Once in a Blue Moon

"Once in a blue moon" means very rarely, or on occasions so infrequent that they are almost exceptional. The phrase is used to describe events that happen far less often than normal, conveying a sense of unusual rarity. This page explores the surprisingly specific astronomical origin of the phrase and how its meaning has developed over centuries.

How to Use This Page

Read the history section for the documented origin of "blue moon" and how the phrase evolved from a calendar term to a rarity idiom. The modern usage section shows how it appears in contemporary speech and writing. The similar phrases section covers related idioms of rarity.

The History

The term "blue moon" has a precise astronomical meaning that directly gave rise to the idiom. A calendar month occasionally contains two full moons: because a lunar cycle lasts approximately 29.5 days and most calendar months are 30 or 31 days, a full moon that falls on the first or second day of the month can be followed by another full moon before the month ends. This second full moon in a single calendar month is called a blue moon, and it occurs roughly every two to three years.

The earliest known use of "blue moon" as a rarity expression appears in a 1528 pamphlet, though with a slightly different nuance suggesting something absurd or impossible, similar to saying "when pigs fly." Over the following centuries the phrase softened into its modern meaning of "very rarely" rather than "never."

The specific "second full moon in a month" definition was popularised by the American magazine Sky and Telescope in 1946, though this was itself the result of a misinterpretation of an earlier, more complex Maine Farmers' Almanac definition. Despite this origin being a mix-up, it has become the standard popular definition and is now widely accepted.

The phrase itself has been in continuous use as an idiom of rarity since at least the nineteenth century, and by the mid-twentieth century it was fully established in everyday English.

How It Is Used Today

In everyday speech: "He rings his parents once in a blue moon, maybe three or four times a year."

In journalism: "Such a combination of political circumstances happens once in a blue moon and the government knows it."

In music and popular culture: the phrase appears in numerous song titles and lyrics, used to evoke rarity, longing, or unexpected good fortune.

Similar Phrases

"Once in a while" means occasionally, though far more frequently than a blue moon. "Once in a lifetime" suggests the event will happen only a single time. "Rarely if ever" is the plain equivalent. "Few and far between" describes things that occur infrequently and in small numbers.

Frequently Asked Questions

How often does a blue moon actually occur? A blue moon in the popular "second full moon in a calendar month" sense occurs roughly every two to three years, or about seven times every nineteen years. This explains its association with rarity.

Can the moon actually appear blue? Yes. After major volcanic eruptions or large wildfires, particles in the atmosphere can scatter red light and allow more blue light through, making the moon appear visibly blue. This rare atmospheric event may have reinforced the phrase's sense of rarity.

Is "blue moon" an English-only expression? The phrase exists primarily in English. Other languages express the same idea differently: in German a rare event happens "alle Jubeljahre" (every jubilee year), and in French it is "tous les 36 du mois" (the 36th of the month, which never comes).

What is the difference between a blue moon and a supermoon? A supermoon occurs when the full moon coincides with the moon's closest approach to Earth, making it appear larger and brighter. These are separate phenomena. A blue moon is defined purely by calendar timing. A full moon can be both a blue moon and a supermoon at the same time.

The two definitions side by side

Two rules share the name blue moon, and they disagree about which full moon counts. The older rule comes from the Maine Farmers' Almanac and works on the seasons. The newer rule works on the calendar month, and it took hold in the twentieth century.

February cannot host a calendar blue moon under either rule, and the arithmetic explains why. A mean synodic month, the interval from one full moon to the next, runs 29.530589 days. A 28-day February falls 1.53 days short of that interval, and a 29-day February in a leap year falls 0.53 days short. Neither can hold two full moons. A 31-day month exceeds one lunation by 1.47 days and can just hold two, which is why the calendar version always lands in a month with 30 or 31 days.

