Once in a Blue Moon – What is it?

It happens only once in a blue moon. What happens? A month containing two full moons, the second of which is commonly referred to as a “blue moon,” occurs only about every 32months. The aphorism “once in a blue moon” is used to denote an occurrence that happens only very rarely. Two full moons in a month are rather rare, considering that over a two-and-a-half-year period there are generally 31 full moons. Only one of them is a blue moon.

But how did the term “blue moon” come to apply to the second full moon in a calendar month? Strangely enough, the nomenclature was arrived at by accident. Originally, a blue moon was used by the Maine Farmers’ Almanac to denote the third full moon in any season that contained four full moons. Generally, a season contains three full moons, and each one is named for an event that is typical of that season. For example, the “Harvest Moon” occurs in the autumn.

Once in awhile, a season will contain four full moons, and the Maine Farmers’ Almanac used the term “blue moon” to describe the third full moon of such seasons. The fourth full moon retained its usual descriptive name. By that original definition, the “Maine Rule,” blue moons could occur only during May, August, November, or February. But the second-full-moon-in-a-month definition of a blue moon allows it to occur in any month.

This definition can be traced back to an article by James Hugh Pruett in the March 1946 issue of Sky and Telescope magazine. In the article, Pruett writes, “Seven times in 19 years there were and still are 13 full moons in a year. This gives 11 months with one full moon each and one with two. This second in a month, so I interpret it, was called Blue Moon.” Pruett misinterpreted an article in the 1937 Maine Farmers’ Almanac about the occurrence of blue moons. Still, Sky and Telescope adopted Pruett’s new definition of a blue moon when it referred to it again in an article in the May 1950 issue of the magazine. Since then, pop culture has adopted the Sky and Telescope rule for the determination of a blue moon – that it is the second full moon in a calendar month. Regardless of which definition one chooses to use, blue moons are still rather rare. They are not unique in appearance, in that they are not actually blue in color, only in terminology. But they’re still an interesting aspect of astronomy. It’s a lunar event that happens only once in a blue moon!