Ever heard that old phrase “once in a blue moon?” But what is a blue moon, really? Contrary to popular belief, a blue moon is not a full moon that appears to have a soft bluish-white glow. People have been using the phrase ‘once in a blue moon’ since the 1800s to describe something that rarely happens, such as the moon appearing to be colored blue, but this was not always the case and isn’t the most likely source of the phrase. The times when the moon has appeared to be blue have usually followed volcanic eruptions or forest fires, and are a result of those conditions in the atmosphere rather than any celestial cause.
Many people believe this phrase first occurred in 1824, but this is also incorrect, since it is found in a 1528 work, The Treatyse of the Buryall of the Masse, by Bishop William Barlow fo Chichester, England. Here, the phrase is used in a similar way to the modern “yes, and pigs may fly.” It implies something not merely uncommon, but utterly absurd and impossible.
In the May 1999 issue of Sky and Telescope the blue moon meant the third full moon in any complete season of the year with four full moons. The seasons in this definition begin according to the exact position of the sun in reference to fixed stars, making spring and summer a bit longer than fall and winter – so that the most likely occurrences of full moons would be during the warmer seasons.
In recent decades, the second full moon in a given month has been designated a blue moon. This may have begun as a spin off the Old English word belewe, which means betray; since it was widely accepted that each month contained but one full moon, a second one in a month obviously betrayed the natural order of things, and thus the moniker “blue moon.”
Despite historical evidence that this meaning of the phrase is not accurate, the second full moon in a month definition is the most widely accepted today. Even the official NASA website supports this definition.
Bonus Factoids:
- A blue moon on New Year’s Eve only happens every 19 years
- The last full moon on a New Year’s Eve was in 2010
- No blue moon of any kind will occur in the years 2011, 2014, and 2017.
- A double blue moon – meaning two blue moons in the same year– only happens in four or five years per century. The last year this happened was 1999.
Sources:
http://www.safefromthestart.org
http://www.wisegeek.com/what-is-a-blue-moon.htm
http://www.phrases.org.uk/meanings/once-in-a-blue-moon.html
http://www.farmersalmanac.com/astronomy/2009/08/24/what-is-a-blue-moon/
http://nssdc.gsfc.nasa.gov/planetary/lunar/blue_moon.html
