A blue moon occurs mainly in two ways:
- A blue moon most commonly refers to the second full moon appearing within a single calendar month. This happens because the lunar cycle is about 29.5 days long, which is slightly shorter than most calendar months. Thus, when a full moon falls early in a month, there can be a second full moon later in the same month. This event happens roughly once every 2 to 3 years and is known as a "monthly blue moon".
- Another, 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.
The term "blue moon" does not usually mean the moon appears blue in color. However, rare atmospheric conditions—such as after large volcanic eruptions or massive forest fires—can cause particles in the air to scatter red light and make the moon look bluish. Such visually blue moons are very rare and unrelated to the calendrical blue moon event.
In summary, a blue moon occurs due to the mismatch between the lunar cycle and calendar months, leading to an extra full moon in a month or season, and only rarely does the moon actually appear blue in the sky due to atmospheric effects.
