Blue moon

Photo: slworking/Flickr

Blue moon confusion

Unlike the much-hyped supermoon, the blue moon is not remarkable for its color, size or brightness. Its claim to fame lies in its rarity, which has been immortalized by the "once in a blue moon" metaphor.

Blue moons aren't as rare as you'd think they are, though.

The most common definition — the appearance of two moons within a single calendar month — is actually a mistake that originated in a 1946 issue of Sky & Telescope. Prior to this misinterpretation, the term "blue moon" was actually used to describe what we now call a seasonal blue moon, which is the third full moon in a single season of four.

These days, skygazers mark both the original seasonal event and the erroneous calendar event, meaning there is double the blue moons to look forward to. Last night's moon is one such example of a seasonal blue moon, and you'll have to wait until May 2016 to witness the next one. Can't wait that long? The next calendar blue moon is slated for July 2015.

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Catie Leary is a photo editor at Mother Nature Network. Follow her on Twitter and Google+.