The Lake County Astronomical Society

A History of "Once in a Blue Moon"

Gary Smith

Today the comment "Once in a Blue Moon" means whenever a full moon occurs twice in a month. There is more to the story than this and if fact the current belief is the result of a misunderstanding.

The "Maine Farmer's Almanac" in 1937 relied on a Tropical Year which starts with the winter solstice. Because the earth's orbit is elliptical, the seasons are not identical. The lunar month is about 29.5 days and that doesn't fit well into a year, which is 365.24 days. The 29.5-day lunar month doesn't fit well into seasons that are three months long either. Occasionally there are 13 lunar months in the year instead of the usual 12, so one of the seasons will have four full moons. This is where the "Maine Farmer's Almanac" called the extra full moon a Blue Moon. Then, in 1946 a writer for Sky & Telescope, J. Hugh Pruett, misinterpreted the almanac to mean the second full moon in a given month, which lead to the current belief in what a "Blue Moon" is. Then, a National Public Radio broadcast of "Star Date" in 1980 repeated the mistake and the current belief stuck. So today when we talk about a Blue Moon, this mistake is repeated again and again.

And now...for the rest of the story (to quote a famous newscaster). "Once in a Blue Moon" was originally meant to convey the idea of almost never, as in "when hell freezes over" or "When the Cubs win the World Series." The volcano Krakatau exploded, in 1883, with great force and spewed tons of dust and gas into the atmosphere. The resulting release of dust and gas turned the moon blue and the sunsets green all over the world. Hence the Blue Moon came to mean something rare, not twice in the same month as we currently understand the meaning of the phrase.