The Phrase That Launched a Thousand Misconceptions
We toss around "once in a blue moon" whenever something is delightfully rare — a perfect parking spot, a quiet Monday morning, a summer without a single mosquito bite. But ask most people what a blue moon actually is, and you'll get a puzzled shrug. Turns out, the answer isn't as simple as you'd hope — and there are, in fact, two completely different definitions fighting for the title.
Definition #1: The Seasonal Blue Moon (The Original)
The older and arguably more legitimate definition comes from the Maine Farmers' Almanac, which tracked moon names tied to the seasons rather than calendar months. Each season normally contains three full moons, and each of those moons carries a traditional name — the Harvest Moon, the Hunter's Moon, the Snow Moon, and so on.
But here's the wrinkle: because the lunar cycle is about 29.5 days and our calendar year is 365 days, we end up with 13 full moons in some years instead of the usual 12. When a season ends up with four full moons instead of three, the third of those four was historically called the Blue Moon. The reasoning was practical: it kept the fourth moon in its rightful named slot at the end of the season.
By this definition, the next seasonal blue moon falls on May 20, 2027 — the third full moon in a spring that stretches from the March equinox to the June solstice.
Definition #2: The Monthly Blue Moon (The Popular One)
The second definition is newer, bolder, and — if we're honest — born from a misunderstanding. In 1946, amateur astronomer James Hugh Pruett wrote an article in Sky & Telescope magazine interpreting the Almanac's rule incorrectly. He defined a Blue Moon simply as the second full moon in a single calendar month.
The error stuck. It was popularized further in a 1980s radio trivia show and eventually cemented into common usage. By this definition, a blue moon happens whenever a calendar month squeezes in two full moons — possible only in months with 31 days (since the lunar cycle is ~29.5 days) or, very rarely, February gets skipped entirely while neighboring months each grab two.
The next monthly blue moon by this definition? January 31, 2028, when January hosts full moons on both the 1st and the 31st.
You can track all upcoming full moons and their exact timing using ChronoKit's Moon Phases calendar — handy for knowing which months are primed for a blue moon by either rule.
So… Is It Actually Blue?
Almost never. The color of the moon depends entirely on what's floating in Earth's atmosphere, not on how many full moons have occurred that month. There are, however, documented cases of the moon appearing genuinely blue:
- Volcanic eruptions — after Krakatoa erupted in 1883, observers worldwide reported a strikingly blue moon for nearly two years as fine ash particles scattered red light
- Large forest fires — massive wildfire smoke can have the same effect, filtering out longer red wavelengths and letting blue light through
- Dust storms — rare but documented in arid regions
In other words, if you ever see an actual blue moon, something dramatic has happened to our atmosphere. It's a sign worth noticing — and possibly photographing.
How Often Does a Blue Moon Happen?
Putting both definitions aside and just asking "how often does an extra full moon occur?" the answer is roughly every 2.5 to 3 years. The phrase "once in a blue moon" is therefore a bit of an exaggeration for something that rare — but it captures the spirit well.
Here's a quick breakdown:
- Monthly blue moons occur about every 2–3 years
- Seasonal blue moons also occur roughly every 2.5 years
- Both can technically occur in the same year (though it's uncommon)
- A full moon on New Year's Eve — always a festive sight — has about a 1 in 19 chance of being a blue moon by the monthly definition
If you want to keep tabs on the moon's current phase, rise and set times, or how full tonight's moon will look from your location, ChronoKit's Moon tracker has you covered in real time.
A Rare Moon Worth Watching
There's something quietly satisfying about knowing the real story behind a phrase you've used your whole life. The blue moon isn't a cosmic anomaly or a celestial glitch — it's a bookkeeping problem, a leftover from centuries of trying to sync the wild lunar cycle to our tidy human calendar. The moon doesn't care about our months. It just keeps cycling, indifferent and luminous, painting the sky silver whether we've named the moment or not.
And if you happen to step outside on the next blue moon and find it looking perfectly ordinary — well, that's exactly the point. The rarity was never about the color. It was about the counting.
Track every upcoming full moon, blue moon, and lunar phase with ChronoKit's Moon Phases calendar so you never miss one.