'Blue moon' refers to when a full moon occurs twice in a calendar month
The moon does not appear blue, but its usual silvery grey colour
Lunar event will occur on Friday night and is the first since August 2012
There won’t be another for three years, until January 31, 2018
By SARAH GRIFFITHS FOR MAILONLINE
PUBLISHED: 07:16 EST, 30 July 2015 | UPDATED: 09:31 EST, 30 July 2015
'Once in a blue moon' is not just a phrase that's used for occasional events.
It is also a 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.
The unusual 'blue moon' lunar event will take place on Friday night and will be the first to occur since 2012. This image shows an image of the last blue moon, which occurred on August 31, 2012. It is seen here above the cloudy night skies of Cincinnati, Ohio
Blue moons are not as rare as other celestial events, but there won’t be another for three years, until January 31, 2018.
Anyone hoping to see a dazzling sapphire orb crossing the sky will be disappointed, however, because a blue moon is usually a silvery grey.
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.
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. A stock image showing different phases of the moon through the lunar months is pictured above
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.
WHY THE MOON CAN APPEAR BLUE AND RED AT TIMES
Confusingly the term 'blue moon' doesn't mean that the Earth's satellite will look blue in colour.
But this can happen in unusual sky conditions when fine particles of dust or smoke act as a filter, only allowing blue light to pass through the tiny particles.
Reports of blue moons followed the Mount St Helens and Mount Pinatubo volcanic eruptions in 1980 and 1991, for example.
In April this year, the moon appeared to glow red in an event called the 'blood moon'. Skygazers in the US, Canada, Australia and New Zealand were treated to a total eclipse of the moon (pictured over New Zealand)
In April this year, the moon appeared to glow red in an event called the 'blood moon'.
Blood moons occur during a lunar eclipse, when the moon passes in the shadow of the Earth, called the umbra, where light from the sun is blocked by our planet.
This gives rise to the moon's ‘blood red’ appearance during a total eclipse, when the entire moon is in shadow and the Earth's atmosphere scatters the sun's red light.
As a result, the red light reflects the moon's surface, casting a reddish rush hue over it.
The last blood moon, which occurred over the Easter weekend, was the third in a series of four, with the final one expected on September 28.