Blue Moon

A 'blue moon" is a term often used to describe a rare event when there are two full moons in a calendar month. This phenomenon occurs because a typical lunar cycle is approximately 29.5 days long, so most months only contain one full moon. However, occasionally, a month will have two full moons, and the second full moon in a single month is referred to as a "blue moon."

The term "blue moon" has nothing to do with the moon's actual color. Moons can appear to have a bluish hue, but this is quite rare and usually caused by atmospheric conditions, such as smoke or dust particles in the Earth's atmosphere. The phrase "once in a blue moon" is used in everyday language to describe something that happens very infrequently or rarely.

It's worth noting that there are other ways to define a blue moon based on different criteria, such as the third full moon in a season with four full moons. These alternate definitions can be a bit confusing, so the most common understanding of a blue moon is the second full moon in a calendar month.