How on Earth did we start using “once in a blue moon”?
The moon does turn blue sometimes, but this idiom doesn't always refer to its color. You’ve heard it, I’ve heard it, but not many people we know have actually seen a blue moon — so what gives? Am image of the moon captured through a blue filter. Image credits steviep187 / Flickr. “Once in a blue moon” refers to events that only happen very rarely, but it’s a tricky idiom. It doesn’t refer to a moon that’s actually blue, although it can appear to be that color under certain conditions and that probably shaped the saying. A blue moon is a real occurrence and, you might be surprised to hear, isn’t actually that rare or unpredictable. Blue moons are ‘extra’ full moons of the regular gray color that pop up every two or three years due to misalignment in the lunar and solar circle. But the phrase was first used to refer to something being absurd — like someone arguing that the moon is blue. So let’s take a look at both halves of this idiom and see why they came to represent the qu...
