A blue moon is an additional full moon that appears in a subdivision of a year: the third of four full moons in a season. The phrase in modern usage 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—for instance, if volcanic eruptions or fires release particles in the atmosphere of just the right size to preferentially scatter red light. Definition The term has traditionally, in the Maine Farmer's Almanac, referred to an "extra" full moon, where a year which usually has 12 full moons has 13 instead. The "blue moon" reference is applied to the third full moon in a season with four full moons, thus correcting the timing of the last month of a season that would have otherwise been expected too early. This happens every two to three years (seven times in the Metonic cycle of 19 years). The author of a March 1946 article in Sky & Telescope attempted to decipher the traditional practice of the editors of the Maine Farmers' Almanac by examining old issues of the almanac. Without enough almanacs to see the correct pattern, he conjectured the wrong rule for 'blue moons', which led to the modern colloquial misunderstanding that a blue moon is a second full moon in a single solar calendar month, with no link to the order it occurs in a season. The phrase "once in a blue moon" is also used idiomatically, which means once after a long time. An example in a sentence: "My sister lives in Alaska, so I only see her once in a blue moon." Owing to the rarity of a blue moon, the term "blue moon" is used colloquially to mean a rare event, as in the phrase "once in a blue moon". One lunation (an average lunar cycle) is 29.53 days. There are about 365.24 days in a tropical year. Therefore, about 12.37 lunations (365.24 days divided by 29.53 days) occur in a tropical year. In the widely used Gregorian calendar, there are 12 months (the word month is derived from moon) in a year, and normally there is one full moon each month, with the date of the full moon falling back by nearly one day every calendar month. Each calendar year contains roughly 11 days more than the number of days