A blue moon is an extra full moon. By the popular modern definition, it's the second full moon to fall within a single calendar month. By the older definition, it's the third full moon in an astronomical season that happens to contain four instead of the usual three. Neither kind looks blue: a blue moon is a calendar quirk, and it turns up every two or three years.
The calendar-month blue moon
The Moon takes about 29.5 days to go from one full moon to the next. Every month except February is longer than that, so if a full moon lands on the first or second day of a 31-day month, there is room for another before the month runs out. The second one is the blue moon.
A few consequences follow from the arithmetic:
- February can never have one. It is shorter than a lunar cycle, so it can't hold two full moons.
- Some years have two. When February has no full moon at all, both January and March can end up with two. That happened in 1999 and again in 2018.
- It depends on your time zone. A full moon happens at one instant worldwide. If that instant is late on the 31st in New York, it is already the 1st in London. So a blue moon in one country can be an ordinary full moon in another, and the calendar pages for different time zones disagree.
On average, a calendar-month blue moon comes around about once every two and a half to three years, which is how the phrase "once in a blue moon" came to mean "not often, but not never."
The seasonal blue moon
The older rule is a little more involved, and it comes from almanac makers who gave each full moon of the year a name tied to its season. An astronomical season, the stretch between a solstice and an equinox, lasts roughly three months, so it normally contains three full moons. Every so often a season holds four.
Almanac writers didn't want to throw off the seasonal names, so they called the third of the four the blue moon. That way the last full moon of the season could keep its traditional name, such as the Moon Before Yule just ahead of the December solstice. The seasonal blue moon shows up at roughly the same rate as the calendar kind, but not usually in the same years.
How a 1946 misreading became the standard
The calendar rule that started it all was the seasonal one, which appears in editions of the Maine Farmers' Almanac from the 1930s. In March 1946, an article in Sky & Telescope magazine by the amateur astronomer James Hugh Pruett tried to explain the almanac's rule and simplified it into "the second full moon in a month." It was an honest mistake, and it was much easier to remember than the original.
The shorter rule spread through popular astronomy writing and then everywhere else. In 1999 Sky & Telescope published a correction, after the astronomer Donald Olson and colleagues, building on work by the folklorist Philip Hiscock, traced the error back to that 1946 article. By then the monthly definition had stuck. Today both are in use, and most news stories mean the monthly one.
There's something pleasing about a tradition that was invented by mistake. It also suggests that a good deal of "ancient" moon lore is younger than it sounds.
Can the Moon actually look blue?
Rarely, yes, and it has nothing to do with the calendar. A genuinely blue-tinted moon needs the right kind of particles in the air: smoke or volcanic ash with grains of a particular size, roughly a micron across, which scatter red light more strongly than blue. After the eruption of Krakatoa in 1883, people in many countries reported a blue or green moon for months. Large wildfires have produced similar sightings.
Most "blue moon" photographs you see online are something else: a camera's white balance set for indoor light, a filter, or a long exposure at twilight when the whole sky is blue. A calendar blue moon looks exactly like any other full moon, bright and pale, a little orange if you catch it rising.
Where "once in a blue moon" comes from
The phrase is older than either definition. In sixteenth-century English, saying someone would claim "the moon is blue" was a way of describing an absurdity, like saying it's made of green cheese. Over time the sense softened from "never" to "very rarely," and by the time almanacs adopted "blue moon" for an extra full moon, the idiom was already familiar. The calendar meaning borrowed the phrase, not the other way around.
Blue moons in astrology
Astrology has no special doctrine of blue moons. An astrologer reads a blue moon the way they would any full moon: the Sun and Moon stand opposite each other in a pair of signs, and the theme of the full moon comes from that pair. Because the Sun moves almost a whole sign between one full moon and the next, the two full moons in a "blue" month usually fall in neighboring signs, so each has its own flavor.
Some modern astrologers like to treat the blue moon as a bonus full moon, a second chance in the same month to finish something or let something go. That's a reflective framing rather than a rule, and it works fine if it helps you. Our full moon calendar lists the sign of each full moon, and the full moon ritual guide has a short way to mark one. For the names of the other full moons of the year, see full moon names by month.
Blue moons also get paired with other labels. A supermoon is a full moon near the Moon's closest point to Earth, and a blood moon is usually a total lunar eclipse. A full moon can be any combination of these, since each label describes something different: the calendar, the distance and the shadow.
Questions readers ask
How rare is a blue moon?
Not very. A calendar-month blue moon happens about once every two and a half to three years on average, and the seasonal kind at a similar rate. Rare enough to be worth a look, common enough that most people will see a few dozen in a lifetime.
What is a black moon?
It's an informal, modern term with no official definition. It is most often used for the second new moon in a calendar month, the dark counterpart of a monthly blue moon. Some people also use it for a month with no new moon at all, which can only happen in February.
Is a blue moon bigger or brighter than a normal full moon?
Not because it's a blue moon. Its size and brightness depend on how far the Moon is from Earth that night, which has nothing to do with the calendar. If a blue moon happens to fall near the Moon's closest approach, it will also be a supermoon.
Does a blue moon mean anything spiritually?
There's no traditional meaning attached to it, since the term itself is a twentieth-century calendar convention. Many people use it as a prompt for something they only do occasionally: a long walk after dark, a letter to an old friend, or a question for the daily tarot card.
