Monday 26 September 2016

July's second full moon makes it a blue night for skywatchers

Published 29/07/2015 | 13:32

The full moon is due to appear for the second time in a month
The full moon is due to appear for the second time in a month

A certain catchy tune will be hard to avoid on Friday night as the full moon sets the nation humming.

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Not just any full moon, a "blue moon" - the first to occur since 2012 and the last until 2018.

But anyone hoping to see the dazzling spectacle of a glowing sapphire celestial body crossing the sky will be sadly disappointed. A blue moon (usually) is silvery grey, like any other.

The term "blue moon" refers to the second of two full moons appearing in the same calendar month.

It has more significance for astrologers than astronomers, marking a time of upset, change and possibilities.

Then there's that mournful ballad, Blue Moon, one of the most infectious earworms ever written. Composed by Richard Rodgers and Lorenz Hart in 1934, it was a hit on three occasions and covered by Elvis Presley in 1956.

The phrase "once in a blue moon" describes an event that is rare or special.

In fact calendar blue moons are not so incredibly rare. The last one occurred on August 31, 2012, and the next will be seen in another three years on January 31, 2018.

Friday night's lunar phase on July 31 fits the calendar definition of "blue" because there was another full moon on July 2.

To make things more complicated, there is a different "seasonal" definition that says a blue moon is the third of four full moons in one season. 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.

Just occasionally the moon really can be coloured blue - but you normally need to be near a volcanic eruption to see the phenomenon.

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 properly blue moon.

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.

Press Association

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