The magical, whimsical idea of a "blue moon" has enchanted humanity for centuries, and with good reason: It's a sign of hope and of rarity. It only happens every few years, after all. But, considering its name, you might be wondering: is a blue moon really blue?
If you're interested in space happenings, you probably know that January 2018 features the first of two blue moons in 2018. But, in case you're wondering, a blue moon isn't really, uh, "blue" — it's just called that because it's a rarity. Throughout a calendar year, there are 12 months that feature 12 new moons and 12 full moons. Every two to three years, though, two full moons fall within the boundaries of one calendar month, and as a result, that second moon is called a "blue moon." The year 2018 is particularly special in that it will feature not one, but two blue moons (after the first on Jan. 31, the second will occur on Mar. 31), something that happens just once every 19 years according to Earth Sky.
So where did the phrase "blue moon" come from? Well, that continues to be a debate among folklore historians. The general consensus is that the term "blue moon" — and its accompanying phrase, "once in a blue moon" (meaning, essentially, that an occurrence is incredibly rare) — existed independently for centuries before being attached to the idea of a second full moon in a month. The first usage, which arose more than 400 years ago, relayed the absurdity of a situation or a person's reasoning abilities, according to Sky & Telescope. Similar to someone saying the moon is made of cheese, the phrase "he would say the moon is blue" embodied a sense of a detachment from reality.
From that initial concept — a "blue" moon being a ridiculous, non-sensical concept — came the phrase of "When the moon is blue," which meant essentially, "Uh, never."
The inkling of possibility that exists in the current blue moon iteration stems from the fact that very rarely, every so often, the moon does kind of, sort of look a little, well, blue. By the mid-19th century, reports Sky & Telescope, several instances of moons that did in fact look blue had been recorded. The celestial sight was caused by an excessive amount of debris in the air, be it from a volcanic eruption, as was true with the 1883 explosion of Krakatoa, or the delay of a monsoon and the subsequent "dust bowl" effect, which swept through India in the 1920s.
But the most common, contemporary usage of "blue moon" — two full moons in a single calendar month — stems from a misinterpretation.
In 1946, astronomer James High Pruett incorrectly used the phrase in an article for Sky & Telescope. Slowly, but surely, that slightly altered, not-quite-correct usage worked its way into our vernacular. In 1980, NPR used the term "blue moon" on their show Star Date, and soon public media nerds everywhere began to incorporate the phrase into their discussions of astronomy until wrong was right, right was, like, also right, and diehard "blue moon" purists just sort of gave up correcting people. And isn't that how language develops anyway?
As humans have been using the sky to track the passage of time since they were, well, humans, there are, of course, a number of names and traditions surrounding every planetary, solar, and lunar event — a blue moon is just one of them. And to have two blue moons in one year, only a month apart? Well, that's just even more special.
So how will you celebrate January's blue moon?