The term "blue moon" has two main definitions and its origins are a bit mysterious and tied to both folklore and calendar calculations. Originally, the phrase "blue moon" might have come from the 16th-century expression meaning something absurd or impossible. Some also believe it may have started from the rare occurrence when volcanic eruptions or large fires send particles into the atmosphere that make the moon appear literally blue for a short time. The two popular meanings of a blue moon are:
- The third full moon in a season that has four full moons instead of the usual three. This comes from old almanacs and calendar adjustments based on the Christian ecclesiastical calendar to keep festivals like Easter in the right lunar phase.
- The second full moon occurring within a single calendar month, which is actually a more modern reinterpretation due to an error in a 1946 astronomy magazine article.
The phrase "once in a blue moon" came to mean a rare event because both types of blue moons happen infrequently—roughly once every 2 to 3 years or so. The moon does not usually appear blue, but the name stuck because of its rarity and some historical atmospheric phenomena.
