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Each year, the moon completes its final cycle about 11 days before the Earth finishes its orbit around the sun. After about two and a half years, there is an extra full moon, which is often called a blue moon. The origin of the term is uncertain, and its precise definition has changed over the years. The term is commonly used today to describe the second full moon of a calendar month, but it was originally the name given to the third full moon of a season containing four full moons.
The occurrence of a blue moon in any calendar month will result in additional photos in that monthly gallery.
Blue has Nothing to do with the Color of the Moon
The title Blue Moon has nothing to do with the actual color of the moon, although a literal blue moon (a moon appearing with a tinge of blue) may occur in certain atmospheric conditions such as when a volcanic eruptions or fires leave particles in the atmosphere of just the right size to preferentially scatter red light.
Too, with my camera settings, there is a Moon setting which often will result in the appearance of a blue colored Moon photo.
(m5h-moon-full-2016-1114.1719) The Full Moon in November is the Beaver Moon
The Thirteenth Moon
A blue moon is an additional full moon that appears in a subdivision of a year that occurs either: (1) the third of four full moons in a single season, or (unofficially) (2) a second full moon during any calendar month.
The term has traditionally referred to an extra moon, where a year which normally has 12 moons, instead has 13 moons. One lunar cycle (lunation) is 29.53 days and there is 12.37 lunations in an average year of 365.24 days. Therefore, each calendar year contains roughly 11 days more than the number of days in 12 lunar cycles.
The extra days accumulate, so every two or three years (seven times in the 19-year
Metonic cycle), there is an extra full moon.
Consequently, the extra moon falls in one of the four seasons, giving that season four full moons instead of the usual three, and, thusly, that season′s forth moon is a blue moon.
Other Common Uses of the Expression "Blue Moon"
Owing to the rarity of a blue moon, the term blue moon is often used colloquially to mean a rare event, as in the phrase once in a blue moon.
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