Blue Moon (Origin)

by Craig Shrives

What Is the Origin of the Saying "Blue Moon"?

The term "blue moon" means a rare event or occurrence.

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Blue Moon (Origin)

Examples of Use:

  • We never see him. He only visits once in a blue moon.
  • Take the job! That kind of opportunity only comes around once in a blue moon.
  • I treat myself to a chocolate cake once in a blue moon.
"Blue moon" is typically seen in the phrase "once in blue moon." A blue moon is the second full moon in any calendar month. A second moon can appear in a month because lunar months (the lunar cycle) and calendar months are not the same. In fact, the lunar cycle is 29.5 days (from new moon to new moon). As a result, some years have 13 full moons, not 12. A blue moon (i.e., the second moon in any calendar month) occurs approximately every three years (specifically, 7 times every 19 years). So, if you're a stickler for accuracy, the term "once in a blue moon" roughly equates to once every 2.7 years.

Of note, a blue moon is not blue. It is no different from any other moon. The adjective "blue" was first recorded in 1946 when amateur astronomer James High Pruett used it to describe the fourth moon in a season (ordinarily, there are three). His choice of word is widely cited as a misinterpretation, although it is unclear what Pruett misinterpreted.

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