A rare event

It’s rare indeed that a real time event coincides with one of my blog posts, but that is exactly the case this week when we will experience a second full moon (i.e., a “blue moon”) in a single calendar month. Hence this “once in a blue moon” post…

It turns out that the term “once in a blue moon” has quite a history. My own interpretation is fairly basic—a rare, almost unheard-of event—but there is way more to the phrase than that.

The term’s earliest usage was in a 1524 pamphlet criticizing English clergy along the lines of “if they say the moon is blue, we must believe it is so.” Another early association is with a “betrayer moon,” when, because the moon’s cycle was out of sync with the church’s calendar, the church would inform its members whether the moon was a true Easter moon or if it was a “blue” or “false” moon—if it was a “blue” moon, such notification kept parishioners from fasting an extra month before Lent actually started.

There are, in fact, rare occasions when the moon does appear blue. The effect is caused by smoke or dust particles in the atmosphere that scatter light differently than when those factors aren’t present. Forest fires in Sweden and Canada in the early 1950s and volcanic eruptions at Mount St. Helens (1980, U.S.), El Chichon (1983, Mexico), and Mount Pinatubo (1991, Philippines) all resulted in reports of “blue” moons.

As shown below, the Maine Farmer’s Almanac assigns names to each full moon in a year. When it happens that there are four full moons in a calendar season, the Almanac calls the third full moon “blue” so the last moon of the season is appropriately named in accordance with their naming convention.

In the mid-1940s a writer for Sky & Telescope magazine misunderstood the Almanac’s system and interpreted it to mean two full moons in a single month. This interpretation was subsequently presented on at least one radio program and in other periodicals and was eventually included in a 1986 edition of the popular Trivial Pursuit board game.With that background, a second moon in a single month is the now the second most understood definition of the term, behind only its long-term definition as an extremely rare event.
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Posted for Jenny Matlock’s AlphabeThursday study of the letter “O.” For Round 5 of this long-running meme, I am focusing on colloquialisms and idioms—words and phrases that are unique to a region or have meanings that aren’t necessarily discernible from the combined meanings of the individual words. I hope you’ve enjoyed this week’s exploration as much as I have.
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Sources for this post include:
Farmer’s Almanac
Sky & Telescope
World Wide Words

8 thoughts on “A rare event

  1. Really interesting to have this explained – and I hadn’t realised each moon had a name this year. I have copied these into my diary! Not looking forward to that Cold Moon. 😦

    • Alexa, I’m pretty sure the full moon names stay the same each year. It’s just the date and time-of-day that change with the year and, of course, with the time zone (the times listed are for U.S. eastern standard or daylight savings time (depending on time of year).

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