A Blue Moon Is the Second Full Moon to Occur in a Single Calendar Month

 
Friday's Urban Legend: DEFINITELY FALSE, MAYBE

The term "blue moon" dates back to the middle ages where it meant something quite impossible. Over time the term came to be used in the phrase "once in a blue moon" to mean "it ain't ever going to happen" [Wikipedia: Blue Moon].

Gradually the phrase took on the meaning of something that happens rarely, instead of never. Following World War II there was an attempt to relate the term "blue moon" to a real astronomical event. The most common explanation was that a "blue moon" was the second full moon in the same calender month. The average time between two successive full moons is about 29.5 days. This means that you can have two full moons in any month except February. This will occur, on average, every two-and-a-half years. This year for example, you might have seen a "blue moon" in May, June, or July depending on where you live [The Blue Moon of 2007]. This interpretation of "blue moon" was promoted by Sky & Telespcope in 1946 and it was due to a misinterpretation of the Maine Farmer's Almanac of the preceding decade.

The Sky & Telescope website has a detailed explanation of the error [What 's a Blue Moon?].

So, if you accept the new definition of "blue moon" then the title statement is correct and this is an example of an urban myth that has transformed the meaning of a phrase. However, if you stick to the original meaning of the term then the title statement is false because a "blue moon" is about as likely as one made out of green cheese.

---

[Photo Credit: The photograph of the "blue" moon is from miramiramazing]