A blue moon is a full moon that arrives one too many for the period it falls in. Under the popular modern definition it is the second full moon inside a single calendar month. Under the older, quieter definition it is the third full moon in a season that happens to contain four. Neither has anything whatsoever to do with the color blue.
The reason two definitions exist at all is a small, genuinely interesting mistake in a 1946 astronomy magazine — which is the rare case of a folklore term with a traceable point of origin.
The monthly blue moon
This is the definition almost everyone means. A lunar cycle runs about 29.5 days; most calendar months run 30 or 31. So occasionally a full moon lands in the first day or two of a month, and a second one squeezes in before the month ends. That second one is the blue moon.
February is the exception that makes the whole system interesting. At 28 or 29 days it is shorter than a lunar cycle, so it can never contain two full moons — and sometimes contains none at all.
The seasonal blue moon
The older definition works on astronomical seasons rather than months. Each season between a solstice and an equinox normally holds three full moons. About every three years one holds four.
When that happens, the traditional almanac names for the season's moons no longer line up — there is an extra one to account for. The third of the four gets called the blue moon, which sounds arbitrary until you realize the point was to keep the last moon of the season in its correct named slot. The extra one had to be absorbed somewhere in the middle.
The naming system needed the final full moon before the equinox or solstice to keep its traditional name. Calling the fourth one blue would have broken that. Calling the third one blue absorbs the extra moon while leaving the anchor points intact.
The 1946 mistake
The seasonal definition is the one recorded in the Maine Farmers' Almanac, and it is the older of the two by a long way.
In 1946, an article in Sky & Telescope attempted to explain the almanac's blue moon rule and misread it — arriving at the far simpler idea that a blue moon is the second full moon in a calendar month. The simpler version was easier to remember, easier to check against any wall calendar, and it stuck.
It spread widely after being used on a popular radio astronomy program in 1980 and in a board game question shortly after. By the time researchers traced the error back to its source in the 1990s, the monthly definition had been in general use for decades and was not going anywhere.
Most folklore has no findable origin. This one has a magazine, a year, and an identifiable misreading — and the mistake is now more widely believed than the thing it got wrong.
How often — with the arithmetic
You can work the frequency out yourself, and the calculation is short enough to be worth doing.
- One lunar cycle takes 29.53 days
- Twelve lunar cycles take 12 × 29.53 = 354.4 days
- A calendar year is 365.24 days
- That leaves 10.9 spare days every year
Those spare days accumulate. Once enough of them have piled up to make a whole extra lunar cycle, a thirteenth full moon appears in the year — and one month has to hold two of them.
29.53 ÷ 10.9 = about 2.7 years between monthly blue moons on average. Seasonal blue moons work out to a similar interval, since both come from the same thirteenth-moon surplus.
Because it is an average rather than a schedule, the actual gaps vary. And occasionally the arithmetic produces two in a single year — which requires a February with no full moon in it at all, letting both January and March hold two. That combination shows up roughly four or five times a century.
When the moon really is blue
The moon can genuinely appear blue. It just has nothing to do with the calendar.
Normally the atmosphere scatters shorter blue wavelengths more than longer red ones, which is why a low moon looks orange. But if the air holds a high concentration of particles close to one micron in width — slightly wider than a wavelength of red light — the scattering reverses. Red gets scattered away and blue passes through.
Documented cases follow major particle-injection events:
- The 1883 eruption of Krakatoa, after which blue and green moons were reported for around two years
- Large forest fires producing droplets in the right narrow size range — a Canadian fire in 1950 is the most-cited example
- Other major volcanic eruptions with high-altitude ash loading
It requires a narrow, uniform particle size, which is why it is rare. Most smoke and dust produce red moons rather than blue ones.
Once in a blue moon
The phrase is older than either astronomical definition, and it started out meaning something stronger than rare.
In sixteenth-century English, saying the moon was blue was a way of asserting something plainly false — the equivalent of saying the sky is green. It was an expression of impossibility, used to mock a claim rather than to describe an interval.
Somewhere over the following centuries it softened from never to very rarely, which is how everyone uses it now. The astronomical definitions arrived afterward and borrowed the phrase, rather than the other way round.
What people read into it
Blue moon symbolism is genuinely modern, and it is worth saying why: unlike the phases, a blue moon marks no change in the moon at all. It is a coincidence between the lunar cycle and a human calendar. The moon that night behaves exactly as any other full moon does.
Most blue moon practice builds on rarity rather than on any lunar quality — treating it as a marked occasion for the thing that keeps getting deferred. The reasoning is that a date which will not return for years is harder to postpone than an ordinary one.
That is a reasonable use of an arbitrary marker, and worth separating from the phase-based traditions, which do track a real change in the sky. If you want those, the eight phases are where the older meaning lives.
Something to do with the rare occasion
An eBook, audiobook and printable workbook for working through a full lunar cycle — including full moon spreads for taking stock of what has actually come to fruition.
Explore Lunar FlowCommon questions
What is a blue moon?
There are two accepted definitions. The monthly blue moon is the second full moon to fall within a single calendar month. The seasonal blue moon, which is the older definition, is the third full moon in an astronomical season that contains four instead of the usual three.
How often does a blue moon happen?
A monthly blue moon occurs on average about every 2.7 years. Twelve lunar cycles take roughly 354.4 days while a calendar year is 365.24 days, leaving about 10.9 spare days each year. Those accumulate until they add up to a full 29.5-day lunar cycle, which takes about 2.7 years.
Is the moon actually blue during a blue moon?
No. The name has nothing to do with color. The moon can genuinely appear blue, but only when the atmosphere holds a high concentration of particles close to one micron across, which scatter red light and let blue through. This has followed major volcanic eruptions and large wildfires — it is rare and unrelated to the calendar.
Where does the phrase once in a blue moon come from?
It began as an expression of impossibility in sixteenth century English — saying the moon is blue meant saying something absurd. Over the following centuries it softened from never to very rarely, which is how it is used now.
Can there be two blue moons in one year?
Yes, though it is uncommon. It requires February to contain no full moon at all, which lets both January and March hold two. This happens roughly four or five times a century.
What does a blue moon mean spiritually?
Because the name describes a calendar coincidence rather than any change in the moon itself, blue moon symbolism is modern and mostly built on rarity — treating it as a marked occasion for something you have been putting off. The moon behaves identically to any other full moon.
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