Blue moon: rare cosmic event to light up the sky
More exciting for astrologers than astronomers, a blue moon is the rare event of a second full moon that appears in a month
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Your support makes all the difference.The sky is set to be lit up by a blue moon. But while the event is rare, it’s also a little badly named — the moon itself will look grey, like any other night.
This evening, the first blue moon since 2012 — and the last until 2018 — is set to be seen in the sky.
The phrase refers to times when a moon appears as full twice in a month. (The full moon on July 31 follows another such night on July 2.)
As such, the event is more exciting for astrologers than astronomers, since it doesn’t change anything about the moon itself.
It also means that the phenomenon will be true everywhere, rather than only on specific parts of the planet.
Sometimes the moons can look blue. But that only really happens when you are stood near a volcanic eruption.
Tonight'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, this 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.
Additional reporting by Press Association
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