A blue moon is an additional full moon that appears in a subdivision of a year. There are two types of blue moons:
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Seasonal Blue Moon: This is the traditional definition of a blue moon and refers to the third full moon in a season that has four full moons.
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Monthly Blue Moon: This refers to the second full moon within a single calendar month.
The phrase "blue moon" has nothing to do with the actual color of the moon, although a visually blue moon (the moon appearing with a bluish tinge) may occur under certain atmospheric conditions, such as if volcanic eruptions or fires release particles in the atmosphere of just the right size to preferentially scatter red light. The term "blue moon" has been around for a very long time and has been used to describe a full moon of a very special kind, but sometimes it is really a description of the moon, which for different reasons turns blue.
Blue moons occur once every two to three years. The next monthly blue moon is on August 30/31, 2023, and the next seasonal blue moon takes place on August 19/20, 2024.