The seasonal rule needs a different explanation. The almanac divided the tropical year, which runs from one winter solstice to the next, into four seasons of three named full moons each. In a year whose season contained a fourth full moon, the extra one had to be named, and the third of the four was given the name so that the special moons kept their places in the calendar. The almanac did not use the second-full-moon-in-a-month rule at all.

Counting the full moons across nineteen years

The reason both rules land on the same rough frequency is the Metonic cycle, and it can be worked from two constants.

A year is about eleven days longer than twelve lunar months, so each year the calendar date of a full moon slips back against the sun. After nineteen years the accumulated difference is 206.6 days, which is almost exactly seven lunar months. That is why nineteen years contains 235 full moons rather than 228: seven of those years carry a thirteenth full moon, and those years are the blue moon years. The two cycles line up to within a tenth of a day, since 235 lunations span 6,939.69 days against 6,939.60 days for nineteen tropical years.

Dividing nineteen years by seven blue moons gives a mean interval of 991.37 days, which is 2.714 years, and that matches the familiar statement that blue moons come two to three years apart. The figure is a mean. The actual gaps vary because the cycle is not evenly spaced, and consecutive gaps of a year and of nearly four years both occur.

The publication chain that fixed the modern meaning

The calendar rule has a recorded paper trail, which is unusual for a folk usage and makes the history checkable.

Lawrence J. Lafleur of Antioch College discussed blue moons in a question and answer column in Sky and Telescope in July 1943, citing the 1937 Maine Farmers' Almanac as his source and quoting the almanac's commentary on its August 1937 page. He did not mention two full moons in one month.

James Hugh Pruett's article titled Once in a Blue Moon appeared in the same magazine in March 1946. Pruett repeated part of Lafleur's material and then added his own reading, writing that seven times in nineteen years there were thirteen full moons in a year, that this gave eleven months with one full moon and one month with two, and that the second of those, as he interpreted it, was called a blue moon. The almanac's August 1937 blue moon fell on the twenty-first, which is not the second full moon of that month, and 1937 held twelve full moons rather than thirteen. The reading was a mistake.

Sky and Telescope adopted the new rule in a note in its May 1950 issue, and the magazine acknowledged the error in its May 1999 issue, after the editors and two researchers examined more than forty editions of the almanac covering 1819 to 1962 and found that not one of the blue moons listed there was a second full moon in a month. The American Heritage Dictionary listed both definitions in its fourth edition in 2000. The broadcast programme StarDate used the calendar rule on 31 January 1980, and a 1986 edition of the board game Trivial Pursuit spread it further, after which the phrase itself became markedly more common in print.

What this page claims and what it leaves open

The history section above states that the calendar definition was popularised by Sky and Telescope in 1946 and that it came from a misreading of the almanac. Both parts of that statement hold up against the record, and the detail worth adding is that the misreading replaced a different rule rather than refining one. The almanac's own blue moon was the third full moon of a season with four, not the second full moon of a month.

What the record does not settle is why the almanac chose the third moon rather than the fourth. The seasonal rule existed to keep a set of named moons in their places across the year, so the choice was a bookkeeping decision rather than an astronomical one. Sources disagree about which modern usage should be called correct, and both definitions now appear in dictionaries.

Two claims sometimes attached to the phrase are not supported here. The idea that the name refers to a moon that appears blue is a separate phenomenon: atmospheric particles after large volcanic eruptions or wildfires can scatter red light and let more blue through, and that effect is unrelated to calendar timing. The idea that the calendar definition is the older one is the reverse of the record.

Sources: Sky and Telescope, in its own account of the March 1946 article by James Hugh Pruett and the July 1943 column by Laurence J. Lafleur, together with the magazine's correction in its May 1999 issue and the examination of more than forty editions of the Maine Farmers' Almanac from 1819 to 1962. The Library of Congress account of the seasonal rule, and the fourth edition of the American Heritage Dictionary for the two definitions it lists. The metonic arithmetic above uses the mean synodic month and the mean tropical year.

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