A "blue moon" is called a blue moon because of its historical and cultural origins related to rare occurrences involving the Moon. The term originally comes from a 16th-century expression "the Moon is blue," meaning something absurd or impossible. Over time, it evolved to refer to a rare event, notably an extra full moon in a season or a second full moon within a calendar month, which happens infrequently. The phrase "once in a blue moon" thus came to mean something that happens very rarely. The naming has two main definitions:
- The traditional meaning, from the Maine Farmer's Almanac, refers to the third full moon in a season with four full moons, ensuring the timing of religious holidays like Easter remains correct.
- The more popular modern definition, a result of a 1946 misinterpretation by astronomer James Hugh Pruett, is the second full moon in a calendar month.
Additionally, the Moon can sometimes literally appear blue due to atmospheric conditions like volcanic eruptions that scatter particles, although this is rare and unrelated to the calendrical meaning of a blue moon. In summary, a blue moon originally meant something rare or impossible, then came to specifically denote certain rare lunar occurrences in a calendar context, and only very rarely does the Moon literally appear blue due to atmospheric effects.
