Blue Moon over North America
30 May 2007
At 9:04 pm Eastern Daylight Time on May 31st, the full moon over North America will turn blue.
Not really. But it will be the second full moon of May and, according to folklore, that makes it a Blue Moon.
If you told a person in Shakespeare's day that something happens "once in a Blue Moon" they would attach no astronomical meaning to the statement. Blue moon simply meant rare or absurd, like making a date for "the Twelfth of Never."
But "meaning is a slippery substance," writes Philip Hiscock of the Dept. of Folklore, Memorial University of Newfoundland. "The phrase 'Blue Moon' has been around a long time, well over 400 years, and during that time its meaning has shifted."
The modern definition sprang up in the 1940s. In those days the Maine Farmer's Almanac offered a definition of Blue Moon so convoluted even professional astronomers struggled to understand it. It involved factors such as ecclesiastical dates of Easter and Lent, tropical years, and the timing of seasons according to the dynamical mean sun. Aiming to explain blue moons to the layman, Sky & Telescope published an article in 1946 entitled "Once in a Blue Moon." The author James Hugh Pruett (1886-1955) cited the 1937 Maine almanac and opined that the "second [full moon] in a month, so I interpret it, is called Blue Moon."
This was not correct, but at least it could be understood. And thus the modern Blue Moon was born. A detailed account of the story may be found here.
Surveying the last four centuries of literature and folklore, "I have counted six different meanings which have been carried by the term," recounts Hiscock. In song, for instance, Blue Moons are a symbol of loneliness; when love conquers all, the Blue Moon turns gold. (See old Elvis records for more information.) "This makes discussion of the term a little complicated," he says.
One complication is that the Moon can turn genuinely blue.
"I had never paid any real attention to the term 'Blue Moon' until one October evening in 2003," he recalls. "I had my telescope set up in the backyard and the moon began rising in the east with a strange blue tint I had not seen before."
The cause of the blue was probably tiny droplets of water in the air. "The air was damp and heavy with moisture," notes King. When water droplets are about 1 micron (one millionth of a meter) in diameter, they strongly scatter red and green light while allowing other colors to pass. A white moonbeam passing through such a misty cloud turns blue.
Clouds of ice crystals, fine-grained sand, volcanic ash or smoke from forest fires can have the same effect. "The key," notes atmospheric optics expert Les Cowley, "is that the airborne particles should all be of very similar size, a micron or so in diameter." Only then do they scatter the correct wavelengths of moonlight and act as a blue filter.
There are other reasons for blue Moons, he notes. "Our eyes have automatic 'white balances' just like digital cameras. Go outdoors from a cozy cabin lit by an oil lamp (yellow light) and the Moon will appear blue until your eyes adjust."
What kind of Blue Moon will you see this week? There's only one way to find out!
Author: Dr. Tony Phillips | Production Editor: Dr. Tony Phillips | Credit: Science@NASA