I have a clear memory from when I was a child in Guildford, Surrey, in the early 1960s. One evening I saw a moon which was distinctly blue. Sadly, I recall no other details such as time or year, but I know I was outdoors.
Could this memory be correct? Is a blue moon possible and if so, what atmospheric conditions are necessary and how often could it happen? There must be something in the saying "once in a blue moon".
In the late autumn and winter of 1963 the Moon had a very bluish tinge. This was due to the eruption of Gunung Agung on Bali which produced a large amount of volcanic dust and exceptional quantities of sulphur dioxide. It was probably also responsible for that winter being unusually cold. There are reports of blue moons following other eruptions.
The questioner's memory is quite reliable: the Moon can indeed appear blue on rare occasions, and so can the Sun. This is caused by anomalous scattering of the moonlight or sunlight by fine particles of ash or dust such as those produced during volcanic eruptions or large forest fires. Blue moons and suns were widely observed after the eruption of Krakatoa in 1883.
More recently, in a paper published in 1951 (Monthly Notices of the Royal Astronomical Society, vol 111, p 477), R. Wilson reported a "blue sun" which was observed over Edinburgh on the afternoon of 26 September 1950 (see below for a first-hand account). The Sun "was observed to be a deep indigo blue" from 3pm, when it was first noticed, until sunset. The following day, the Sun's colour had returned to normal.
Wilson, who worked at the Royal Observatory, had the presence of mind to take a spectrogram of the blue sun. This shows a marked extinction of the red part of the solar spectrum when compared to a spectrogram of the "normal" Sun, so the effect was not a product of the observer's imagination.
Wilson noted that extensive forest fires had been burning in Alberta, Canada, on 23 September. The smoke clouds had reached eastern Canada on 24 September, when they were thick enough to blot out the Sun.
When the Sun did become visible again, it was purple or blue. From Atlantic weather charts, Wilson calculated that the smoke would have reached Scotland by 26 September.
Normally, blue light is scattered more strongly than red light by dust in the atmosphere. This is why the Sun appears red at sunrise and sunset. However, when the particles have a mean radius between 0.4 and 0.9 micrometres and an almost uniform size distribution, they scatter longer wavelengths (red light) more strongly, giving rise to blue moons and blue suns.
According to Bohren and Huffman in Absorption and Scattering of Light by Small Particles published by John Wiley and Sons, the uniform size distribution is a key factor in anomalous scattering. Few processes in nature give rise to such a size distribution, which is why blue moons and blue suns are so rare.
As for the old expression "blue moon", it is at least five hundred years old. Philip Hoscock has written an entertaining article about both its history and its folklore which can be found at www. griffithobs.org/IPSBlueMoon.html.
I was flying near St Andrews in Scotland in early October 1950 when I noticed that the Sun was clearly blue. After opening the cockpit canopy and removing my sunglasses the Sun still looked blue so I radioed Air Traffic Control at RAF Leuchars to ask what they thought of the situation. They said it looked blue to them too, and they sent off a Meteor aircraft to investigate.
After a while they told me that their pilot had reported climbing through what looked like a smoke layer at about 11 000 metres and that above it the Sun looked normal. The RAF's meteorological office thought that it was almost certainly smoke brought across on a jet stream of air from some severe forest fires that Canada was experiencing at the time. They said that had there been a visible Moon that night, and it had been viewed through the same smoke layer, it too would have looked blue.
There is no clear link between historical blue moons and the expression "once in a blue moon". The first recorded reference to a blue moon in literature appears in the 1528 poem Rede Me and Be Not Wroth:
"Yf they say the mone is blewe
We must believe that it is true"
Thereafter the expression appears to mean a patent absurdity--waiting for a blue moon would be like waiting for hell to freeze over.
In the last few decades, a blue moon has come to mean the second full moon in a calendar month; something that occurs seven times every 19 years or about once every two and half years on average. There were two such blue moons in 1999. This modern meaning is attributed to a 1946 article in Sky & Telescope magazine, and is a misinterpretation of an earlier meaning which was used to describe the third full moon in a season containing four full moons.