Bebi
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Calendar Blue Moon - 30 June 2007Tomorrow is going to be a Calendar Blue Moon in the UK. Some countries have already had theirs at the beginning of the month.
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The term blue moon has at least four related meanings. One is a common metaphorical phrase for a rare event. Full moons are given names in folklore, and two definitions of blue moon are a name for a rare full moon that does not have a folk name. One modern blue moon definition is a result of a misinterpretation of the Maine Farmer's Almanac, where a second full moon occurs in a calendar month. The older definition of blue moon is for an extra full moon that occurs in a quarter of the year, which would normally have three full moons, but sometimes has four. Oddly, it is the third full moon in a season that has four which is counted as the "extra" full moon and named blue moon. According to certain folklore, it is said that when there is a blue moon, the moon has a face and talks to the items in its moonlight.
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The older meaning of blue moon to name an extra full moon, as was used in the Maine Farmer's Almanac, was the third full moon in a quarter of the year when there were four full moons – normally a quarter year has three full moons. The division of the year into quarters for this purpose has the dividing line set between March 21 and March 22. This has to do with the rule for setting the date for the Christian Holy Day of Easter, which depends on the last full moon - as calculated by the computus, a somewhat inaccurate formula - on or before the Equinox on March 21, which is also somewhat inaccurate.
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Calendar blue moons occur infrequently, and the saying once in a blue moon is used to describe a rare event. However, they are inevitable because of the mis-match between the solar and lunar cycles. Each calendar year contains twelve full lunar cycles, plus about eleven days to spare. The extra days accumulate, so that while most years contain twelve full moons to match the twelve months, every two or three years there is a year with thirteen full moons. On average, this happens once every 2.72 years. Additionally, in some years there is no full moon in February at all, since February is slightly shorter than the time from one full moon to the next. This condition, known as black moon, gives additional 'blue' moons in the preceding and following months.
http://en.wikipedia.org/wiki/Blue_moon
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