The decade and this year will end with a “Blue Moon.” The idiomatic expression “once in a blue moon” metaphorically refers to a rare event. By modern definition, a blue moon is the second full moon in a calendar month. Blue moons are not entirely rare and occur about once every two years.
Most years have twelve full moons, but since a lunar cycle is 29.5 days, we wind up with almost 11 leftover days. Eventually the days add up, and we have two full moons in a single month — like we do this month.
The next blue moons will occur August 2012, followed by July 2015 and October 2020.
The expression has been in use for about 400 years.
The whole thing may have started in the dark ages which followed the fall of the Roman empire, one of the few lifelines for science and mathematics in Europe was the desire to calculate the correct date of Easter, following clues in the bible. This must fall on the Sunday immediately following the 14th day of the Paschal Moon, which in turn falls on or after the day of the ecclesiastical vernal equinox which is fixed as 21st March. It can never occur before 22nd March or after 25th April. Lent, which begins 46 days before Easter, contains the Lenten Moon.
By tradition all twelve full moons of a year have names, some religious like those above and the Moon Before Yule and the Moon After Yule, and some relating to seasonal activities and events such as Flower Moon, Hay Moon, Harvest Moon and Hunter’s Moon. The main function of a Blue Moon is to preserve the relationships between these religious and lunar events in those years when there is a thirteenth full moon.
By this rule, a Blue Moon is the third full moon of four in a fixed season usually containing three, and will always fall on the 20th - 23rd of May, February, August or November. Here, again using Universal Time, are the months in which blue moons will occur over a full 19 year cycle using this more traditional rule –
Sometimes dust or smoke from forest fires or a volcanic eruption somewhere can change the moon’s color. But the origin of the phrase referring to the moon’s hue is lost in time. NASA inks the “blue moon” term to the 1883 explosion of the Indonesian volcano Krakatau, when ash rose to the top of the Earth’s atmosphere, making the moon appear blue. But the phrase was in use before that.
Triskaidekaphobiacs. those who fear the num,ber 13. says that because the Blue Moon is the 13th full moon of the year it will be unlucky. I guess that makes as much sense as any other part of it all.