Clouds in a Glass of Beer: Simple Experiments in Atmospheric Physics by Craig F. Bohren

Author:Craig F. Bohren
Language: eng
Format: epub
Publisher: Dover Publications

12

Once in a Blue Moon

Yf they saye the mone is belewe

We must beleve that it is true

Anonymous—16th century

Charles E. Funk: A Hog on Ice

Rare events are sometimes said to occur only “once in a blue moon.” This saying is familiar enough, but how many of us have ever thought about its meaning? Is it merely whimsical, or is it a reflection of folk wisdom which has a basis in fact?

We usually see the moon as white or perhaps yellow. Low in the sky it may appear orange or red. It never is blue. Never? Well, never in my experience, but I have not been so lucky as Robert Wilson, who in September of 1950 observed a blue moon (and a blue sun) in Edinburgh. Of course, these unusual events were witnessed by many other residents of this city. But Wilson had a unique opportunity to do more than just note the curious fact that the sun was blue and then go about his business: he was an astronomer on the staff of the Royal Observatory (now professor of astronomy in University College, London) and had the presence of mind to make quantitative observations with a telescope. Moreover, on the basis of his and other observations and some calculations, he concluded that the blue sun and moon were caused by clouds of small particles from forest fires in Alberta, which had been carried by winds across the Atlantic to Europe. These particles were probably small oil droplets formed in the combustion products of the fire. Oil, unlike water, has a sufficiently low vapor pressure that small droplets of it can survive a trip across the Atlantic.

To my knowledge a blue moon has not been reported since 1950. So I must admit with some sadness that it is unlikely that I shall ever see one. But blue moons, however rarely they may occur naturally, can be produced artificially at will with the simplest of apparatus. This was happened upon by Donald Huffman, a physics professor at the University of Arizona, during a lecture to astronomers. His topic was interstellar dust, the particles responsible for, among other things, the dark patches in the Milky Way. Dust between us and a star reddens its light: the starlight we receive is redder than it would be if there were no dust (this is not to be confused with the red shift, which is an entirely different matter). A familiar example is the sun, the star nearest to earth, which may be red at sunrise and sunset because of selective scattering by atmospheric molecules and particles.

Huffman usually illustrates his lectures with simple demonstrations. To demonstrate reddening of starlight he blew a puff of cigarette smoke into a beaker inverted on the glass plate of an ordinary overhead projector. The image of a clear beaker transmitted onto a screen is white; that of a smoke-filled beaker is reddish. At least that is what it is supposed to happen. And so it did—at first. But to Huffman’s surprise the image of the smoke-filled beaker began to change from red to blue.

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