The idiom ‘once in a blue moon’ implies that something rarely happens. Here’s what you should know about the popular phrase.
You probably heard the popular idiom, “once in a blue moon” many times over the years.
An idiom is a popular phrase whose meaning often can’t be ascertained from the combination of words that make it up.
So let’s talk about what blue moons are and how the idiom came to be.
First, blue moons don’t happen that often. In fact, they occur about once every 2.7 years. It’s the second full moon occurring in the same month. At the time I write this, the next one will occur on Halloween 2020. This moon doesn’t look blue; it looks like any normal full moon. It’s just a matter of timing as to whether a second full moon would fall within any single month.
But that’s only one way they’re determined. Some use the term to mean the third of four full moons within a single season, with a season being between a solstice and equinox. If you rely on that definition, the next seasonal blue moon will be August 22, 2021. But most people don’t rely on that definition for the astronomical occurrence.
Either way, it’ll be a while before we see one.
The relative rarity of the event makes it a good subject for an idiom used to mean “once in a while.”
But that’s not the whole story.
The idiom’s current understood meaning is actually its second. Originally, it meant something completely different.
Its original meaning was something that’s absurd. This usage made its debut in a 1528 anti-clerical pamphlet that seemed to use the phrase to indicate that if a clergyman claimed something was so, it should be blindly believed to be so.
You can read that story here.
In any case, this original meaning fell out of our common understanding. By the 1820s, the currently-understood meaning appeared.
So if you refer to a “blue moon” with that phrase, you’re referring to something that rarely happens.