With all the talk of the Super Blue Blood Moon that will grace the sky over certain parts of the world on January 31, it’s easy to wonder why the heck it’s being called a ‘blue moon’ if it’s supposed to turn red.
Well, “blue moon” is just a weird naming convention for the second full moon of the month; it doesn’t actually have anything to do with the moon’s hue. But as the senior editor for the magazine Sky & Telescope J. Kelly Beatty explains, that hasn’t always been the case.
“Imagine an astronomical season,” Beatty tells Inverse in a video. “Say from a solstice to an equinox, every once in a while we get four full moons in that three month season and traditionally the third of those was referred to as ‘the blue moon’.”
Kelly explains that above that, it was an error in an old issue of Sky & Telescope that gave the second full moon in a month its now-accepted name.
“In 1946, Sky & Telescope blew it,” said Beatty. “We misinterpreted that traditional definition and started calling a blue moon the second full moon in a month and I guess it stuck. Whatever definition you use, these things happen around every 2.7 years, on average.”
So it turns out that the former and contemporary definition for a blue moon aren’t actually all that rare, which is kind of a let down if you’ve made a habit of saying using the idiom, “Once in a blue moon.”
So maybe we should start saying, “Once in a Super Blue Blood Moon?” That would make a lot more sense.