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Blue Moon

July 30, 2015

Folklore doesn’t have to be old to be popular. Consider, for example, the modern definition of Blue Moon: the second full Moon in a calendar month. Although that definition began as a mistake, it became popular more than two decades ago and has ingrained itself in skywatching lexicon.

And by that definition, there’s a Blue Moon the next couple of nights. There was a full Moon on July first, and there’s another early tomorrow morning — the second full Moon of the month.

Blue Moon already had several definitions, including the thirteenth full Moon in a calendar year, and the fourth full Moon in a calendar quarter. The phrase could also be taken literally. Under some rare atmospheric conditions — when there’s a layer of ash high in the air, for example — the Moon can actually look blue. And the phrase “once in a blue moon” is unrelated to the Moon itself, and simply means that an event is rare.

The “second-full-Moon-in-a-month” definition entered the culture in the 1980s, thanks to a mistake in a decades-old magazine article. It was popularized by both Star Date and the game Trivial Pursuit.

Some have tried to stamp out the definition, but it’s unlikely they’ll succeed. Like Super Moon, which is an unusually close full Moon, and Blood Moon — a reference to the Moon’s red color during an eclipse — Blue Moon is probably here to stay — a fun bit of modern folklore about the night sky.

Tomorrow: on the wing of the swan.

Script by Damond Benningfield, Copyright 2015

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