Concept in Definition ABC

On astronomy A blue moon is the succession of two full moons that occurred during the same month, according to the Gregorian calendar. This phenomenon is not common and in fact occurs periodically every three years. Knowledge of the phases of the moon and blue moon dates back to ancient times, when Egyptian astronomers and then Greeks observed the moon in its different phases (the great astronomer of antiquity was Aristarchus of Samos, who is part of the school Hellenistic that flourished from the third century BC. C).

What explains this curious phenomenon is a mathematical probability calculation, since if the full moon cycle occurs every 29.5 days and a full moon occurs on the first day of the month, there are a probability for a second full moon to appear at the end of the same month. In other words, some years have one more full moon than normal (13 instead of 12). If we take today as a reference, the last blue moon occurred on July 31, 2015 and, therefore, the next will appear in 2018.

The origin of the expression blue moon

Its name comes from the English blue moon, but it has no relation to the color of the moon, although under certain conditions atmospheric yes it is possible that the moon presents a bluish aspect (for example, when there is a full moon and the effect of a volcano is produced erupting). In this way, the blue moon is not blue, as this name has a history more related to the role that the moon was given in ancient times. Specifically, it was the English who began to use the expression blue moon and blue is believed to come from the evolution from the term belewe, the Old English word that expressed the concept of betraying. In this way, the second full moon that appeared sporadically in the sky appeared "treacherously", it is say, unexpectedly, which created the "treacherous moon" pun that came to be known as blue moon. In fact, in English there is a very popular expression, once in a blue moon, which means that something happens very infrequently (in Spanish we would say "from figs to figs" or "from Easter to Ramos").

The English expression "once in a blue moon" reminds us that in each language the expressions are literally untranslatable, so they must be adapted to the idiosyncrasy of each language.

Regarding the first time the expression blue moon appears in the literature English, according to the Oxford English Dictionary the reference occurred in the seventeenth century in a play on words colloquial.

The blue moon as a rare astronomical phenomenon has an effect that goes beyond astronomy itself. Throughout history, the rarity of the blue moon has inspired poets, musicians or mystics. We should not forget that the moon is associated with mystery, romanticism and all kinds of ancestral popular beliefs.

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