domingo, dezembro 27, 2009

Blue Moon: 31/12/2009

When two full moons occur in any calendar month, the second is called a blue moon. The term (in its modern usage) therefore has nothing to do with the Moon's actually color. A blue moon occurs about once in 2.5 years on average. A blue moon can occur in January and the following March if there is no full moon at all in February, as is the case in the years 1999, 2018, and 2037.

There are several other meanings ascribed to the term "blue moon" (the most common being "a very uncommon event"), but the one given here seems first to have appeared in The Maine Farmers' Almanac of 1937. The phrase "blue moon" has been around for several hundred years, but its meaning has changed a few times. The earliest use of "blue moon" meant an obvious absurdity which everyone knew never happened. However, the moon does occasionally turn blue as a result of smoke from forest fires or particles from a volcanic eruption. Since these blue-looking moons were rare but did happen from time to time, the phrase "once in a blue moon" was coined, meaning that an event is unusual, but can happen occasionally (Kibbey). "Blue moon" has also been used as a symbol of sadness and loneliness. That appears to have a history of its own among musicians and songwriters (Kibbey).

The following table lists all blue moons from 1990 to 2010 for dates in universal time. Note that because the full moon occurs at different times (and therefore potentially on different calendar days and in different calendar months) in different time zones, the occurrence of blue moons is time zone (and daylight saving time) dependent. For example, a blue moon occurs on May 31, 2007 in the Eastern Daylight Time zone, but on June 30, 2007 in universal time. Blue moons cannot occur in February, since the Moon's synodic period is 29.531 days, but February is 29 days at its longest (during a leap year).


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