“Once in Blue Moon” – Phrase Origin

Phrase etymology is a sometimes challenging, always exciting mystery to pin down. Some English vernaculars lead short lives and, as such, are products of their time. Others, like “once in a blue moon,” thrive from generation to generation and may never go out of style.

“Once in a blue moon” has its roots in how often the moon appears blue, which is only very, very rarely. The moon can appear blue after major ecological events such as volcano eruptions or forest fires. The phrase means “not very often” or “hardly ever at all.”

As to who first decided to use this clever phrase and when, we must page back through hundreds of years. In this article, I invite you to dig deep into the roots of today’s idiom. Let’s find out where it all began.

The Story Behind “Once in a Blue Moon”

The expression “once in a blue moon” goes back to the 1600s. In those days, few people bothered to consider the scientific meaning of “blue moon” (more on this below). Instead, they viewed a blue moon as something ridiculous, akin to flying elephants or dancing hippos.

For pity’s sake, sir! I’ll pay that kind of money when the moon turns blue!

Go inside and change your clothes! You look silly as a blue moon!

Commentary like this was the spark upon two stones—anger and sarcasm—that created today’s oh-so-commonly ejected idiom.

In those days, no one considered a blue moon to be, well, a real thing. Today we know better, but since blue moons are still pretty rare, the phrase remains intact and stronger than ever.

When exactly can one look into the sky and expect to see a blue moon? I have some answers.

What Exactly Is a Blue Moon?

There are two definitions of a blue moon. One of the definitions is literal, while the other makes sense by appellation only.

In the literal sense, you can—and perhaps have—look up into the night sky and see a gorgeous and extremely rare blue moon. This happens not because the moon has suddenly turned blue but due to what’s happening beneath it. (In other words, the moon is the same as it always has been. It only appears to be blue because of what’s happening in the sky and the atmosphere.)

Blue Moons Sometimes Appear After an Ecological Event

The meaning of “once in a blue moon” makes perfect sense when we consider how often the moon can appear blue. Most of the time, a volcano’s eruption is behind a blue moon.

When a big mountain goes boom, vast clouds of ash spew into the sky with the force of a massive bomb. We’re talking megatons of power here. Some particles in the ash are actually large enough to block red light while still allowing blue light to pass through. These particles are what cause the moon to appear blue.

How often do these phenomena occur? Well…once in a blue moon.

Indonesia’s Krakatoa blast of 1883 turned the moon’s rays blue worldwide for two years. The moon turned blue again in 1980 when Mount St. Helens erupted, in 1983 after Mexico’s El Chicon eruption, and in 1991 when Mount Pinatubo erupted in the Philippines.

Large forest fires can also release ash that blocks red light. In 1950 a massive forest fire in Canada actually turned the moon and the sun blue for residents in Canada, the United States, and clear over into the United Kingdom.

All of these events were years and sometimes decades apart, supporting the idiom we examine today.

The Second Full Moon in a Month

In a typical month, the moon goes through a total of 8 phases: 

  • New
  • Waxing crescent
  • First quarter
  • Waxing gibbous
  • Full
  • Waning gibbous
  • Third quarter
  • Waning crescent

You can learn more about the phases of the moon in this Youtube video:

When the fifth phase—full—occurs twice in the same month, it’s known as a blue moon. These “blue moons” happen about every two to three years, so while they are not precisely rare, we can’t necessarily call them common. This occurrence led to the contemporary meaning of “once in a blue moon.”

It’s possible for two full moons to occur within the same month. This is because the phases of the moon take about 29.5 days from new to waning crescent. Twelve cycles take 354 days—not quite enough for our 365-day years. So we get an extra full moon every two and a half years.

That’s one extra werewolf change for those of you afflicted. Spare hunting time for you October food-gatherers. Or just a little extra light if you’re one of those who like to read by the window.

How Much Sense Does the Phrase “Once in a Blue Moon” Actually Make?

With the above information in hand, the first source makes perfect sense. It takes a powerful volcano blast—or a sizable forest fire—to block atmospheric red light while allowing blue light to pass. Indeed, most people go their whole lives without seeing one of these “blue moons.”

The second source, however, is more contemporary and doesn’t make as much sense. As stated, two full moons in a month are hardly rare, but they still happen far enough apart that the popular phrase remains. 

The idiom lives on regardless, likely because of its power to pierce right to the heart of what we’re trying to say. Other sayings are not so lucky. The early 1900s gave us “twenty-three,” which means to scram or go away. That one sure didn’t last long. Nor did “square,” an idiom from the 1960s, meaning someone is a bore or not up on the latest trends.

For now, “once in a blue moon” is safe from the curtain calls of one generation giving way to the next. Indeed, the term “blue moon” even has other meanings.

Other Uses of Blue Moon

The color blue has been associated with sadness for hundreds of years. In his 1385 poem “The Complaint of Mars,” poet Geoffrey Chaucer used the word “blewe” to describe the sadness of his tears. Looking up blue in a dictionary will surely bring about one definition of looking sad or disappointed.

And then there’s the song “Blue Moon,” composed in 1934 by Richard Rodgers and Lorenz Hart. The song tells of a man standing alone beneath the moon “without a dream in my heart, without a love of my own.” The man prays to the moon to please send him someone to love forever and ever—and the moon obliges, turning to gold upon granting the prayer.

If only it were that easy in the real world. Perhaps because finding the love of your life is so rare, we can say it only happens once in a blue moon. That would link the song neatly to its narrator’s fate.

Today, many companies use the phrase ‘blue moon’ to market their products. Some examples of these include:

  • Blue Moon Brewing Company
  • The Blue Moon Restaurant
  • Blue Moon Software
  • Blue Moon Auto Sales
  • And more!

There is even a blue moon ice cream flavor! Even if the phrase ‘once in a blue moon’ falls out of fashion sometime soon, no one is likely to ever forget it, thanks to all of these associations. 

In Conclusion

Some idioms come and go. They are like leaves blown on an autumn wind, enchanting us—however briefly—with the music of their passing. All too soon, they gather at some other place down the road, never to be heard from again.

Others, such as “once in a blue moon,” endure, perhaps because no other combination of words, regardless of their melody, so well captures the feel of what we’re trying to say. It is a rare feat. A trait that comes along in the English language only so often. One time or even less in a blue moon.

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