A blue moon is not a myth but a real astronomical event, and its mythology is a mix of folklore, calendrical quirks, and a famous misinterpretation. The term “blue moon” has two modern meanings—a second full moon in a single calendar month, or the third full moon in a season that has four—and neither has ancient roots. The folklore surrounding blue moons is largely a 20th-century invention, though it draws on older traditions of naming full moons and on the rare atmospheric conditions that can make the moon actually appear blue.
What Exactly Is a Blue Moon?
Today, most people use “blue moon” to mean the second full moon in a calendar month. This happens because a lunar cycle (about 29.5 days) is shorter than most calendar months, so a month can occasionally squeeze in two full moons. The second one is called a blue moon. This definition became popular after a 1946 article in Sky & Telescope magazine misinterpreted an earlier, more complex rule from the Maine Farmers’ Almanac.
The older, seasonal definition is the third full moon in a season that has four full moons. Normally a season has three, but because the lunar cycle is shorter than a season, every two to three years a season gets a fourth. The third of those four is called a blue moon. This rule was used by the Maine Farmers’ Almanac in the 1930s, but the magazine’s 1946 article simplified it incorrectly, and the monthly definition stuck.
Origins of the Phrase “Blue Moon”
The phrase “once in a blue moon” has been used in English since the 19th century to mean a rare event, but it did not originally refer to a specific astronomical occurrence. The earliest known use in print is from 1821, where it appears in a British publication as a metaphor for something absurd or never happening. The connection to an actual second full moon in a month came later, and the modern calendrical meaning is a 20th-century development.
Some folklorists suggest the phrase may have older roots in a now-lost medieval English usage, but there is no solid evidence for that. The idea that a blue moon was a calendar oddity seems to have emerged in the 1800s, possibly from farmers’ almanacs, but the exact chain of transmission is unclear. What is certain is that the phrase predates the astronomical definition by over a century.
When the Moon Actually Turns Blue
The moon can genuinely appear blue in color, but this is an atmospheric effect, not a calendrical one. Volcanic eruptions, forest fires, and dust storms can inject particles into the upper atmosphere that scatter red light and let blue light through, making the moon look blue or sometimes green. The most famous recent examples include the 1883 eruption of Krakatoa, after which the moon reportedly looked blue for months, and the 1950 Canadian forest fires, which produced blue moons and even blue suns.
These events are rare, which is likely why “once in a blue moon” became a metaphor for rarity. The connection between the color and the calendar is purely coincidental—a blue-colored moon has nothing to do with a second full moon in a month. The two meanings are separate and only share the same name.
Folklore and Superstitions
Unlike many full moons, the blue moon has no deep body of ancient mythology. There are no Greek or Norse myths about a blue moon, no Hindu or Chinese legends that specifically mention it. The folklore that exists is modern and mostly playful: some say a blue moon is a time for wishes, others claim it brings chaos or change. These ideas are not rooted in any historical tradition.
However, the blue moon does inherit some of the general folklore attached to full moons. Many cultures have named each full moon of the year—the Harvest Moon, the Wolf Moon, the Strawberry Moon—and the blue moon is sometimes treated as an extra, unnamed moon that breaks the usual cycle. In that sense, it carries a sense of the uncanny or the unexpected, but this is a contemporary framing rather than an ancient belief.
Common Misconceptions
One common misconception is that a blue moon is always the second full moon in a month. That is true only under the modern definition; the older seasonal definition makes it the third of four. Another misconception is that the moon actually appears blue during a blue moon. It does not—the color is unrelated to the calendrical event. A third error is that blue moons happen very rarely; in fact, they occur about every two to three years, which is frequent enough that “once in a blue moon” exaggerates their rarity.
There is also a persistent myth that the Maine Farmers’ Almanac defined a blue moon as the second full moon in a month for centuries. In reality, the almanac’s rule was the seasonal one, and the monthly definition came from the 1946 Sky & Telescope article. The mistake was then popularized by a 1980s radio program and later by a popular children’s book, cementing the wrong definition in public memory.
How to See a Blue Moon
Seeing a blue moon requires no special equipment—just a clear sky on the night of a full moon. To know when one is coming, check a lunar calendar or an astronomy website. The next blue moon under the monthly definition is the second full moon in a calendar month; under the seasonal definition, it is the third full moon in a season with four. Both are listed in advance by almanacs and observatories.
If you want to see the moon actually appear blue, you need a major atmospheric event like a volcanic eruption or a large forest fire. These are unpredictable and not tied to the calendar. Most people will never see a blue-colored moon in their lifetime, but the calendrical blue moon is easy to witness—just mark the date and look up.
Comparisons with Other Full Moon Names
Unlike the blue moon, most full moon names have genuine folk origins. The Harvest Moon is the full moon nearest the autumn equinox, named for its help to farmers. The Wolf Moon is the January full moon, named by some Native American tribes for wolves howling in winter. These names are tied to seasonal activities and were passed down orally. The blue moon stands apart because it is not a seasonal name but a calendrical anomaly—it has no agricultural or cultural purpose.
Some traditions, like the Hindu calendar, have their own lunar months and full moon names, but they do not have a concept of a blue moon. The idea of an extra full moon breaking the usual naming scheme appears to be uniquely English-speaking and modern. This makes the blue moon more of a linguistic curiosity than a mythological one.
Frequently Asked Questions
Is a blue moon actually blue?
No. The calendrical blue moon looks the same as any other full moon. A blue-colored moon is a separate phenomenon caused by atmospheric particles, usually from volcanic eruptions or fires, and it is extremely rare.
How often does a blue moon occur?
Under the monthly definition, a blue moon happens about every two to three years. Under the seasonal definition, it also happens about every two to three years, but not in the same years as the monthly version. The two definitions rarely coincide.
What is the origin of “once in a blue moon”?
The phrase dates back to at least 1821 in English, where it meant something rare or absurd. It was not originally tied to an astronomical event. The connection to a second full moon in a month came only in the 20th century.
Can a blue moon affect human behavior?
There is no scientific evidence that any full moon, blue or otherwise, affects human behavior. The idea that full moons cause madness or crime is a persistent myth with no reliable supporting data.
What is the difference between a blue moon and a blood moon?
A blue moon is a calendrical event—a second full moon in a month or a third in a season. A blood moon is a total lunar eclipse, during which the moon appears reddish due to Earth’s shadow. They are unrelated.
Why is the blue moon called “blue” if it isn’t blue?
The name likely comes from the rarity of the event, not its color. The phrase “once in a blue moon” predates the astronomical definition, and the color may have been chosen simply because blue is associated with rarity or sadness in English. The atmospheric blue moon is a separate coincidence.
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