A blue moon occurs in two main ways related to the timing of full moons, not the moon's actual color:
- Monthly Blue Moon: This is when there are two full moons within a single calendar month. Since the lunar cycle is about 29.5 days, slightly shorter than most calendar months, occasionally a full moon appears at the very beginning of a month and another at the end, resulting in two full moons in one month. This happens roughly every 2 to 3 years
- Seasonal Blue Moon: This occurs when a season (spring, summer, fall, or winter) has four full moons instead of the usual three. In this case, the third full moon of the four is called a blue moon
The term "blue moon" does not mean the moon literally appears blue in color during these events. However, on very rare occasions, the moon can appear blue due to atmospheric conditions. This happens when particles of a specific size—usually from volcanic eruptions or large forest fires—scatter red light and allow blue light to pass through, giving the moon a bluish tint. Historical examples include the 1883 Krakatoa eruption and other volcanic events like Mount St. Helens in 1980
. In summary, a blue moon usually refers to an extra full moon in a month or season, and only very rarely does the moon actually look blue due to atmospheric particles.
