What is a blue moon and when does it occur?

Decoding the Blue Moon: It’s Not Actually Blue (Usually!)

Ever heard the term “blue moon” and pictured a giant, sapphire-colored orb hanging in the sky? Yeah, me too. Sadly, the moon rarely turns blue, but the name does signify something pretty special in the lunar cycle. So, what is a blue moon, really? And when can you catch one? Well, buckle up, because it’s a bit more complicated than you might think. Turns out, there are actually two definitions floating around!

Two Definitions? Seriously?

Yep! The definition most people know is that a blue moon is the second full moon we get in a single calendar month. Think about it: a lunar cycle (from full moon to full moon) takes about 29.5 days. That means it’s kinda rare to squeeze two full moons into a 30 or 31-day month. Poor February, with its measly 28 or 29 days, never gets a blue moon this way.

But here’s a fun fact: this “second full moon” definition isn’t the original one! The older, more traditional meaning – sometimes called a seasonal blue moon – refers to the third full moon in an astronomical season that has four full moons. Now, astronomical seasons (spring, summer, fall, winter) are defined by equinoxes and solstices. Usually, each season gets three full moons. But when a season gets a bonus moon, making it four total, that third one gets the “blue moon” title. Confusing? A little, I know.

Blame it on a Misunderstanding

So, how did we end up with two definitions? Well, the “second full moon in a month” thing actually came from a misinterpretation of the Maine Farmers’ Almanac. Back in 1946, an amateur astronomer named James Hugh Pruett wrote about it in Sky & Telescope magazine, referencing the almanac. Basically, he simplified things a bit too much, and the “monthly” blue moon was born! Even though it’s based on a mistake, it’s now the definition most people recognize. Go figure!

How Often Can I See One?

Despite the saying “once in a blue moon,” these events aren’t super rare. On average, you’ll see a blue moon (either kind) every two to three years. Monthly blue moons happen because, as we said, the lunar cycle is a tad shorter than most months. Seasonal blue moons are just a result of the slight mismatch between the lunar cycle and the solar year.

I read somewhere that, on average, a blue moon occurs about once every 33 months, which works out to roughly 41 times a century, or about seven times every 19 years. And get this: sometimes we even get two blue moons in the same calendar year! That only happens about four times a century, though. Talk about rare!

Will it Actually Look Blue?

Okay, let’s get this straight: despite the name, a blue moon almost never looks blue. The name has nothing to do with the moon’s color and everything to do with its position in the calendar. However, under very specific atmospheric conditions, the moon can appear bluish. This happens when there’s a bunch of dust or smoke particles hanging out in the atmosphere. These particles scatter red light, allowing more blue light to get through. Volcanic eruptions and big wildfires can create these conditions. For example, after the Krakatoa eruption way back in 1883, people reported seeing a blue moon for almost two whole years! Imagine that!

So, When’s the Next One?

Mark your calendars! The next blue moon will be on May 31, 2026. And it’ll be the “monthly” kind.

A Little Blue Moon Trivia

Did you know the phrase “once in a blue moon” was around before the astronomical definition? It just meant something that hardly ever happens. And contrary to what you might think, there’s no real evidence that blue moons were ever considered unlucky.

The Bottom Line

Whether you’re a fan of the modern “monthly” definition or you prefer the old-school “seasonal” one, a blue moon is a cool reminder of the celestial dance that shapes our calendar. Even if the moon doesn’t turn blue, it’s a great excuse to step outside and appreciate the amazing night sky. Who knows, maybe you’ll even get lucky and see a real blue-ish moon someday!