You’ve heard the phrase too many times to count: “Once in a blue moon.” But what is a blue moon, exactly? (Hint: The moon doesn’t actually turn blue, unfortunately.)
Despite the misleading name, blue moons are actually fascinating for plenty of reasons—beyond being a great excuse to revel in the beauty of the night sky, they reveal way more about our culture and our history than you might think.
Even though the next blue moon is still pretty far off, there’s plenty to keep you glancing up in the meantime. Here’s everything you need to know about the lunar event, plus when you can catch the next one.
What is a blue moon, exactly?
A blue moon, by the most common definition, is a second full moon that appears in a calendar month. This moon looks no different than any regular full moon, explains Walter Freeman, associate professor of physics and astronomy at Syracuse University in New York.
But since full moons occur roughly 29.5 days apart (just enough to typically appear only once per month), blue moons are rare, only showing up about once every 2.5 years, Freeman says. If a full moon occurs in the first day or two of most months, there will be a blue moon at the end of the month. Blue moons cannot occur in February, even during leap years, because the month is shorter than a lunar cycle.
The term can also be used to describe the third of four full moons in a calendar season, Freeman notes, because there are usually only three full moons per season. If four full moons happen to fall between a solstice and an equinox, the third of the group will be a blue moon. Fittingly, these are called seasonal blue moons.
This is actually the older of the two definitions, but our cultural understanding of “blue moon” has changed over time: “Because it’s not a term of art in astronomy, with some precise, technical definition, it means whatever people think it means,” Freeman explains.
A misunderstanding of the term originally published in the 1940s eventually caught on in pop culture, according to EarthSky, leading to calendrical blue moons being the most widely recognized.
Two blue moons can occur in one calendar year, but that’s even rarer, only happening about four times each century, per NASA. They’re most likely to occur in January and March, and only when there is no full moon in February, which is known as a black moon. The last such event was in 2018; the next one isn’t until 2037.
Why is it called a blue moon?
The origin of the name “blue moon” is unclear, but NASA notes that it could be traced back to the 1883 eruption of the volcano at Krakatoa—an island in the Sunda Strait between Java and Sumatra, Indonesia—which tinted the atmosphere with ash, making the moon appear blue. The phrase “once in a blue moon,” used to describe something that happens infrequently, was likely coined in the wake of this event. Over time, any extra full moon came to be known as a blue moon.
Does a blue moon actually look blue?
Blue-colored moons are even rarer than blue moons, and they’re not necessarily full. “The moon does’t make its own light; it just reflects sunlight,” Freeman explains. “Anything that would change the appearance of the moon is something that happens here, rather than something that happens to the moon.”
Smoke, dust, and other particles in the atmosphere can act as a filter for moonlight, creating special circumstances in which the moon will appear blue from our vantage point. This is why the moon might have looked blue following the Krakatoa eruption, and it’s similar to the spectacular sunsets caused by dust clouds.
This can happen during any moon phase, not just full moons. Anyone hoping to catch a glimpse of a genuine blue-colored moon might be out of luck, though: Freeman says that he’s never seen this happen, since it’s such a rare phenomenon.
What does a blue moon symbolize?
If you want to get deep into the meaning behind blue moons, chew on this: They aren’t special for any cosmic reason—and some cultures don’t even recognize them.
Blue moons are essentially a side effect of the way we organize time, Freeman explains. “These things in the sky, they were the first clocks,” he says. “We know the sun rises every 24 hours, we know the seasons change about every 365 times the sun rises, and then we know the moon goes through a cycle of phases about every 29 or 30 days.” But these lengths of time don’t fit together evenly.
When humans started organizing time into calendars, different groups accounted for these “random amounts of time” in their own ways. “It’s an interesting window into the world’s cultures, looking at the different choices people made,” Freeman says.
The Gregorian calendar, the most widely used one today, is based primarily on the sun, meaning it prefers matching up with the seasons over following the full moon. Each season roughly lines up with a lunar cycle, but not perfectly.
Other cultures had different ideas: The Islamic calendar is purely lunar, with each year measured by a 12-month cycle of 354 or 355 days. (The discord between the seasons and the lunar cycle is why holidays like Ramadan can occur in the summer or the winter.) The lunisolar Chinese and Hebrew calendars also measure months by the full moon, but account for that difference by adding a leap month every few years. By design, these calendars don’t include blue moons.
Before the invention of the Gregorian calendar, Freeman says, blue moons didn’t exist; they are a relatively recent concept. Because of this, they don’t have much symbolism outside of astrology.
When is the next blue moon?
Since the last calendrical blue moon occurred on Halloween in 2020, the next blue moon will take place on Friday, August 30, 2023. And to help pass the time, the next seasonal blue moon will occur on Sunday, August 22, 2021.
Go here to join Prevention Premium (our best value, all-access plan), subscribe to the magazine, or get digital-only access.
You Might Also Like