Have you ever heard someone say that a situation happens only “once in a blue moon”? This usually means that whatever is being described is so rare that you’re not likely to see it for a very long time. But where did this phrase come from? What exactly is a blue moon — and are they really that rare?

So, what’s a blue moon?

Full moon with tree branches in front
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It turns out that the term “blue moon” can have multiple definitions, so we’ll describe them and then clarify. The first version of a blue moon is when there are two full moons within a calendar month. The second (and older) definition refers to the third full moon in a season with four full moons. And the last usage describes a moon that appears bluish in color.

If you're confused, we understand. It’s a little odd that three naturally occurring lunar phenomena have the same name. And while all three of these scenarios are rare and don’t happen every month, depending on the definition, they’re not that infrequent. So, let’s break down the various blue moons and why they happen.

Blue moon: two full moons in a calendar month

Full moon in the dark sky
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This one is probably the most confusing to understand because it has to do with the total number of days in a calendar month as compared to the total number of days in a lunar cycle. However, this version of a blue moon is what most people are thinking of when they use the "once in a blue moon" saying. As you know, months within the Gregorian calendar can have anywhere from 28 to 31 days, but the lunar phase cycle doesn’t waiver. There are always 29.5 days. (To clarify, a lunar phase refers to the shape of the illuminated part of the moon as seen from Earth. This is how we track things like a new moon, crescent moon, quarter moon, gibbous moon, and full moon.)

Two full moons can appear in a single calendar month because of the inconsistent number of days in our months. If a lunar phase is always 29.5 days, but some months have an additional half-day or more, over time the lunar phase would grow to be out of sync with our calendar months, and a second full moon could occur in one month. The term "blue moon," then, can refer to either that second full moon or the overall phenomenon. By this definition, the most recent blue moon was March 31, 2018, and the next one will appear on October 31, 2020.

Blue moon: the third full moon in a season with four full moons

This definition of a blue moon is what was originally meant by the term. It refers to the third full moon in a season with four full moons, where seasons are delineated by the solstices and equinoxes. A typical season has only three full moons (one per month), so a season with four full moons is relatively rare, occurring every two-and-a-half years. The most recent seasonal blue moon was in May 2019, and the next one is expected to occur in August 2021.

Blue moon: when the moon appears blue

Full moon
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Of the blue moon definitions, this is the rarest (and most literal). For this scenario to occur, atmospheric conditions have to be perfect. There must be smoke or dust particles in the atmosphere that are slightly bigger than 900 nanometers in size. For some reason, in this size range, the particles scatter any red light and cause the moon to appear blue. This occurs only after a major geological or weather event such as a dust storm, volcanic eruption, or a forest fire. Blue moons were reported shortly after the eruptions of Mount Saint Helens in the United States, El Chichon in Mexico, and Mount Pinatubo in the Philippines.

Blue moon combinations

There are times that a blue moon can combine with other rare moon events to create super events. For example, on January 31, 2018, there was a total lunar eclipse. These types of events usually cause the moon to look red, giving it the common name “Blood Moon.” But on that particular night, the moon was at one of its closest positions to the Earth, almost making it a supermoon, when the moon looks exceptionally large in the night sky. Because that full moon was a blood moon and a supermoon, it was dubbed the Super Blue Blood Moon.