The term "Blue Moon" has two main meanings and origins.
- The traditional and original meaning comes from the Christian ecclesiastical calendar and the Maine Farmer’s Almanac, where a "seasonal Blue Moon" is the third full moon in an astronomical season that has four full moons instead of the usual three. This designation was made to keep church holidays properly aligned with lunar phases. The "blue" in the name is associated with something rare or unusual, such as the appearance of an extra full moon in a season.
- The more popular modern definition of "Blue Moon" is the second full moon in a single calendar month. This meaning arose due to a misinterpretation by amateur astronomer James Hugh Pruett in 1946, which was published in Sky & Telescope magazine. Though based on an error, it became widely accepted and is now a fully recognized use of the term. It reflects rarity, hence the phrase "once in a blue moon" meaning something that happens very rarely.
The term "Blue Moon" also has a historical figurative usage from as early as the 16th century, where a "blue moon" meant something absurd or impossible. In rare cases, the Moon has actually appeared blue in color due to atmospheric effects like large volcanic eruptions or forest fires, but this literal blue moon phenomenon is uncommon and unrelated to the calendrical meanings.
In summary, the name "Blue Moon" refers to the rarity of the event—either an extra full moon in a season or a second full moon in a month—and originated from traditional lunar calendars as well as a modern misunderstanding that became popularized. The phrase also has roots in old expressions meaning something rare or absurd.
