What Exactly is a “Blue Moon”?

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The phrase “blue moon” is used metaphorically to refer to a rare event, as in the expression “once in a blue moon.”  It has nothing to do with the actual color of the moon, although any moon can appear blue if the air is full of particles from volcanic eruptions or forest fires.

A “blue moon” is usually defined as the second full moon occurring during a calendar month. But until 1946, a “blue moon” was the third full moon in any season in which there were four full moons.  It was a sort of Leap Day for moons, necessitated by the fact that the average lunar cycle is 29.53 days, slightly shorter than an average month.

Although most months have just one full moon, over time the shortfall in the lunar cycle accumulates.  So not only do the dates of the full moon change from year to year, but every two to three years there is a 13th full moon.

Blue Moon is just one of the the full moon nicknames used by certain Native American tribes, including the Algonquins.  Historically, many Native Americans calculated seasons by the lunar calendar, and these nicknames referred to the activities that took place in a given lunar month.  For instance, the full moon closest to the autumn equinox was the “Harvest Moon,” and the next one the “Hunter’s Moon,” the time to store up meat for winter.

In March 1946, however, Sky and Telescope magazine mistakenly defined a Blue Moon as the second full moon in a calendar month, rather than a season.  Although this was corrected in 1999, many people still hold to this erroneous definition.

There will be no Blue Moon in 2014. The next Blue Moon (by the second full moon in a month method) won’t occur until July 31, 2015.   But you can still impress that certain special someone by telling him or her that the full moon on February 14, 2014, Valentine’s Day, is the Snow Moon. While this is a romantic day for many, for native Americans the Snow Moon (also called the Hunger Moon, Famine Moon or Bony Moon) was a reminder that the weather could be harsh and food scarce.

And no, in case you were wondering… they couldn’t just shoot a chocolate bunny.

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