New Year – in a blue moon
You may have gathered that I dabble in astronomy. The old year was designated the International Year of Astronomy as a celebration of the 400th anniversary of Galileo's first use of the telescope and of the publication of Johannes Keplar's book Astronomia Nova, in which he gave the first two of his three laws of the planets.
It is entirely fitting that this New Year's Eve should be a blue moon.
Everyone is familiar with the expression 'once in a blue moon.' It means, of course, something that doesn't happen very often. All sorts of folk lore surround it. It is supposed to be a time to make wishes, make vows and make offerings. Farmers used to be concerned about them, for it was said that a blue moon could affect hen's egg laying, or the harvests.
The Moon will probably not appear blue, however. The term is used to describe the occasion when you have two full moons in a month. We had one at the start of December and now on New Year's Eve we have a second.
The term 'blue moon' was first used by an astronomer in 1946, although the phenomenon of an extra full Moon has been known about for millennia. The Greek astronomer Meton of Athens described this extra full Moon in about 2,500 BC. He noticed that in a nineteen year period there would be seven more full moons than calendar months. Thus, there are seven extra full moons in the so-called Metonic cycle of nineteen years, meaning that you can expect a full moon at some stage about every two and a half years.
The Moon has been observed to be blue at various times in the past, and sometimes even green. It will only do this when there is a great deal of atmospheric pollution, such as after a cataclysmic volcanic eruption.
Back in 1883 when the volcano on the Indonesian island of Krakatoa erupted with devastating effect – the so-called 'bang that was heard around the world' – the Moon did appear green for a short period and then blue for two years. The reason is similar to the little milk experiment that I described last week. Particles of about one micron were spewed into the atmosphere.
This size is just right to scatter red wavelengths of light, but allow the rest to pas through. Thus moonlight, or the reflected light from the sun off the Moon was affected to make it look blue. Interestingly, it is recorded that the sun at times looked lavender colour.
In areas subject to other sudden atmospheric pollution, such as occurs when there are – heaven forbid - forest fires, then the Moon can appear pink or even red. This is because the particles in the atmosphere are considerably smaller than one micron, so red light waves are not scattered, but blue ones are.
Enjoy your New Year. And if the Moon is slightly blue, make a wish.
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Weather for Wakefield
Thursday 09 February 2012
Today
Light snow
Temperature: 0 C to 2 C
Wind Speed: 8 mph
Wind direction: South
Tomorrow
Light snow
Temperature: -2 C to 2 C
Wind Speed: 15 mph
Wind direction: South east
