Meaning Of Idiom ‘Once in a Blue Moon’
Once in a blue moon means not very often; very rarely; only once in a long period of time; practically never. 1Heacock, Paul. Cambridge Dictionary of American Idioms. Cambridge: Cambridge UP, 2010. ,2Ammer, Christine. American Heritage Dictionary of Idioms. Boston: Houghton Mifflin Harcourt, 2013. ,3Kirkpatrick, Elizabeth M. The Wordsworth Dictionary of Idioms. Ware: Wordsworth, 1995.
Examples Of Use
“I only get to see my family once in a blue moon now that I live in France.”
“I used to paint all the time. Now I only do it once in a blue moon.”
“Once in a blue moon someone surprises you and does something decent.”
Origin
Although the moon can appear blue and a full blue moon is not very rare, the color blue was used arbitrarily in this phrase, since the 1500s, to say that something could not occur or be true. Since the moon is never actually blue (or red, yellow, green, etc.) to say that something occurs once in a ‘blue’ moon is to say that it [almost] never occurs. 4Kirkpatrick, Elizabeth M. The Wordsworth Dictionary of Idioms. Ware: Wordsworth, 1995.
Today, a ‘blue moon’ refers to the second full moon in (in other words, an additional one) within a calendar month. It is claimed that this is the origin of the idiom ‘once in a blue moon.’
That the second full moon is called a blue moon is true. Furthermore, since this is so rare, it must be the correct etymology of the idiom. This is almost certainly not true. To start, this is the second definition of ‘blue moon.’ There is an older one that involves the third full moon in a season with four.
Regardless, neither of these appear to be the origin of the idiom. This idiom has existed for 400 years, long before anyone ever referred to these rare additional full moons as blue moons. As well, most full moons are NOT blue! They are plain old white or pale gray. Since the idiom existed long before the modern use of blue moon to refer to rare additional full moons, and since a full moon is no more likely to be blue than pale white, yellow (due to atmospheric contamination) I submit, as do language scholars, that the use of blue is quite arbitrary as the moon can take on many shades, including red and gold. Philip Hiscock says this idiom is akin to saying the moon is made of green cheese.
People 400 years ago may have witnessed the moon appear to be blue and understood this to be a rare phenomenon, but the term would not have been specific to a second full moon as a second full moon can still appear to be other colors and the modern name is not an accurate reflection of any color appearance. Keep in mind, of course, that the moon is always the same color. It’s the atmosphere that determines what we see, due to the scattering of light.
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