Blue moon
A rare full moon
Let’s start by saying the a blue moon is not actually blue. Although appearances of really blue moons have been reported the have nothing to do with the blue moon I’m discussing here.
In fact there are two common definitions of a blue moon.
• The monthly blue moon, which is the now most commonly used and known definition calls the second full moon in a calendar month a blue moon.
• The seasonal blue moon is third full moon in an astronomical season when that season happens to have four full moons instead of the usual three.
Although the first definition mentioned above is the most common one it actually originated from a misinterpretation in the March 1946 issue of Sky and telescope magazine. Currently this definition is so widespread that it no longer can be considered a mistake but rather a second definition.
The seasonal blue moon originated from the Maine Farmer’s almanac. In this almanac each full moon has a name which originate in fact from the names given to each full moon by native American tribes. European settlers took over most of these names though some names may have been adjusted.
Every full moon has a name. If however during a calendar year a thirteenth full Moon occurs there is no name for it and it will be called a Blue Moon. Which moon will be called the Blue moon is determined on a seasonal base. A regular season has three full moons. If a fourth full moon occurs during a season then the third full moon will be called a blue moon.
Now how rare is a full moon then? Well again it depends on which definition one uses. Following the definition of the monthly full moon a blue moon occurs on average every 2,7 years. However the seasonal is a much rarer occurrence. This happens only about every nineteen years. 
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