phrase "once in a blue moon"? The answer might not be what you expected.

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

You ever wonder the origins of the phrase, "once in a blue moon." You might hear the familiar phrase sprinkled among 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?

When was the last time you have seen a Blue Moon?

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 appeared after other major volcanic events of 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.


Did the Earth Move for You Last Full 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: "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 1821. 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 it this insert from the story, “How’s Harry and Ben? – haven’t seen you this blue moon.” Starting from here, the phrase developed into a relatively common expression.

Another Definition

You are probably more familiar with the idea that a blue moon is the second full moon in a calendar month. This idea started in 1946 by mere accident when in his article Sky and Telescope, James Hugh Pruett, accidentally incorrectly referenced an idea from Maine Farmer’s Almanac. The idea gained more steam in the 1980s.

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. What do you think came first the actual blue moon, or the phrase?