The sight being so rare, the phrase ‘once in a blue moon’ was coined because of it.
But rather than the moon actually changing colour, a blue moon refers to when two full moons appear in the same calendar month.
The first full moon was spotted on July 1 and the second will appear on the last day of the month – Friday, July 31.
timeanddate.com on blue moons: “A Moon that actually looks blue, however, is a very rare sight. The Moon, full or any other phase, can appear blue when the atmosphere is filled with dust or smoke particles of a certain size.
“Eruptions like on Mount Krakatoa, Indonesia (1883), Mount St Helens (1980) and Mount Pinatubo (1991) are all known to have caused blue moons.”
This appearance is the first such occurrence in three years since the last blue moon month in August 2012, the next will not be until 2018.
Astronomers predict it will only happen on 12 occasions between now and 2043.
Friday’s weather is set to be mostly clear across Sussex with just a few clouds around, so there is a good chance of getting a good view of the phenomenon.
If you miss this Friday’s blue moon, here are your next opportunities:
31 January 2018
31 March 2018
31 October 2020
31 August 2023
31 May 2026
31 December 2028
30 September 2031
31 July 2034
31 January 2037
31 March 2037
31 October 2039
31 August 2042
More about blue moons at timeanddate.com.
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