"What is a blue moon?" All of us here at the Newton Falls Public Library had heard the expression "Once in a blue moon," but none of us were exactly sure what a blue moon was either.
The answer turned out to be a little complicated. Doing an online search for "What is a blue moon?" brought up articles from Weather.com and news.Discovery.com. The most common definition of a blue moon puts it as the second full moon in a month. However, this is actually a misconception. Going by the original definition, a blue moon is actually the third full moon of a season that has four moons. Typically, there are three moons in a season, and they're referred to as the early moon, the mid-season moon, and the late moon. When there are four, referring to the extra as a "blue moon" allows the late moon to be the fourth and final in the season.
According to "Blue Moon Tuesday, But Not the Kind You Think," written by Michele Berger for Weather.com, and "Blue Moon Rises: What Does It Mean?" written by Joe Rao for news.Discovery.com, the misunderstanding comes from a 1946 article in Sky & Telescope magazine. The article was then referenced on a popular National Public Radio program in 1980, and people have been using both definitions ever since.
Given the mutability of the English language, both definitions are listed as correct in The American Heritage Dictionary, 4th Edition
There's also such a thing as a literally blue moon. According to "Blue Moon Rises," the ash from volcanic eruptions like Mount St. Helens and Krakatoa can cause the moon to take on an azure hue.