You’ve said it. I’ve said it. Everyone’s grandmother has definitely said it. But if you actually stop to think about the meaning of once in a blue moon, the logic starts to feel a bit shaky. Does the moon actually turn blue? Is it some weird atmospheric glitch, or are we all just repeating a phrase that lost its literal meaning four hundred years ago?
Basically, when someone tells you they hit the gym "once in a blue moon," they aren't talking about astronomy. They're telling you they’re lazy. It’s a polite way of saying "almost never." But the history of how we got there is a messy mix of folklore, volcanic eruptions, and a massive mistake in a 1946 issue of Sky & Telescope magazine.
The Two Ways a Moon Becomes "Blue"
There isn't just one definition. That’s where the confusion starts.
Most people today use the "Monthly Blue Moon" rule. This is when you get two full moons in a single calendar month. Since the lunar cycle is roughly 29.5 days and our months (mostly) last 30 or 31 days, the math eventually forces a squeeze. Every two or three years, a month ends up with a "bonus" full moon. That second one is the Blue Moon.
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But there’s an older, more complicated version called the Seasonal Blue Moon. Back in the day, the Maine Farmers' Almanac defined it as the third full moon in a season that has four. Usually, a season (Spring, Summer, Fall, Winter) has three full moons. When the timing shifts and a fourth one sneaks in before the equinox or solstice, the third one is dubbed "blue."
Why the third and not the fourth? It’s kind of a quirk of ecclesiastical logic. The fourth moon needed to keep its proper name so the calendar for holidays like Lent and Easter wouldn't get wrecked.
Did the Phrase Always Mean "Rare"?
Not exactly. Language is weird.
If you go back to the 16th century, saying the "moon is blue" was a way of saying something was impossible or absurd. It was like saying "when pigs fly." In 1528, a couple of friars wrote a pamphlet attacking the English clergy, using a rhyme that basically said if the church told people the moon was blue, they’d have to believe it.
It was about deception. It was about things that literally could not happen.
Somewhere along the line, "impossible" softened into "rare." By the 19th century, the meaning of once in a blue moon shifted toward the temporal. It stopped being a joke about a fake color and started being a benchmark for time.
The Great 1946 Blunder
We have to talk about James Hugh Pruett. He was an amateur astronomer writing for Sky & Telescope. In an article titled "Once in a Blue Moon," he misinterpreted the Maine Farmers' Almanac rule. He simplified it, stating that the second full moon in a month was the Blue Moon.
He was technically wrong according to the traditional source.
However, his "mistake" was so much easier to understand than the seasonal calculation that it stuck. The StarDate radio program picked it up in 1980. Then Trivial Pursuit put it on a card. Suddenly, Pruett’s error became the global standard. Today, even NASA acknowledges both definitions because language is defined by how people actually use it.
Can the Moon Actually Turn Blue?
Yes. But you probably don't want to be around when it happens.
For the moon to physically look blue, the atmosphere has to be filled with particles of a very specific size—slightly wider than the wavelength of red light (about 0.7 micrometers). When these particles are present, they scatter red light and let the blue light pass through.
It’s rare. Like, "global catastrophe" rare.
- Krakatoa (1883): When this Indonesian volcano blew its top, the ash stayed in the atmosphere for years. People across the globe reported sunset colors that looked like blood and a moon that appeared vividly blue or even green.
- Mount St. Helens (1980): Similar deal. Small-scale reports of a bluish moon followed the eruption.
- Canadian Forest Fires (1950): A massive fire in Alberta sent a plume of smoke across North America and over to Europe. For a few nights, the sun and moon looked blue through the haze.
So, while the meaning of once in a blue moon is usually metaphorical, it has a literal, slightly terrifying cousin rooted in environmental upheaval.
Why We Love This Phrase So Much
There is something deeply human about tying our habits to the sky. We could just say "infrequently." We could say "seldom." But those words are boring. They lack the imagery of a lone, glowing orb defying its usual ivory hue.
In modern pop culture, the phrase has become a staple. Bill Monroe wrote a bluegrass classic about it. Frank Sinatra crooned about it. It represents the "hail mary" moment—the one-off chance that shouldn't happen but does.
Honestly, the phrase works because it feels significant. If you tell your boss you check your personal email "once in a blue moon," it sounds much more poetic and excusable than saying "I check it twice a month."
How Often Does It Actually Happen?
If we're going by the calendar definition (the second moon in a month), a Blue Moon occurs roughly every 33 months.
If you want to get really nerdy, you can look for a "Double Blue Moon." This is when two Blue Moons happen in the same calendar year. This only happens about four times a century. It occurred in 2018 and won't happen again until 2037. During those years, January and March both have two full moons, while February—the short, unlucky sibling—usually has no full moon at all.
That’s called a Black Moon, just to make things more confusing.
Put the Phrase to Use
The meaning of once in a blue moon is essentially a tool for managing expectations.
If you’re writing or speaking, use it to highlight something that feels special or out of the ordinary. Don’t waste it on things that happen every week. It loses its punch. It’s for the friend who finally calls from overseas. It’s for the local team winning the championship. It’s for that specific, rare steak you only treat yourself to on birthdays.
Actionable Insights for the Curious
- Track the next one: Don't just take my word for it. The next calendar Blue Moon is slated for May 31, 2026. Mark your calendar. It won't be blue, but it’s a good excuse for a party.
- Check the source: If you see a "Blue Moon" mentioned in an old book (pre-1940s), remember they are likely using the seasonal definition, not the two-in-a-month rule.
- Watch the sky after fires: If there are major wildfires in your region, look up. You might actually see the physical phenomenon that inspired the literal side of the phrase.
- Audit your "Never": Start noticing how often you use this idiom. Are you using it for things that actually happen monthly? If so, you’re being more astronomically accurate than you realized.
The beauty of the English language is that it’s a living, breathing mistake. We took a 16th-century joke about an impossible moon, mixed it with a 19th-century almanac, added a 1940s magazine error, and turned it into one of the most recognizable phrases in the world. It’s a mess. But it’s our mess.
Next time you see a full moon, remember that it doesn't need to change color to be rare. It just needs to show up at the right time.