Once in a blue moon - what does it mean?

We've all heard the saying, but what does "once in a blue moon" mean and where does it come from?

Ste Wright | 3 min read

Every once in a while, we hear the phrase "once in a blue moon" used to describe something that is rare or happens infrequently. But what exactly does it mean? Is there a connection to the lunar phenomenon known as a blue moon? Let's delve into the origins and history of both the blue moon and the saying itself.

What is a blue moon?

First and foremost, let's clarify what a blue moon actually is. Contrary to popular belief, a blue moon is not blue in colour. Instead, it refers to the occurrence of an additional full moon within a specific time period. Normally, there are 12 full moons in a year, with one appearing each month. However, due to the mismatch between the lunar and solar calendars, occasionally, an extra full moon sneaks into the cycle, creating what we call a blue moon.

The origin of the term "blue moon" can be traced back to the 16th century. It was commonly believed that the moon could turn blue, although it is exceedingly rare and typically requires unusual atmospheric conditions, such as volcanic eruptions or large-scale fires. In fact, the last widely documented instance of a truly blue moon was caused by the eruption of Mount Krakatoa in 1883, which filled the sky with ash and caused the moon to take on a bluish hue.

However, the modern definition of a blue moon has nothing to do with the moon's colour. It can be attributed to a misinterpretation of an article James Hugh Pruett published in the March 1946 edition of Sky and Telescope magazine. In the article, Pruett referred to a blue moon as the second full moon occurring within a calendar month. Although Pruett's interpretation was incorrect, the idea caught on and eventually became the accepted definition of a blue moon.

History of the saying "once in a blue moon"

two people standing on a beach with the moon in the background
Once in a blue moon basically means something's rare

Now that we understand the origins of the term "blue moon," let's explore how it became associated with the saying "once in a blue moon." The phrase has its roots in the 19th century and was initially used to describe something considered rare or uncommon. Over time, people began to connect the infrequency of a blue moon to this saying, further reinforcing its association with rare occurrences.

The phrase "once in a blue moon" gained widespread usage in the early 20th century and has since become a part of everyday language. It is often employed to convey a sense of rarity or to emphasize the infrequency of an event or situation. For example, one might say, "I only see my favourite band perform live once in a blue moon."

Interestingly, the saying "once in a blue moon" has transcended its lunar origins and is now commonly used in various contexts, both literal and figurative. It has become a part of our collective vocabulary, representing those extraordinary moments that happen unexpectedly or after a long time.

So, the next time you hear someone mention "once in a blue moon," you can appreciate the connection to the rare lunar phenomenon from which the saying emerged. While the blue moon itself may not occur often, the impact of the phrase has undoubtedly left a lasting mark on our language and culture, reminding us to cherish those exceptional and uncommon experiences that come our way.

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