Rare blue moon to grace the sky tonight

A blue moon was last seen in 2012 and will appear again in 2018

A rare blue moon, which was last seen in 2012, will appear on Friday night.

Blue moons are rare and the next blue moon is not expected until 2018. The term ‘blue moon’ does not mean that that the moon will change colour, it actually refers to the second of the two full moons that appears in one month. The first full moon appeared on July 2.

According to NASA, a blue moon is more likely to be red because minute particles called aerosols in our atmosphere filter out the blue light.

'Blue moon' over the ancient Acropolis hills in Athens, picture taken August 31, 2012 PHOTO: AFP

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The English phrase ‘once in a blue moon’ which has been used since the 15th century and means something rare, did not use to have a scientific meaning. There is no clear knowledge of how and why the astrological definition for the blue moon arose.

Interestingly, blue-coloured moons have been seen before, although they do not have any connection to the ‘blue moons’ under discussion here. Usually, these blue-tinged moons can be seen after wildfires and volcanoes, when smoke in the air shatters red wavelengths of light, acting like a blue filter.

Read: NASA spacecraft shows Pluto wrapped in haze, ice flows

A blue-coloured moon was once seen when Mount Krakatoa erupted in 1883 in Indonesia. Some believe that the dust from the volcano which caused spectacular sunsets around the world, inspired Edvard Munch's The Scream.

This article originally appeared on BBC.