A blue moon occurs when there are two full moons in a single calendar month. The lunar cycle is about 29.5 days, which is slightly shorter than most calendar months, allowing for the possibility of two full moons in one month if the first full moon happens at the very beginning of the month. This happens approximately every two to three years. There is also a seasonal definition of a blue moon, which is the third full moon in a season that has four full moons instead of the usual three. Despite the name, a blue moon does not actually appear blue in color; the term is simply used to describe this rare timing phenomenon. Occasionally, the moon can appear blue due to atmospheric particles from events like volcanic eruptions or large fires, but this is unrelated to the calendrical blue moon. In summary:
- A blue moon is most commonly the second full moon in a calendar month.
- It occurs roughly once every 2 to 3 years.
- It does not mean the moon is actually blue in color.
- Rarely, the moon can appear visually blue due to atmospheric effects.
This explanation combines the monthly and seasonal definitions and distinguishes the visual phenomenon from the calendrical one.
