By: Joan Whetzel
"Once in a Blue Moon", an idiom used since the 1600s, currently implies something far different than it did back in Shakespeare's day. The various meanings for "Blue Moon" change with the events and circumstances to which the expression is applied as well as the people using it - poets and English teachers, astronomers, songwriters or anyone who ever observed the night sky. Does the moon actually turn blue? That depends on how you use the adage - and whether you picture the moon through the eyes of a poet.
"Once in a Blue Moon"
The phrase "Once in a BLue Moon" originally meant that something occurred rarely or was seldom witnessed, like a 100 year flood. It also hinted at the absurdity of certain things occurring. It's like saying I'll go out with John Doe on the twelfth of never, meaning a date with John Doe is never going to happen. In this case, the moon doesn't actually turn blue any more than it is made of green cheese. Over the years, the Blue Moon adage has evolved to take on another meaning for poets and songwriters. A blue moon for them symbolizes loneliness or someone who is blue or sad over the loss of a love. This blue moon only transforms into a bright gold when that person finds love again. It wasn't until much later that the phrase "Once in a Blue Moon" took on an astronomical meaning.
The Blue Moons and the Calendar
When comparing lunar and calendar months, it appears that both are nearly equal lengths. Lunar months (one full cycle from new moon to new moon) equals 29.53 days, which matches up with the calendar month of 30 to 31 days. The monthly calendar length only allows for the occurrence of one full moon in most months. About once every two to three years, the astronomical Blue Moon occurs, highlighting the rarity of this event. The moon does not turn blue, it simply denotes the appearance of a second full moon in a calendar month. The last Blue Moon, under this astronomical definition, transpired in the Northern Hemisphere on December 31, 2009. The next takes place August 31, 2012.
Where Did the Current Definition of Blue Moon Come From
The current definition of a Blue Moon, meaning two full moons in one month, took off in the 1940s due to a misinterpretation by a writer for Sky & Telescope Magazine. The writer, James Hugh Pruett, researched the term Blue Moon in the 1938 Maine Almanac, looking for facts on the blue moon for his readers. In the Almanac, Mr. Pruett found one vague reference to two full moons in one calendar month, and interpreted it to mean that the double full moon must be a Blue Moon, at least in his opinion. His opinion, repeated often enough, soon became fact.
When the Moon Appears Blue
In, reality there are only two instances where the moon appears to turn blue. In the first case, a volcano explodes (e.g. Mt. St. Helens in 1980), spewing tons of ash into the air. The second case involves a massive fire (i.e. a Canadian Forest Fire in 1951) that fills the air with tons of bluish smoke, ash and debris. In both cases the smoky haze, filled with particulate matter, spreads across large regions of air space. The haze and particulates caused by the volcanic eruptions and fires, come between the moon and the earthbound observer, making the moon appear blue. What's really happening is that the haze and particulate matter scatter the blue light. similar to the way the Earth's atmosphere scatters blue light, giving the daytime sky its familiar blue color. On the other hand, one could always take a photograph of the moon through a blue filter, or manipulate a photo of the moon to give it a faux finish. With a little help from the poet's eye, anything is possible - once in blue moon.
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