What a 'Blue Moon' Really Is — and the 1946 Mistake That Changed Everything

Here's the twist: the definition of "Blue Moon" that basically everyone uses today came from a mistake. According to research by Philip Hiscock and Donald Olson published in Sky & Telescope, a 1946 article by James Hugh Pruett misread the older Maine Farmers' Almanac rule — and the misreading stuck so hard it rewrote the cultural rulebook entirely.

So what did the almanac actually say?

The Maine Farmers' Almanac in the 1930s used a completely different definition. According to that historical record, a Blue Moon was the third full moon in a season that had four. (Most seasons have three full moons. When a fourth showed up, the third one got the Blue Moon label — probably to keep the traditional named moons like Harvest Moon and Hunter's Moon landing on their usual schedule.)

It's a niche calendrical workaround, honestly. But it made sense for people who were literally farming by the moon.

Wait, so how did the mix-up happen?

In 1946, a writer named James Hugh Pruett published a piece in Sky & Telescope that referenced the almanac's Blue Moon concept — but interpreted it as the second full moon in a single calendar month. As Hiscock and Olson's research for Sky & Telescope documents, that reading was simply wrong. Pruett had misread what the almanac was actually tracking.

But the "second full moon in a month" idea was so much easier to explain and remember that it caught on. It spread. It became trivia. It became song lyrics. It became the thing your coworker confidently announces in the break room.

Honestly? Relatable. A clean, memorable rule will always beat a technically-correct complicated one in the wild.

Which definition is the "real" one now?

Both definitions exist in circulation, and Sky & Telescope's own research acknowledges the historical layering here. The seasonal rule (third of four full moons in a season) is the older one, rooted in the Maine Farmers' Almanac tradition. The monthly rule (second full moon in a calendar month) is the newer, more popular one — born from a 1946 misreading.

Neither version actually makes the moon turn blue, by the way. The color is figurative — a way of marking something rare and outside the usual rhythm. Which, when you think about it, is kind of a perfect metaphor for a definition that itself broke all the rules.

So the next time someone brings up the Blue Moon at a party, you now have the full, delightfully messy story. You're welcome.

For entertainment purposes only. Not a substitute for professional, medical, legal, or financial advice.

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