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There is a common expression in English language referring to a blue moon. When people say that something happens “only once in a blue moon”, they mean that it happens only very rarely, once in a great while. This expression has been around for at least a century and a half. There are references to this expressions that date from the second half of the nineteenth century.

The expression “a blue moon” has come to refer to the second full moon occurring in any given calendar month. A second full moon is not called a blue moon because it is particularly blue or is any different in the hue from the first full moon of the month. In stead, it is called a blue moon because it is so rare. The moon needs a little more than 29 days to complete the cycle from full moon to full moon. Because every month except February has more than 29 days, every month will have at least one full moon (except February, which will have a full moon unless there is a full moon at the very end of January and another full moon at the very beginning of March). It is on the occasion when a given calendar month has a second full moon that a blue moon occurs. This does not happen very often, only three or four times a decade.

The blue moons of today are called blue moons because of their rarity and not because of their color; however, the expression “blue moon” may have come into existence in reference to unusual circumstances in which the moon actually appeared blue. Certain natural phenomena of gigantic proportions can actually change the appearance of the moon from the Earth. The eruption of the Krakatao volcano in 1883 left dust particles in the atmosphere, which clouded the sun and gave the moon a bluish tint. This particular occurrence of the blue moon may have given rise to the expression that we use today. Another example occurred more than a century later. When Mount Pinatubo erupted in the Philippines in 1991, the moon again took on a blue tint.

One of the most (51. _____ ) -used reference books is a dictionary, which provides information about words.