You’ve probably looked up at a perfectly normal, pearly-white moon and heard someone say, "Look, it’s a Blue Moon!"
It’s confusing. Honestly, it's a bit of a letdown if you’re expecting a giant sapphire floating in the night sky. Most of the time, a Blue Moon isn't blue at all. It’s just a regular moon that happens to be a bit of a mathematical rebel.
The phrase "once in a blue moon" makes it sound like some incredibly rare, mystical event that happens every fifty years. In reality? It’s just a quirk of our calendar. Humans decided a year should be twelve months long, but the moon didn't get the memo.
So, why called blue moon? The answer is a weird mix of 19th-century farm almanacs, a massive mistake by a 1940s amateur astronomer, and a very literal volcanic eruption that actually turned the sky colors.
The Two Modern Definitions (Blame Sky & Telescope)
There are actually two ways to define this event, and depending on who you ask, you'll get a different answer.
The most common version—the one you probably see on social media—is the "Monthly Blue Moon." This is when you get two full moons in a single calendar month. It happens because the lunar cycle is about 29.5 days, while our months are usually 30 or 31 days. Eventually, those leftover hours and days pile up. You get a full moon on the 1st or 2nd of the month, and then another one squeezes in right before the month ends.
But here’s the kicker: this definition was basically a typo.
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In 1946, a guy named James Hugh Pruett wrote an article for Sky & Telescope magazine. He was trying to interpret the Maine Farmers' Almanac, but he oversimplified it. He wrote that the second full moon in a month was a Blue Moon. He was technically wrong according to tradition, but his version was so simple that it stuck. It’s the version we use today because, let’s be real, it’s way easier to track on a kitchen calendar.
Then there’s the "Seasonal Blue Moon." This is the "correct" traditional version. A typical season (Spring, Summer, Fall, Winter) has three full moons. But every once in a while, a season will have four. When that happens, the third one is called the Blue Moon. Why the third and not the fourth? It was a way for the almanac makers to keep the names of the other moons—like the Harvest Moon or the Hunter’s Moon—aligned with the right time of year.
Why the Name "Blue" Specifically?
Nobody is 100% certain where the word "blue" came from, but etymologists have some solid theories.
One of the strongest theories points toward the Old English word belewe, which meant "to betray." The idea was that this extra moon was a "Betrayer Moon." It tricked people who were trying to calculate the dates for Lent and Easter. If you were a monk in the Middle Ages trying to set the religious calendar and an extra moon popped up, it threw your whole system out of whack. It betrayed the standard cycle.
Over hundreds of years, belewe likely morphed into "blue." It’s a classic linguistic game of telephone.
When the Moon Actually Turns Blue
Okay, so we know why the calendar event is called that. But does the moon ever actually look blue?
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Yes. But you usually don't want to be around when it happens.
For the moon to take on a blueish tint, the atmosphere has to be filled with specific particles. We’re talking about dust or smoke particles that are slightly wider than the wavelength of red light (about 0.7 micrometers). When these particles are present, they scatter the red light and let the blue light through.
The most famous instance happened in 1883. The volcano Krakatoa erupted with a force so violent it was heard 3,000 miles away. It blasted so much ash into the stratosphere that people all over the world saw blue moons—and green suns—for years.
Forest fires can do it too. In 1950, massive muskeg fires in Alberta, Canada, sent a thick plume of oily smoke across the Atlantic. People in England looked up and saw a lavender-blue moon. It’s an eerie, beautiful sight, but it usually means there’s an environmental disaster happening somewhere nearby.
The Math Behind the Rarity
If you're wondering how often you can catch one of these (the calendar kind, not the volcano kind), it’s pretty predictable.
Blue Moons occur roughly every 2.7 years.
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Because February is the shortest month, it’s the only month that can never have a Monthly Blue Moon. In fact, every 19 years, February has no full moon at all—a phenomenon known as a Black Moon. When that happens, you usually get two Blue Moons in the same year (one in January and one in March).
The last time we had a "Double Blue Moon" year was 2018. We won’t see that again until 2037.
Misconceptions That Just Won't Die
You'll often see "Super Blue Blood Moon" in news headlines. This sounds like a rare celestial alignment from a fantasy novel, but it’s just a pile-up of terms.
- Super: The moon is at perigee (closest to Earth), so it looks bigger.
- Blue: It’s the second moon in the month.
- Blood: There’s a lunar eclipse, which actually turns the moon red.
So, a "Blue Moon" that is also a "Blood Moon" is actually a red moon that is also the second moon of the month. It's confusing. It’s also a perfect example of how much we love to give the moon dramatic nicknames.
Why It Matters to Us
We’ve been obsessed with naming the moon for as long as we’ve been looking at it. Native American tribes, Anglo-Saxon farmers, and Hindu observers all have specific names for every full moon of the year.
Naming the "extra" moon was just a way to make sense of the chaos of the universe. We like patterns. We like things to fit into twelve neat boxes. When the universe gives us thirteen, we have to call it something special to feel like we’re still in control.
Whether it's a "betrayer" moon or just a mistake in a 1940s magazine, the Blue Moon represents the little glitches in our human systems. It's a reminder that our calendars are just an approximation of the wild, messy movements of space.
Next Steps for Moon Watchers:
- Check the Date: Look up the next Seasonal Blue Moon—it’s often a better "true" astronomical event than the monthly version.
- Gear Up: You don't need a telescope to see the color, but a pair of 7x50 binoculars will reveal craters even in a bright full moon.
- Check Local Air Quality: If the moon actually looks blue tonight, check the news for nearby wildfires or volcanic activity, as atmospheric particles are likely the cause.
- Track the 19-Year Cycle: Mark 2037 on your long-term calendar if you want to witness the next rare "Double Blue Moon" year.