segunda-feira, julho 02, 2007

You have a blue moon in your eyes
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The term blue moon has a number of related meanings. One is a common metaphorical phrase for a rare event.
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A Calendar blue moon is the second of two full moons to occur in the same calendar month. They are inevitable because of the mis-match between the solar and lunar cycles. The months of the gregorian calendar are all very close to the 29.5306-day period of the moon's phases: the synodic month, or lunation. Most of the months are longer than this by one or two days, except February, which is the only month which cannot contain a calendar blue moon. Since February is one or two days shorter than the moon's cycle, very occasionally it has no full moon – there is a full moon at the end of January, and the next one is at the beginning of March. What this means is that both January and March will have blue moons. This happens, on average, once every thirty-five years.
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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.

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