We use “once in a blue moon” for something that happens very rarely. My parents said it all the time when I was growing up, and I always wondered what it really meant, and where on earth it came from.
The origin of the phrase turns out to be stranger than its modern meaning suggests. It passed through three very different stages: a 16th-century joke about an impossible claim, a 19th-century expression for a long time, and a 20th-century astronomical definition that began with a misunderstanding. And despite the name, a blue moon usually looks completely ordinary.
1528: a moon that was only blue because someone said so
The oldest known appearance of “blue moon” in English comes from a 1528 satire called Rede Me and Be Nott Wrothe. It was written by the friars William Roy and Jerome Barlow, printed in Strasbourg, and aimed at Cardinal Thomas Wolsey and the Catholic clergy, as Wikipedia’s blue moon article explains. It contains the lines “Yf they saye the mone is belewe, / We must beleve that it is true”. In other words, if they tell us the moon is blue, we must believe it.
The point wasn’t astronomy. A blue moon was offered as an obviously absurd claim, something people might accept only because someone in authority said it. It was much closer to “the moon is made of green cheese” than to anything about lunar calendars, so saying something happened once in a blue moon was originally the same as saying it never happened.
There’s no evidence the 1528 line evolved directly into the modern phrase, though. It’s best seen as an early ancestor of the same image.
1821: a blue moon becomes a long time
Almost three centuries later, the phrase turns up with a different meaning. In his 1821 book Real Life in London, the writer Pierce Egan has one character tell another he “haven’t seen you this blue moon”. He adds a footnote explaining that it usually implies a long time, and the Oxford English Dictionary treats it as the earliest example of that sense.
Exactly how this meaning arose is uncertain. One theory is that it grew out of the older phrase “once in a moon”, meaning once a month or occasionally, with “blue” making the interval seem rarer still. Either way, “once in a blue moon” itself is recorded by 1833, in essentially the sense we use today. The idiom was well established long before anyone used it as an astronomical term.
1937 and 1946: an almanac, and a magazine mistake
In 1937, the Maine Farmers’ Almanac used “blue moon” for a particular full moon. An astronomical season normally contains three full moons, but occasionally it has four. The almanac called the third of those four the blue moon, so the traditional seasonal names of the others could stay in their expected places. Researchers haven’t found an earlier source for this calendar sense. This is now called the seasonal blue moon.
Then came the mistake. In March 1946, amateur astronomer James Hugh Pruett wrote an article for Sky & Telescope called “Once in a Blue Moon”. He worked from a brief earlier mention in the same magazine rather than the almanac itself, and he took a blue moon to mean the second full moon in a calendar month.
The error is obvious in hindsight. Researchers Donald Olson, Richard Tresch Fienberg and Roger Sinnott eventually tracked down the 1937 almanac. As Sky & Telescope later explained, that year had only 12 full moons, and the one the almanac flagged, on 21 August, clearly wasn’t the second full moon of that month. In their 1999 article, the researchers reported finding a dozen almanac blue moons, and not one was a second full moon in a month.
Pruett’s simpler definition stuck anyway. Folklorist Philip Hiscock traced how it spread, as World Wide Words recounts. It went from the 1946 article to a radio program in 1980, then to The Kids’ World Almanac of Records and Facts in 1985, and then into the board game Trivial Pursuit in 1986. In 1999, Sky & Telescope published the research tracing the modern definition to its own pages, admitting to what it called its “Blue Moon blooper”. The meaning millions of people know today is, in effect, a popular misunderstanding that became standard usage.
| Year | What happened |
|---|---|
| 1528 | A satire uses a blue moon as an example of an absurd claim |
| 1821 | Pierce Egan uses the phrase to mean a long time |
| 1833 | “Once in a blue moon” is recorded in its modern sense |
| 1937 | The Maine Farmers’ Almanac uses it for the third of four full moons in a season |
| 1946 | James Hugh Pruett misreads it as the second full moon in a month |
| 1980-1986 | Radio, a children’s almanac and Trivial Pursuit spread the new definition |
| 1999 | Sky & Telescope traces the error back to its own pages |
Two kinds of blue moon
Today, both definitions are in use, so different sources may give different dates. A monthly blue moon is the one most people mean: the second full moon in a calendar month. Because the lunar cycle is about 29.5 days, most months have one full moon, but every so often the timing squeezes in two.
A seasonal blue moon is the older almanac version: the third full moon in a season that has four. Neither has anything to do with colour.
Can the moon actually look blue?
Yes, but that’s a completely different thing. A genuinely blue-looking moon needs huge amounts of very fine particles in the atmosphere. Sky & Telescope notes that it happens only exceedingly rarely, when volcanic eruptions or forest fires send lots of smoke and fine dust into the sky. The eruption of Krakatoa in Indonesia in 1883, and huge Canadian forest fires in the early 1950s, are both reported to have tinted the moon blue in parts of the world. A calendar blue moon, by contrast, looks just like any other full moon.
The next blue moons, and a surprise for Australia
The most recent monthly blue moon peaked at 8:45 UTC on 31 May 2026, according to EarthSky. The next is a seasonal blue moon on 20 May 2027, followed by a monthly one on 31 December 2028.
There’s a catch for Australians, though. Full moon times are calculated in Universal Time. As Astronomy magazine explains, converting to local time far from Greenwich can push a full moon into the next day, or the next month, so it’s no longer a blue moon.
The December 2028 full moon happens at 16:48 UTC on 31 December, according to Star Walk, which is already 1 January 2029 everywhere in Australia. That’s about 12:48 am in Perth, 2:18 am in Darwin, 2:48 am in Brisbane, 3:18 am in Adelaide and 3:48 am in Sydney and Melbourne. So for us, December 2028 won’t have a blue moon at all.
Here’s the surprise: we actually do better than the rest of the world. Star Walk notes that in time zones at least eight hours ahead of Greenwich, which includes all of Australia, February 2029 has no full moon at all. So Australia gets two blue moons instead, on 30 January and 30 March 2029, while most of the world gets just one. Even something as simple as two full moons in a month depends on where you’re standing.
Older than the moon it names
What I love about this story is that it runs the opposite way to what most people assume. The saying didn’t come from the second full moon in a month. The saying came first, by more than a century, and the astronomical meaning was attached to it later, partly by accident.
Words often drift like that, starting with one meaning and ending up somewhere else entirely, as I found when I looked into Shakespeare’s supposed 1,700 invented words. And if you want to stay with the moon, my post on the barycentre explains why it doesn’t simply orbit the centre of Earth.
Sources
- Wikipedia: Blue moon
- Sky & Telescope: True blue moon occurs Sunday, August 22nd
- Olson, Fienberg and Sinnott: What’s a blue moon?, Sky & Telescope, May 1999 (Texas State University digital library, PDF)
- World Wide Words: Blue moon
- EarthSky: When is the next blue moon?
- Astronomy: What is a blue moon? When’s the next one?
- Star Walk: Next blue moon dates