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Blue Moon: That Once In A Blue Moon Moment Is Now

Blue moon

Why is November's full moon blue?

Tonight’s full moon was a rare “blue moon”, even rarer than a “blue moon” typically is.

We’re all familiar with the phrase “once in a blue moon”. It refers to a rare event, something that happens, but not very often. What exactly makes a moon blue?

“Blue Moon” is the colloquial term for a second blue moon occurring within one month. Tonight’s moon is, of course, the only full moon in November. How does that work?

Here’s an explanation for the miniature astronomers in your house (and for you, if you’re the one with the telescope habit).

According the Christian Science Monitor, a blue moon happens when there are 13 full moons in a given year. The 13th full moon will cause one season to have an extra full moon in it. Normally, each season gets three full moons, but in a “blue moon” year one season will have four.

Tonight’s full moon is the third out of four full moons this fall, making it a blue moon by that standard. Each moon in a season has a particular name: the early moon, the mid-season moon, the late moon. The blue moon is the one that falls between the mid-season moon and the late moon, if there is one.

In fact, the whole “two full moons in one month” thing started out as a factual error in an article almost 60 years ago. It’s a typo that took on a life of it’s own.

This is the kind of trivia that fascinates 10-year-old history buffs. Did you check out the Blue Moon with your kids? Will you share this wacky story with them?

Photo: AudreyJM529

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