‘Once in a blue moon’ a celestial object genuinely changes colour
At the end of August there were announcements that the full moon would be a “blue supermoon”. You may have been disappointed that no matter how hard you looked, the moon just wasn’t blue.
So, are there ever actually “blue” moons that appear blue? The answer is yes.
The American magazine “Sky & Telescope” first used “blue moon” in 1946 to refer to a second full moon in a calendar month. This caught the popular imagination and has stayed with us. This past August had a full moon on August 1 and then again on August 31, hence the second one was called a “blue” moon, even though it wasn’t blue.
Two full moons in one month happens occasionally in the Gregorian calendar that is used internationally. That is because all months in the Gregorian calendar are 30 or 31 days (except February). Since the time from one full moon to the next is 29.5 days, when there is a full moon right at the start of the month, another one occurs at the end.
This never happens in the Islamic, Jewish, and Chinese calendars, since their months are exactly as long as the time it takes the moon to go through its phases, and they begin with new moon, so there can only ever be one full moon in a month. Of course, the origin of the word month comes from “moonth”, the time for the moon to go through its phases.
Because there are slightly more than 12 “moonths” in a year, the Gregorian calendar stretches a month to be longer than 29.5 days to fit in exactly 12 in a year.
The expression, “once in a blue moon”, meaning “very rarely”, goes back five centuries.
It was from that expression that the second full moon in a
month became a “blue” moon.
But real blue moons are much rarer.
Normally, the sky is blue because the molecules of air and particles floating in the atmosphere are slightly smaller than the wavelength of blue light, which is about 400 nanometres, that is, 400 billionths of a metre.
Red light has a much longer wavelength of about 700 nanometres. This match between the size of the particles in the air and the wavelength of blue light means the air scatters blue light much better than red light.
That is why the sky is blue. For the same reason, when you look directly at the sun at sunset, or at the moon at moonrise, the blue light is being scattered away and the red comes through, so sunsets turn red and moonrises are often golden.
But “once in a blue moon,” the air fills with particles that are slightly bigger than the wavelength of red light. And then the red light is scattered better than the blue, the sky goes strange, eerie colours and the sun and moon appear a light blue in colour. These are the real blue moons.
This can happen after large forest fires and volcanic eruptions. I myself saw both blue moons and blue suns on many days and nights from the SA Astronomical Observatory at Sutherland after the eruption of Mount Pinatubo in the Philippines in June 1991.
Aerosols and ash from the volcano rose high into the atmosphere and spread around the world in the months after that eruption giving purple and egg-yolk yellow sunsets, and blue moons!
In 1950, a blue moon and blue sun were seen in Scotland and the north of England as reported by the newspaper, The Scotsman, on September 27 1950.
Those arose after smoke from massive wildfires in British Columbia, Canada drifted across the north Atlantic.
With the terrible wildfires in Canada in the last few months, I have been wondering if anyone would see a real blue moon or blue sun, but I have not seen any report of this yet.
Those are so rare that they really do happen only “once in a blue moon”.
Donald Kurtz is extraordinary professor at North-West University in Mahikeng. He has an A-1 rating from the SA National Research Foundation, its highest rating. He also holds appointments in the UK of emeritus professor at the University of Central Lancashire and visiting professor of astrophysics at the University of Lincoln.