What Is A Blue Moon

blue moonA blue-colored moon? No, unless matters such as smoke particles are present in the atmosphere, you may never see a blue-colored moon. You’ve used or heard this term- a blue moon. In language, the term means something that rarely occurs. However, in astronomy, the term means a completely different thing. The definition of a blue moon has changed significantly following the previous gloomy definition. A few decades ago, the term referred to the occurrence of four full moons within one season. For instance, the third full moon was the blue moon. Currently, when two full moons occur in the same month, the 2nd full moon is referred to as the blue moon.

How did the change in definition occur? In 1946, James Hugh wrote an article about the Maine Farmer’s Almanac, which by then called the third of the four full moons in a season ‘Blue Moon.’ He expounded that 13 full moons in a year occurred seven times every 19 years. Each of the eleven months had one full moon and one month had two. He decided to call the second one ‘Blue Moon’. From then on, the term was frequently used leading to the adoption of his idea of the 2nd full moon in a month being the Blue Moon.

Now, how many blue moons can occur in a year? To answer such a question, we’ll have to deploy some arithmetic skills. Let’s consider the current definition of a blue moon. A year contains approximately 365.25 days. The lunar cycle takes around 29.5 days. Dividing the days of the year by the days of the lunar cycle we get 12. 38 moon. Since there is no decimal moon(s), the figure denotes an existence of a year with 13 full moons. The 13 full moons indicate the presence of two blue moons during the same year. The year 1999 is an example. Two full moons occurred in both January and March. There was no full moon during February. The next occurrence will be in January and March of 2018, 2037, etc. If we consider the earlier definition of a blue moon, only a maximum of two blue moons can occur in the same year. It is much rare to have the blue moon, the 2nd full moon in the month, and the blue moon, the third full moon in the season, occurring in the same year. 1948 is the year this will occur.

Despite the two definitions being different, they agree on one thing- 13 full moons can occur in the same year.

Once in a blue moon is a common saying.  It refers to things happening out of the ordinary.  You could say “Once in a blue moon, I hired a Baltimore errand service.”  Meaning, that you hire a company to help you once in a while.