A "blue moon" generally refers to either the second full moon in a calendar month or the third full moon in a season that has four full moons, rather than the moon actually appearing blue in color. The term "blue moon" originates from this calendrical phenomenon, which occurs because a lunar cycle (about 29.5 days) is shorter than most calendar months. Thus, occasionally two full moons happen in the same month, roughly every 2 to 3 years, which is called a blue moon. The moon can sometimes literally appear blue in rare atmospheric conditions, such as after volcanic eruptions or large forest fires, which release particles that scatter red light and make the moon look bluish. However, this is unrelated to the more common calendar-related meaning of "blue moon." The phrase "once in a blue moon" reflects the rarity of these events, especially the atmospheric blue moon phenomenon, although the calendar blue moon itself is not extremely rare. The modern "blue moon" concept as the second full moon in a month was popularized in the mid-20th century and is mostly a product of American folklore. In summary:
- A blue moon usually means a second full moon in one month or the third full moon in a season with four.
- It can rarely refer to a moon that looks blue due to special atmospheric conditions.
- The term and concept of blue moon derive from calendrical peculiarities and rare optical effects.
