How many times have you said or heard someone say “once in a blue moon”. The common phrase means not very often and for good reason. A blue moon occurs approx once every two and a half years. So what is a blue moon?
The term refers to a full moon that is the second full moon in that month. Is it actually a blue colour? Not at all. Although, occasionally, any full moon can have a hint of blue in it. This is caused by dust particles or fires.
So where did this term come from then? Apparently, in 1819, The Maine Farmers’ Almanac listed the dates of future blue moons. They had their own definition of what a blue moon is which is taken from the fact that lunar and calendar months are different. Some years have 13 full moons but typically only 12 moons in a year.
Each moon in a 12 moon year has a name like “Harvest Moon”. They called a 13th moon a blue moon just to keep track. Author James Pruett printed Sky & Telescope Magazine in 1946 using the term blue moon to try to explain the the “second moon in one month” and why the Almanac called it a blue moon. The term stuck and most people believe it’s because it was an answer to a question in an early version of Trivial Pursuit.
The next “Blue Moon” you will see will be on July 31st, 2015.