Manama: 'Once in a blue moon' is not just a phrase that's used for occasional events.
It is also an unusual astronomical phenomenon that occurs when a full moon occurs twice in a calendar month.
The lunar event will take place on Friday night and will be the first blue moon to occur since 2012.
Friday night's lunar phase on July 31 fits the calendar definition of ‘blue’ because there was another full moon on July 2. Blue moons are due to the difference between calendar and lunar months. While a calendar month lasts from 28 to 31 days, a lunar month - the time interval between two full moons - is always 29.53 days long.
Confusingly, there is also a ‘seasonal’ definition that says a blue moon is the third of four full moons in one season. According to this definition, tomorrow’s full moon is not ‘blue’ at all. The next seasonal blue moon is not due until May 26, 2016. While the blue event is of interest to astronomers, it is of more significance to astrologers, who say blue moons mark a time of upset, change and possibilities.
Occasionally, the moon really does appear blue, but that usually happens because of an erupting volcano. In 1883, blue moons appeared almost every night after the Indonesian volcano Krakatoa exploded with the force of a nuclear bomb. Plumes of ash rising high into the atmosphere acted like a filter only allowing blue moonlight to pass through the tiny particles.
Blue coloured moons were also seen in 1983 after the eruption of the El Chichon volcano in Mexico. There were other reports of blue moons caused following the Mount St Helens and Mount Pinatubo volcanic eruptions in 1980 and 1991. Wildfires producing smoke containing one micron-wide ash particles can also produce a blue coloured moon.
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