What is the atmospheric process that causes a 'Blue Moon'?

American folk lore has names for each of the Full Moons seen during the year. A Blue Moon is the name of the second Full Moon that happens in any given month, at least that was what many of us believed until recently. As it turns out, this calendar definition is itself not the one used by people earlier than the 1940's. It was introduced by mistake by an author of Sky and Telescope magazine, and since the 1980's has become the common 'technical' definition. Even many astronomers prefer to use this 'second full moon of the month' definition, otherwise this second full moon goes without any name, unlike the second new moon ( Black Moon, Spinner Moon etc).

There is a book, in German, called "Die Welt des Mondes" by R. Oldenbourg published in 1957, which includes an article apparently written by Patrick Moore in 'Guide to the Moon' printed in 1953 by Eyre and Spottiswoode. Steffens remarks that at his institute there are witnesses ( a Professor Isserstedt) who actually saw a Blue Moon in the year 1954. It was BLUE, not bluish or powder blue, but BLUE. Patrick Moore's article described several sightings of Blue Moons in 1944 in America, in 1949 in Queensland, and in England on September 26, 1950. According to Moore who witnessed the 1950 event,

"The moon was in a slightly misty sky and had a kind of lovely blue color comparable to the electric glow discharge. I never saw something similar before"

Apparently this phenomenon has been reported by many people all over the world. It is believed that dust in the atmosphere at very high altitudes causes it, and the event in 1950 seen in England may have been produced by an unusually heavy season of forest fires in Canada around the same time. The actual cause may have to do with selective absorption of moonlight by soot particles of the right size, rather than by scattering which accounts for why the sky is blue in the daytime. The term 'blue moon' may indeed have something to do with rare atmospheric conditions perhaps caused by distant forest fires or other 'soot like' particle sources which may be effective in preferentially scattering red light rather than blue light (the normal situation during a sunset).


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