Blue moon: origin of the name, a 1946 error and its symbolism
A blue moon is not blue: it is a rare full moon, named after a 1937 almanac and then a 1946 error. Origin of the expression, two definitions, dates 2026-2034, symbolism.
Key points
| Calendar meaning | Second full moon in the same calendar month (the most common meaning today) |
|---|---|
| Seasonal meaning | Third full moon of an astronomical season that has four (meaning from the Maine almanac, 1937) |
| Atmospheric meaning | A genuinely bluish Moon, very rare: caused by smoke or volcanic ash |
| Frequency (calendar meaning) | About every 2.7 years, according to Wikipedia |
| Most recent and upcoming | 31 May 2026; 31 December 2028; 30 September 2031; 31 July 2034 (calendar meaning, UTC dates) |
| Status | Popular expression, with no official definition in astronomy |
A Moon that is not blue
A blue moon (lune bleue in French) is, most of the time, not blue at all. The expression refers to an extra full moon, which the calendar makes rare, and not to a colour. For the detailed definitions and a comparison with the black moon, read the page black moon and blue moon. This page focuses on one precise question: where does the name come from, and why are there two meanings?
“Once in a blue moon”: the expression
In English, people say “once in a blue moon” to describe something that almost never happens. The figurative use of “blue moon” appears as early as the beginning of the 19th century: Wikipedia cites a use from 1821 in Pierce Egan's Real Life in London, to mean a long period of time. The expression is therefore much older than any astronomer's definition. In French, the equivalent is “tous les 36 du mois” (literally “every 36th of the month”). An etymology that links “blue” to an old English word meaning “to betray” still circulates, but it has no basis.
1937: the Maine almanac
The seasonal meaning comes from the Maine Farmers' Almanac. An astronomical season lasts on average three months and usually contains three full moons. When it contains four, the 1937 almanac called the third one a “blue moon”, so that, according to the magazine Sky & Telescope, the names of the moons would stay aligned with the religious calendar (Lent, Easter). According to Wikipedia, this use of the term seems to have been invented by the publisher of the time, H. P. Trefethen, and the almanac's explanation is considered doubtful: at best it is modern American folklore, not an ancient rural tradition.
1946: an error that becomes the rule
In 1946, amateur astronomer James Hugh Pruett published an article in Sky & Telescope that misread the almanac's rule and stated that the blue moon is the second full moon of the same month. This version, easier to spot on a calendar, spread through radio in the 1980s, then through a well-known board game. It is today the most common meaning, and some dictionaries now recognise several. This short episode shows how an error can become a custom: there is no “true” meaning, only two meanings to know.
When a Moon really is blue
The Moon sometimes looks bluish, but that is not its colour: it is the atmosphere. Particles about a thousandth of a millimetre in size, suspended in the air, scatter red light more than blue: the Moon then looks blue or green. This happened after the eruption of Krakatoa in 1883, which produced coloured suns and moons for months, and in 1950, when smoke from large forest fires in Canada gave rise to blue moons observed in Europe. The phenomenon is very rare, and a truly blue Moon has nothing to do with calendar definitions.
The dates to remember
The dates below are those we calculated (UTC dates; depending on the time zone, the date may change by one day). Check them against an ephemeris before organising anything.
| Meaning | Date | Note |
|---|---|---|
| Calendar | 31 May 2026 | Second full moon of May (the first was on 1 May) |
| Seasonal | 20 May 2027 | Third of the four full moons of spring 2027 |
| Calendar | 31 December 2028 | Coincides, according to the tables, with a total lunar eclipse |
| Seasonal | 24 August 2029 | Third of the four full moons of summer 2029 |
| Calendar | 30 September 2031 | |
| Calendar | 31 July 2034 |
The two meanings almost never coincide: for one full moon to be blue under both rules, a rare combination of circumstances is needed.
How to check a blue moon date
To know whether a full moon is “blue”, two points matter. First, the time zone: a full moon that falls on the 31st at 11 p.m. in Europe may be on the 1st in Asia, which changes the count for the month. Published tables are often given in Coordinated Universal Time (UTC), and the date in France is sometimes one day different. Then, the definition used: calendar or season. To check, consult an ephemeris (for example the one from the IMCCE, at the Paris Observatory) and compare the two rules. Finally, the song “Blue Moon” (Rodgers and Hart, 1934) has no link with these dates.
Modern symbolism and gestures
In practices inspired by esotericism, the blue moon is a “bonus”: an opportunity to do what you always put off, or to conclude a project. There is no ancient basis: popular names create meanings after the fact.
- Do what you have been postponing: write a letter, call a friend, repair an object.
- Note what is rare in your recent life: an encounter, an achievement, a calm moment.
- Have dinner with loved ones and watch the Moon rise.
For other ideas, with no promises, see simple moon rituals.
Frequently asked questions
Why do we say “once in a blue moon”?
To speak of a very rare event. The English expression is attested as early as the beginning of the 19th century, long before the calendar definitions.
What is the right definition of a blue moon?
There are two. The seasonal meaning is the older one (1937 almanac); the calendar meaning, born from an error in 1946, is today the most common.
How often is there a blue moon?
In the calendar sense, about every 2.7 years according to Wikipedia, that is roughly 37 per century.
Is a blue moon bigger or brighter?
No. It is an ordinary full moon, which just happens to be the second of the month.
Is the next blue moon in 2028?
In the calendar sense, according to our calculation, yes: 31 December 2028. In the seasonal sense, one is expected on 20 May 2027.
Also read
The meanings mentioned belong to tradition and symbolism. They are neither promises of results nor medical advice. Back to The Wheel of the Year.