There are two astronomical definitions of a Blue Moon; both are a type of Full Moon. When the Moon very rarely actually looks blue, it's because of a certain size dust particles in the atmosphere.
The next Blue Moons
Year Date
2015 Friday, 31 July
2016 Sunday, 22 May
Once in a blue moon means very rarely. But just how rare depends on your definition.
In astronomy, Blue Moon is defined as either the third full moon of an astronomical season with four full moons or the second full moon in a calendar month.
Such a blue Moon (second full Moon in single calendar month) will next occur on Friday, July 31, 2015 at 10:43 am UTC.
Contrary to popular belief, a blue moon is not actually blue in color. Blue moon is a term that is used to describe the third full moon of a season that has four full moons.
A year has four seasons - Spring, Summer, Fall (Autumn), and Winter - with three months and three full moons each. When one of the seasons in a year has four full moons, instead of the usual three, the third full moon is called a blue moon.
These days, the second full moon in a calendar month is also often referred to as a blue moon. This particular use was popularized due to a misinterpretation in a 1946 article in Sky and Telescope magazine. Such blue moons occur rather frequently - at least once every two or three years. The next such blue moon will occur on July 31, 2015.
Are blue Moons rare?
Blue colored moons do rarely occur when dust or smoke particles in the air are of a specific size. Such particles help create a blue colored moon by scattering blue light.
Red moons, which can be caused by other sizes of dust particles or lunar eclipses, are much more common than blue moons.
The phrase, once in a blue moon, is colloquially used to suggest that something is very rare.
Why the third Moon?
There are different accounts of why the third full moon of a season of four full moons is called a blue moon.
For instance, the Ecclestical calendar, which indicates the dates of the Christian fasts and festivals, uses the phases of the moon to determine the exact dates for holidays like Lent and Easter. The month of Lent contains the Lenten Moon. The first full moon of Spring – also known as Easter Moon or Paschal Moon – falls a week before Easter. In order to ensure that Lent and Easter coincides with the phases of the moon, the calendar has termed the third moon of the season as the blue moon.
Another version of this is that since each full moon of a normal year already has a given name, for instance Harvest Moon, the 13th nameless full moon in a year was named a blue moon. This way the lunations and calendars werw aligned to make sure celebrations and customs would still fall during their "proper" times.
Did you know?
About once every 19 years, the month of February does not have a full moon. The years when this happens, also have two full moons in two different months. This phenomenon will occur next in 2018.
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