You have probably heard someone say something happens “once in a blue moon.” It is one of those phrases everyone uses and almost nobody stops to unpack. It means something very rare, something that hardly ever happens. But what is a blue moon, exactly? Is it a moon that turns blue? Does it happen once in a lifetime? And when is the next one?
The honest answer is simpler than the phrase suggests, and a little stranger. A blue moon is an extra full moon. It is not a different kind of moon, and most of the time it looks exactly like any other full moon — big, bright, and white. What makes it special is timing, not colour. The name comes from the calendar, not from the sky.
In this guide, we answer the question of what is a blue moon, explain both definitions of a blue moon, why the name stuck, how often blue moons really happen, whether the moon can ever look blue, and the exact dates of the next ones so you can plan to see one yourself.
The short answer: A blue moon is an extra full moon. In the definition most people use today, it is the second full moon in a single calendar month. In the older definition, it is the third full moon in a season that contains four full moons. Either way, the moon does not normally turn blue.
What Is a Blue Moon?
What is a blue moon? It is what happens when the rhythm of the lunar cycle and the rhythm of our calendar fall out of step with each other. The moon takes about 29.5 days to go from one full moon to the next. Our calendar months, though, are 30 or 31 days long (except February). That small mismatch adds up.
Twelve lunar cycles take about 354 days, which is roughly 11 days shorter than a calendar year. So every two or three years, the extra days pile up enough to squeeze a thirteenth full moon into a single year. That thirteenth full moon is the blue moon. It is a perfectly ordinary full moon that simply arrives in an unexpected slot.
Nothing about the moon itself changes on a blue moon night. It is the same moon, at the same distance, shining with the same reflected sunlight. The only thing that is unusual is where it falls on the calendar. That is why astronomers sometimes call it a calendar event rather than an astronomical one — although it is still a lovely excuse to go outside and look up.
More Information: When Is the Next Lunar Eclipse? (2026 Guide)
The Two Definitions of a Blue Moon
Here is where it gets confusing, and where the question of what is a blue moon gets two answers: there are two different definitions of a blue moon, and both are in common use. A full moon can count as a blue moon under one definition but not the other. Knowing both saves you from arguments, because most disagreements about blue moons come down to people using different definitions without realising it.
The Monthly Blue Moon
The monthly blue moon is the definition most people know today. It is the second full moon in a single calendar month. Since a full moon comes around every 29.5 days and most months are 30 or 31 days long, a month can occasionally hold two of them — one near the start of the month and one near the end.
The most recent monthly blue moon fell on May 31, 2026. May had already seen a full moon on May 1, so the second one on May 31 earned the title. If you missed it, you have a long wait for the next monthly one: December 31, 2028. That New Year’s Eve blue moon will be extra special, because a total lunar eclipse is expected on the same night.
The Seasonal Blue Moon
The seasonal blue moon is the older, traditional definition. It comes from the Maine Farmer’s Almanac, a publication that named full moons for farming and seasonal purposes. A season — spring, summer, autumn or winter — normally has three full moons. Occasionally, a season has four. When that happens, the third full moon of the four is called the blue moon.
This definition is the original one, and it is still used by almanacs and many astronomers. The next seasonal blue moon falls on May 20, 2027, the third full moon of the spring season that year. Notice that this is a different date from the next monthly blue moon. That is normal. The two definitions run on different clocks, so they rarely agree.
Why Is It Called a Blue Moon?
Nobody knows for certain where the phrase came from, but the most widely accepted story starts with a mistake. In 1946, an article in Sky & Telescope magazine tried to explain the old almanac definition and got it wrong, describing a blue moon as the second full moon in a month instead of the third in a season of four. The error stuck. Radio programmes and newspapers repeated the simpler monthly version, and within a few decades it had become the definition most people knew.
The phrase “once in a blue moon” is older than either definition. It has been used since the 19th century to describe something extremely rare, in the same spirit as “when pigs fly.” The idea was that a blue moon was something so unusual it might as well be impossible. The phrase caught on because it paints a vivid picture: everyone knows the moon is white, so a blue one sounds like a once-in-a-lifetime sight. That history is part of what is a blue moon in popular language, not just in astronomy.
Is the Moon Ever Actually Blue?
Very rarely, yes — but not because of the calendar. The moon can genuinely appear blue when the air is full of tiny particles of a very specific size, roughly one micron across. Particles that size scatter red light and let blue light through, tinting the moon.
The most famous example followed the eruption of Krakatoa in 1883. The volcano threw so much ash into the atmosphere that the moon looked blue for years afterwards, and sunsets turned strange colours around the world. Large wildfire smoke can do the same thing on a smaller scale: smoke particles of the right size have turned the moon blue over parts of North America after major forest fires.
So a blue moon can be blue, but the colour has nothing to do with what is a blue moon in calendar terms, or whether it is the second full moon of the month. On a normal blue moon night, expect a bright white-gold moon, just like any other full moon. If it ever does look blue, it is the atmosphere doing it, not the calendar.
How Often Do Blue Moons Happen?
Blue moons are uncommon, but if you ask what is a blue moon and how often it happens, the answer is that they are not as rare as the saying suggests. On average, a blue moon of one kind or the other comes around every two to three years. Monthly blue moons and seasonal blue moons each appear roughly that often, so if you count both definitions, you get a blue moon a little more often than the saying implies.
