According to Sky and Telescope magazine, this term has a very complicated lineage. In one way or another, its usage seems to go back several hundred years. Its most recent definition as the second full moon in a single month, seems to be only a few decades old because of a mistake made in an article published in Sky and Telescope back in the 1940's. This definition has only really become popular since the 1980's. You can read more about this mix up in the article by Philip Hiscock.. As an astronomer, I will continue to use the mistaken definition for a Blue Moon as the second full moon in a month. I do this merely because this unique phenomenon begs to be called SOMETHING, and to call it a Blue Moon seems just as good as any other name I can come up with. I know this will get me into lots of trouble with the purists...so sue me!
Copyright 1997 Dr. Sten Odenwald
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