Blue moon: the story of an expression born of a mistake
Second full moon in a month or third in a season with four? The two definitions of blue moon, the 1946 error and the rare moons that really look blue.
Few astronomical expressions have such a curious history. The blue moon has nothing blue about it, is not a particular physical phenomenon, and its most widespread definition arose from a misreading. Yet the term appears in the newspapers every few years and keeps a charm that derives from an old English saying and from a few rare atmospheric events.
Two definitions
Today the name blue moon is mostly given to the second full moon in a calendar month. Since the synodic month lasts about 29.5 days, while almost all calendar months have 30 or 31, it can happen that one full moon falls in the first days of a month and another in the last. This happens on average every two and a half to three years; in 2026, for example, May had two full moons, on the 1st and the 31st.
There is, however, an older definition, called seasonal. An astronomical season, from a solstice to an equinox, usually contains three full moons; when it contains four, the third is called a blue moon. This rule appeared in the Maine Farmers' Almanac, an American almanac, in the first half of the twentieth century, and served to keep the traditional names of the full moons aligned with their seasons.
The 1946 error
In 1946 an article in the magazine Sky & Telescope, trying to explain the almanac's rule, wrongly interpreted it as the second full moon of the month. The new definition was later taken up by radio programmes and quiz games and spread quickly. In 1999 the same magazine published a reconstruction of the affair, acknowledging the mistake, but by then the monthly version had entered common usage, and since then the two definitions have coexisted.
An English saying
In English the expression «once in a blue moon» means «very rarely» and is attested at least from the nineteenth century. Earlier still, in an English satirical pamphlet of 1528, saying that the moon is blue amounted to asserting an absurdity one was forced to believe. The original idea, then, was not that of a rare event but of something impossible, and only later did the sense shift towards rarity.
When the Moon really is blue
And yet the Moon can really look blue. After the eruption of Krakatoa in 1883 and during the great Canadian forest fires of September 1950, particles of a particular size suspended in the atmosphere filtered out red light, making the Moon and the Sun look bluish in several regions of the world. These are rare events, tied to the size of the particles and not to the calendar, which no almanac can predict.
In popular tradition the blue moon has no ancient, codified symbolic meaning: its aura of exceptionality is largely modern. Its history nevertheless teaches something valuable about the way traditions are born, which sometimes take shape not in the mists of time but between the pages of a magazine and the waves of a radio broadcast.
Anyone who wants to observe a blue moon, by either definition, will therefore see an ordinary full moon. What makes it special is not its colour but the calendar: a small mismatch between the rhythm of the Moon and that of human months, which every so often leaves room for one extra full moon.