A relatively rare New Year’s Eve blue moon will be shining down on revelers as they ring in 2010. Blue moons are relatively common but it has been 19 years since one was coupled with December 31st and it will be another 19 years before another occurs.

Since the 1940s the term ‘blue moon’ has come to mean the second full moon in any given month. Full moons occur every 29.5 days so most years have 12 full moons. When the calendar aligns just right though, an ‘extra’ full moon can occur. The blue moon occurs every 2 years, 7 months so given its relatively infrequency, one can understand where the phrase ‘once in a blue moon’ got its meaning.

That current definition of ‘blue moon’ actually came about as a mistake. The phrase itself has been around for at least 400 years. Prior to the 1940s the Maine Farmers’ Almanac tied the event to the seasons saying a blue moon was the fourth full moon in a season rather than the usual three. Its explanation however was entirely convoluted and difficult to understand.

In 1946 the magazine Sky & Telescope published an article titled “Once in a Blue Moon” and in it the author misinterpreted the almanac saying the “second [full moon] in a month, so I interpret it, is called Blue Moon.” This mistake caught on in modern folklore and continues to this very day.

Get more details about whether or not a ‘blue moon’ is really blue and other interesting history in the complete article on the Denver Weather Examiner.