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Picture of the week: By the light of the silvery moon

By Helen Carson

This is the awe-inspiring sight of a full moon rising over people gathered on Glastonbury Tor in Somerset ahead of this week's so-called 'Blue Moon'.

The rare event, which was last seen in 2012, refers to the second of two full moons occurring in the same calendar month - but the moon is definitely not blue.

While the expression "once in a blue moon" may appear to be connected to the unusual lunar cycle, the common phrase was not coined from science, but is simply a term applied to something unexpected, according to historical records.

Stargazers ensured they got a good view of Thursday's blue moon, as there won't be another one until 2018.

Amateur astronomer James Hugh Pruett, who studied blue moons in the 1940s, said they occur seven times every 19 years, because the lunar month is shorter than most of our familiar calendar months.

This occasionally means there will be 13 full moons in one year, instead of 12.

And the term blue moon has been sourced back to the Maine Farmers' Almanac, in the early 19th century, whose definition was given to the third full moon in a season containing four.

But the earliest explanation appears in the Morris Dictionary of Word and Phrase Origins in 1528, which defines 'blue moon' as a rare occurrence.

Meanwhile, the native American Algonquin people named all full moons - from the Wolf Moon in January right through to the Cold Moon in December.

Scientists say moons only have a bluish tinge if there are specific-sized particles in the air, like dust or water droplets.

So blue-coloured moons have been seen after wildfires and volcanoes, when smoke in the air scatters red wavelengths of light, acting like a blue filter.

One time there was said to be a blue moon was after the eruption of Mount Krakatoa in Indonesia in 1883.

The dust in the atmosphere caused spectacular sunsets around the world - some think it's the inspiration for Edvard Munch's famous painting The Scream.

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