Indians are really privileged to witness two full moons in August. This is an exciting astronomical spectacle that happens rarely. When two full moons are perceptible in a month, then the event is called ‘Bluemoon’. A blue moon can refer to the third full moon in a season with four full moons. Normally one can witness twelve full moons that occur approximately monthly.
In addition to those twelve full lunar cycles, each solar calendar year contains roughly eleven days more than the lunar year of 12 lunations. The term "blue moon" comes from folklore. Different traditions and conventions place the extra ‘blue’ full moon at different times in the year.
A bluemoon is the occasional blue colour of the Moon, due to effects in the Earth’s atmosphere. It can be caused by dust particles, from volcanoes or forest fires, high in the upper atmosphere, which scatter light, making it appear blue.
The expression sometimes refers to the occurrence of a second full moon in a calendar month, something which occurs about seven times every 19 years. A ‘blue moon’ is also used colloquially to refer to a ‘rare event’, reflected in the phrase ‘once in a blue moon’. The key to a blue moon is having lots of particles slightly wider than the wavelength of red light 0.7 micrometre - and no other sizes present.
This is rare, but volcanoes sometimes produce such clouds, as do forest fires. Ash and dust clouds thrown into the atmosphere by fires and storms usually contain a mixture of particles with a wide range of sizes, with smaller than 1 micrometre, and they tend to scatter blue light. This kind of cloud makes the moon turn red; thus red moons are far more common than blue moons.
“It’s extremely rare that we would see an actual blue-colored moon, although unusual sky conditions can create it. Blue-colored moons aren’t predictable. In the 21st century, according to folklore, the name Blue Moon has come to have two other meanings, both of which are predictable - and very popular. A Blue Moon can be the second full moon in a calendar month or it can be the third of four full moons in a single season. There will be two full moons in the month of August this year. Thus the next scheduled blue moon — second full moon of a calendar month — will be August 31. Every month has a full moon, and, most of the time, the names for full moons coincide with particular months or seasons of the year. By either definition, the name Blue Moon accounts for times when there happen to be more full moons than is convenient," said amateur astronomer S A Mohan Krishna.
In recent decades, many people have begun using the name Blue Moon to describe the second full moon of a calendar month.
The time between one full moon and the next is close to the length of a calendar month. So the only time one month can have two full moons is when the first full moon happens in the first few days of the month.
This happens every 2-3 years, so these sorts of Blue Moons come about that often. The idea of a Blue Moon as the second full moon in a month stemmed from the March 1946 issue of Sky and Telescope magazine, which contained an article called 'Once in a Blue Moon' by James Hugh Pruett.
“There were two full moons in January and two full moons in March and no full moon in February. So both January and March had Blue Moons.
The next year of double blue moons is 2018. The full moon of August 31, 2012 will be considered a Blue Moon. The next scheduled blue moon is in July 2015,” he added.
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