The cosmos are full of miraculous phenomena that have inspired some of our most beloved and popular songs, poems, and turns of phrase. From “wishing on a star” to “you are my sunshine,” the planets and galaxies above and around us have always influenced human language and have formed the basis of many a charming, star-dusted metaphor. Few celestial phrases are more common than “once in a blue moon”—but where does this one come from?

The Origins of the Phrase “Once in a Blue Moon”

Like many colloquialisms, the exact, precise moment when someone said or wrote “once in a blue moon” for the very first time remains unclear. We do know that the phrase stems from the 16th-century expression “the moon is blue,” which referred to something that is not possible.

It is widely believed that the first recorded mention of this term appears in a 1528 pamphlet written by William Roy and Jeremy Barlowe. The pamphlet’s purpose was to challenge the Roman clergy, and it contained a rather eloquent couplet that read, “Yf they say the mone is belewe / We must believe that it is true”—or, in modern English, “If they say the moon is blue / We must believe that it is true.” In this context, a “blue moon” signified something entirely impossible or untrue, and so here it was a clear way to criticize the Roman church for demanding absolute adherence to its dogma without question. 

The meaning of the phrase began to shift slightly when, in 1883, the Krakatoa volcano erupted in Indonesia, and people began to see moons that actually did look rather blue due to discolorations in the clouds. This led to “blue moon” becoming a phrase that refers to something that is highly unlikely but still possible. 

Something similar occurred in 1951 when wildfires in Alberta, Canada leached smoke into the sky, which gave the moon a pale aqua tint. All this gave rise to the phrase “once in a blue moon” as we know it today.

What Is a Blue Moon in Astronomy?

According to the Library of Congress, the Maine Farmer’s Almanac began referring to “blue moons” as early as the 19th century, using the term to describe a year that has 13 full moons instead of 12. The almanac specifically used the term “blue moon” to describe the third moon that appears in a season with four moons.

However, today, a “blue moon” in the astronomical sense commonly describes a full moon that appears in the same month as another full moon. This comes from a slight misinterpretation of the Maine Farmer’s Almanac definition that appeared in a 1946 issue of Sky & Telescope. In an article called “Once in a Blue Moon,” James Hugh Pruett wrote, “Seven times in 19 years there were—and still are—13 full moons in a year. This gives 11 months with one full moon each and one with two. This second in a month, so I interpret it, was called Blue Moon.” 

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