A blue moon happens primarily because of the way the lunar cycle aligns with the calendar. There are two main definitions:
- The most common meaning is the second full moon in a single calendar month. Since the lunar cycle is about 29.5 days, and most calendar months are longer than that, occasionally two full moons occur in one month. This happens roughly once every two to three years. It is the reason behind the phrase "once in a blue moon" to describe a rare event.
- A less common definition is a seasonal blue moon, which is the third full moon in a season that has four full moons instead of the usual three. This is based on the naming tradition of full moons in American almanacs.
The term "blue moon" has nothing to do with the actual color of the moon in these cases. However, the Moon can rarely appear blue in color due to atmospheric conditions, such as after large volcanic eruptions or forest fires, when small particles in the air scatter red light and allow blue light to dominate. But this visual phenomenon is unrelated to the calendrical blue moon. In summary, blue moons happen because of the mismatch between the lunar cycle and the calendar, and the term mostly refers to these rare timing occurrences rather than the Moon being literally blue in color.
