Blue Moons
By R. d'E. ATKINSON ALTHOUdFf people nowadays are fairly blase about mere eclipses, and few would regard the spectacle of the moon being" turned to blood as an indication that the end of the world was imminent, most of us have still thought of a blue moon "as something almost more unlikely than the Day of Doom itself. But now large areas of Europe as far south as Switzerland have been startled, not merely by a blue moon, but by a blue sun in addition. Some reports claimed only a bluish tinge, but some said " as blue as a neon light," and there can be no serious doubt that the truth lay somewhere in this range. One thing is immediately clear; the moon, at any rate, must be acquitted of all primary responsibility. If the sun were really to turn blue, the moon would do the same, of course, since all its light is merely reflected sunlight ; but the reverse process is out of the question. The sun did in fact look blue to us, and the moon cannot have been the cause of that. However, the blueness cannot really have been due to the sun either, nor to any colouring agent in the regions of space near the sun, since the spectacle was seen only over a small fraction of the earth's surface. Most parts saw the sun and moon their normal colour, and the cause of the blue- ness must have been located in the earth's atmosphere, some- where up above the countries involved. We know a considerable amount about the way in which light can become coloured; and we can say with certainty that in ordinary cases the process is always one of subtraction or deflection, rather than one of creation. White light, of course, contains all colours, and a beam of white light which is turned blue by being passed through a piece of blue glass has no nuke blue in it than before; it simply has much less red and yellow than it had. Nothing has been added, and ordinarily nothing can be. The blue colour was due to some agent in the atmosphere which removed red and yellow from the sunlight and moonlight more readily than it removed blue, producing light with a blue preponderance much as a blue glass does. In a piece of coloured glass the colours which are stopped are genuinely absorbed. But we are quite familiar with a second process which produces colours in h different way; This is the process called," scattering," and it gives us the blueness of a clear sky an the redness of a setting sun as two different aspects of a single operation. In the earth's atmos- phere a small fraction of any beam of light is "scattered," or deflected abruptly from the straight path which light usually follows, by the molecules of the gas itself, and in clear air this happens much more readily for blue light than red. Thus when we look at the sun or moon, even if they are high in the sky, they appear a little redder than they really are, or than we would see them if we could get outside the atmosphere. When they are high, only a little light is lost in this way, and the reddening is very slight. When they are very low, of course, the rays have to come through a much greater amOunt of air, and the loss of blue light becomes extremely serious; the sun appears orange or even pure red as it sets, practically all of the blue and most of the yellow having been removed in this way. There is, of course, some true absorption as well (a great deal, over a city), but a considerable portion of the blue is only scattered ; it goes on its way in some new direc- tion, but since this direction is now very much a matter of chance, there is blue light travelling in all cross-directions; wherever one looks, with a sky free of clouds, blue light is corning from that direction, and one therefore sees the whole sky blue. If the air did not scatter at all, but were "really" " blue in the sense in which blue glass is, we should see blue .sunsets instead of red ones, and a black sky by day. Evidently, this normal atmospheric scattering cannot have been the cause of the blue moon and sun, for its effect is opposite in kind; it reddens the direct beam hnd we are now looking for something that will "blue" " it. We definitely want something which will remove red and orange light more readily than it will remove green and blue; and if it does this by scattering, the red light would show diffusely in cross-directions, just as the blue which has been scattered shows in cross-directions with ordinary air. Some time ago the American physicist, R. W. Wood, who has made many very ingenious contributions in the field of optics, experimented with various vapours and smokes, trying to produce a cloud which would look red when seen sideways to the light. He found that if he built up smoke-particles to larger sizes by letting iodine vapour condense on' them, the resulting cloud did in some cases look red when lit from the side—but not every kind of smoke would do. Professor Wood found that " the smoke which appeared best was that from hot sealing-wax; tobacco smoke did not work well." He suggested that the smoke-particles must perhaps be trans- parent themselves; whether or not this is correct, at any rate he was able to produce clouds which glowed vividly red when white light was shone on them. (We do, of course, often get red clouds quite high in the sky, near sunset, but in that case the sunlight shining on them is itself red. Wood's light was white). He suggests that the redness was due to absorption of blue within each particle or droplet, followed by scattering of the light that had been reddened in this way. This may indeed have been what happened, especially as the source of light did not seem coloured either way when directly viewed through the cloud; it would in fact have lost all colours in nearly equal amounts. This shows that in order to produce a blue moon it is not enough to subtract red light ; one must also avoid subtracting blue. A genuinely blue layer of gas, cloud or smoke would, of course, do this if it were dense enough, absorbing the red light exactly as blue glass does; but it seems more likely that something along the lines of Wood's work must have been involved. Not, of course, that actual iodine could have been present for the blue sun and moon; but water might condense on to smoke particles if such particles were present, building them up to suitable sizes, and, as many commentators pointed out, there had been enormous forest fires in Canada shortly before. Smoke itself, of course, and especially wood smoke, does look blue; but here we are definitely on the wrong track again. It looks blue when seen sideways to the light, and it reddens any source of light seen through it, behaving, in fact, rather like ordinary air only more so. and exactly the wrong way for what we want. It is perhaps a significant "pointer," however, that the burning forests will have consisted largely of conifers, which have a high resin-content; for what it is worth, the smoke may have resembled that from sealing-wax much more than that from tobacco. It is also noteworthy that on a different occasion, following fires in Australia, there were reports from New Zealand of abnormally reddish clouds by day. It may well turn out that the most interesting aspect of this phenomenon will be the light it can throw on the movements of upper-air masses. Meteorologists can follow masses of air whose temperature or humidity is different from that of neighbouring 'masses, but they seldom get the chance to follow a specific mass that has been really "labelled" as this was. If the observations, when collected, prove complete enough, one might get a very clear picture in this particular case; there are, for example, reports that high-flying aircraft got above the layer responsible; if this is correct, we know the thickness as well as the other limits of the mass. But it is, of course, a very expensive method of doing meteorology, rather like burning one's house down to get roast pig. Much as they.might like to work faster, the weather-men cannot indulge in this kind of work oftener than (just exactly) once in a blue moon.
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