A blue moon can refer to the second full moon in a month, the third full moon in a season containing four full moons, or a moon that appears blue due to atmospheric conditions
. The phrase "once in a blue moon" describes something that happens rarely
. Meanings of a blue moon:
- Second Full Moon: The second full moon to appear in a single calendar month is called a blue moon
. This occurs because the lunar cycle (approximately 29.5 days) is shorter than the length of most calendar months, so occasionally two full moons can occur in the same month
- Third Full Moon: Blue moon can traditionally refer to the third full moon in a season that has four full moons
. In the past, the Maine Farmer's Almanac used the term blue moon to identify the third full moon in a season with four full moons so other full moons with special names would occur at the correct times of the year
- Appearing Blue: The moon can actually appear blue, although this is rare
. This can happen due to smoke and dust particles in the atmosphere, such as after volcanic eruptions or large fires