A few quirks are worth knowing. February can never have a monthly blue moon, because the month is shorter than the 29.5-day lunar cycle — it simply cannot hold two full moons. In fact, February can have no full moon at all, as happened in 2018. When that happens, January and March each get two full moons, giving two monthly blue moons in one year. Astronomers call that a double blue moon. The next one is due in 2037, which will also be a year with two blue moons.
When Is the Next Blue Moon?
If you want to catch the next one, here are the dates that matter. These are the next blue moons under each definition, checked against current lunar calendars:
| Date | Type | Notes |
|---|---|---|
| May 31, 2026 | Monthly | Already passed — second full moon of May 2026 |
| May 20, 2027 | Seasonal | Third full moon of spring 2027 — the next blue moon |
| December 31, 2028 | Monthly | Second full moon of December 2028, same night as a total lunar eclipse |
The seasonal blue moon on May 20, 2027 is the next one to look forward to, and it is the clearest answer for anyone asking what is a blue moon and when the next one will appear. It will be visible from everywhere on Earth where the sky is clear, and you will not need any equipment to enjoy it. Mark the date, and check the weather closer to the time.
Blue Moon vs Supermoon vs Blood Moon
People often mix up the names of special full moons. They sound similar, but they describe completely different things. This table sorts them out:
| Moon | What it means | How often |
|---|---|---|
| Blue moon | An extra full moon by calendar or season | Every 2–3 years |
| Supermoon | A full moon near perigee, the closest point of its orbit | Several times a year |
| Blood moon | A full moon during a total lunar eclipse, coloured red by Earth’s shadow | A few times per decade |
A single full moon can be more than one of these at once, which is why what is a blue moon is a question about the calendar, not about size or colour. The May 31, 2026 blue moon was also the most distant full moon of the year, making it a micromoon — the opposite of a supermoon. And the December 31, 2028 blue moon will double as a blood moon during a total lunar eclipse. The names describe different features, so they can overlap.
More Information: Under Armour Men’s Unstoppable Fleece Joggers (2026 Guide)
How to See a Blue Moon
Seeing a blue moon needs no planning beyond knowing the date. A full moon rises around sunset and stays up all night, so it is the easiest moon phase to catch. Step outside shortly after sunset and look towards the eastern horizon. The moon will be large, bright and hard to miss.
For the best view, find a spot away from streetlights with a clear view of the horizon. The moon looks biggest when it is low in the sky, just after moonrise, thanks to a trick of perception called the moon illusion. Your brain compares the low moon with trees and buildings and judges it larger than the same moon high overhead. It is not actually bigger, but it certainly feels that way.
If you want a photo, use the longest zoom you have and include something in the foreground — a tree, a rooftop, a hill — for scale. A phone camera will capture it, but the moon will look small without zoom. A small tripod or a steady surface helps keep the shot sharp.
Common Myths About Blue Moons
A few myths follow blue moons around, so let us clear them up. First, the moon does not turn blue on a blue moon night, except in the rare atmospheric conditions described above. Second, blue moons do not happen once a century or once in a lifetime — they come around every two or three years. Third, there is no good evidence that a blue moon affects tides, weather, sleep or human behaviour any more than an ordinary full moon does. The tides follow the moon’s phase and distance, not its place on the calendar. Understanding what is a blue moon helps to separate the calendar fact from the folklore.
The real appeal of a blue moon is simpler than any of that. It is a reminder that the sky keeps its own time, slightly out of step with our calendars, and every few years the two drift far enough apart to give us an extra full moon. That is worth looking up for.
Frequently Asked Questions
What is a blue moon in simple terms?
A blue moon is an extra full moon. Most often it means the second full moon in one calendar month. In the older definition, it is the third full moon in a season with four full moons.
Is the moon actually blue during a blue moon?
Almost never. The moon normally looks white or pale gold. It can look blue only when volcanic ash or wildfire smoke fills the air with particles of just the right size to scatter red light.
How rare is a blue moon?
Not very rare. A blue moon comes around every two to three years on average. The phrase “once in a blue moon” makes it sound rarer than it really is.
When is the next blue moon?
The next blue moon is the seasonal one on May 20, 2027. The next monthly blue moon falls on December 31, 2028, on the same night as a total lunar eclipse.
Can February have a blue moon?
No. February is shorter than the 29.5-day lunar cycle, so it can never hold two full moons. In some years February has no full moon at all.
What is a double blue moon?
A double blue moon is a year with two monthly blue moons. It happens when February has no full moon, so January and March each get two. The next one is due in 2037.
Is a blue moon the same as a supermoon?
No. A blue moon is about the calendar — an extra full moon. A supermoon is about distance — a full moon near perigee, the closest point of its orbit. A full moon can be both, but the terms mean different things.
Final Thoughts
What is a blue moon? It is a small calendar quirk with a great name. It is an extra full moon that appears when the 29.5-day lunar cycle drifts out of step with our months and seasons, giving us thirteen full moons in a year instead of twelve. It usually looks like any other full moon, and that is fine — the pleasure is in knowing what you are looking at and why it is there.
The next one arrives on May 20, 2027. Put it in your calendar now, and when the night comes, step outside just after sunset and look east. You will be watching a piece of sky history that people have named, argued about and celebrated for generations — and it will cost you nothing but a few minutes of your evening.
Read Also: What Is a Blood Moon? Meaning, Causes and How to See One
1 Comment