Once in a Blue Moon

There's an old phrase that goes "once in a blue moon". We all know it to mean once in a great while. But what is a blue moon and why does this phrase occur?

The term blue moon goes back a long time, but the first traces of it in written English are from a pamphlet written in 1528. In the late 19th and early 20th centuries, the Maine Farmers' Almanac published the times of full moons for farmers. This publication was the first to explain the definition of a blue moon. But before their definition, you have to understand full moons first.

If you take a calendar year and split it into quarters, you are looking at four blocks of three months. Typically in a quarter, there are three full moons. Technically, there are 12.37 full moons a year. No, there aren't partial full moons, but mathematically, it works out to 12.37 full moons. This means that some years, there will be three full moons in a yearly quarter all year long. That year won't have a blue moon. But sometimes, there is a fourth full moon in the quarter. In order for the names of the three full moons to be kept in check, the third full moon in a quarterly amount of four full moons is considered to be the blue moon. This was the definition given by the Maine Farmers' Almanac.

However, in the 1940's, the magazine Sky & Telescope had an article about the blue moons. The author misinterpreted the definition that the almanac had made. He assumed that it meant some years there are 13 full moons instead of 12, and the second full moon in a month is considered to be the blue moon. Oddly enough, this inaccurate definition became the definition that people know.

So, last night, we had a blue moon, and the next one is scheduled to be for 2015. Since there is such a long period of time between blue moons, that's where the phrase "once in a blue moon" came to be known as once in a great while.

Also, don't forget the great classic song "Blue Moon".