A lunar phenomenon which really does only happen once in a Blue Moon, is set to light up the night sky on Friday 31st July.
The spectacle appears only once every three years, according to the National Oceanic and Atmospheric Administration.
The blue moon will have more significance for astrologers, people who claim they can glean meaning from the movement of the stars, than for scientific astronomers.
A blue moon traditionally marks a time of change and possibility in the astrological world.
Tomorrows blue moon is the first since August 31, 2012, and won’t be seen again until January 31, 2018.
But is the moon actually blue? No. The term is a misnomer as the moon actually appears gray or silver, just like any other full moon.
What is a “Blue Moon”?
The weather network explains: There are actually two definitions of “blue moon.”
One definition states that the “blue moon” is the third of four full moons in one season. There are three full moons during Summer 2015 – July 2, July 31 and August 28 – so this full moon doesn’t satisfy that condition. The next full moon that will qualify is on 26 May 2016.
The second definition (by the calendar) is what we’re seeing on Friday – the second of two full moons that occur within the same calendar month.
How rare is “rare”?
The phrase “once in a blue moon” is closely linked with rare and special events. How rare is a blue moon, though?
Seasonal blue moons happen just about once every three years. The last one was on 20 August 2013. The next will fall on 26 May 2016.
The last calendar blue moon was on 31 August 2012. This next full moon, coming on July 31, will be the 36th full moon since then, and after this there are 30 full moons until the next one, on January 31, 2018.
That’s not exceedingly rare, but waiting nearly 3 years for something can certainly make it seem so.
The reason for the rarity is due to the difference between our calendar months and a lunar month.
While our calendar months last anywhere from 28 to 31 days, depending on the month and the year, a lunar month – the time between reaching the same phase, Full to Full, New to New, etc – is always 29.53 days. The difference not only throws off when different lunar phases happen from month to month, but the timing of full moons and even how many full moons during a year changes over time. It’s not often when that 29.53 day lunar month happens to squeeze into one of our calendar months, though. It takes awhile for everything to line up properly.
A truly blue moon?
As for the full moon actually looking blue, as the Science@NASA video above details, that is exceptionally rare.
This only happens when tiny particles – volcanic ash particles or smoke particles from forest fires – measuring around a millionth of a metre (a micron) across get lofted up into the sky. Particles of this size scatter red light stronger than other wavelengths, while allowing blue to pass through. So, when moonlight shines through a layer of these particles, it can make the Moon look blue.