Popular Science Monthly/Volume 28/February 1886/Bishop's Ring Around the Sun
| BISHOP’S RING AROUND THE SUN. |
By WILLIAM M. DAVIS.
IF there is nothing new under the sun, there is at least something new around it. For the last two years close observers of the sky have noticed that the noonday sun has been surrounded by a corona of dusky, coppery, or reddish light, as it has been variously described, the circle of most distinct color having a radius of about fifteen degrees, and inclosing a brilliant, silvery or bluish glow close around the solar disk. A similar appearance of much less intensity has been occasionally noticed around the full moon on very clear winter nights.
The most experienced observers of sky-colors are agreed that this corona was not visible before the latter months of 1883. Von Bezold, of Munich, who was considered the most competent meteorologist to prepare a schedule for observations on the colors of the sky for the recent German Arctic Expedition, says that, in spite of the close attention he had previously given to the appearance of the usual whitish glow around the sun, he had never till recently seen the dusky ring. Thollon, of Nice, who had made a special study of the sky around the sun for a series of years, declares confidently that a change occurred in November, 1883. Backhouse, of Sunderland, who has a careful record of parhelia for twenty-five years, confirms this opinion. We may, therefore, safely accept the conclusion that the change of color from the blue of the open sky to the intense glare of whitish light close around the sun, was until lately effected without the appearance of any reddish tinge in the transitional area.
The new corona, to which the name of "Bishop's ring" has been given after its first observer, has never been a very conspicuous affair, and therefore has not attracted the popular attention that it deserves; but it could easily be seen every clear day last winter, and has repeatedly been noticed since then in the latter months of 1885. The hazy days of summer are not favorable to its visibility. It is best seen from elevated stations, which gain their sky-colors chiefly from the finer particles floating at great altitudes, as they are above the lower strata of the atmosphere where the relatively coarse, haze-making dust is suspended. Forel, of Morges, one of the most acute observers of terrestrial physics in Switzerland, reports the distinct visibility of the ring from mountain-tops, while it is not to be seen from the valleys, where the whitish, hazy light overpowers its delicate colors. He adds that many of his countrymen in the higher Alps had been struck with the appearance of the new color in the sky before they had heard mention of it. For the same reason Tissandier found the distinctness of the corona greatly increased when viewed from a balloon high above the dusty air of Paris. At low-level stations it is best seen during the persistence of that type of weather known as "anti-cyclonic" among modern meteorologists. Such weather is characterized by high barometric pressure, and consequently has descending currents of pure, clean upper air. The sky is then brilliantly clear and free from haze, and at such times last winter the ring was of remarkable distinctness. Thin cirrus clouds generally hide it; but the presence of scattered, sharp-edged cumulus clouds adds to its visibility in the clear spaces between them. Let one of them stand before the sun, so that its heavy shadow darkens the lower air, whose reflecting particles ordinarily add much white light to the blue of the sky; then, looking between the clouds in the neighborhood of the sun, a broad arc of the ring appears with its colors blending in what may be fairly called the most delicate intensity. Just before a moderate thunder-storm early last June, the ring was thus presented with most beautiful effect. It was seen in Cambridge with extraordinary distinctness on the afternoon of November 2, 1885, when the lower clouds of a heavy rainstorm rapidly broke away in the west, about two o'clock, leaving the sun well hidden behind a sheet of upper cloud and a space of open sky below it. The lower air was thus well shaded from direct sunlight, and the strength of the colors was most remarkable. There was first the margin of the glowing central area at the edge of the cloud, soon turning pale brassy yellow, and then strong reddish gold at about fifteen to twenty degrees from the sun; farther out yet was the delicate rosy or purplish-pink, and at last the pure blue of the sky. The colors were wonderfully vivid for the time of day, although, of course, not so brilliant as those of a well-developed sunset; but it unfortunately seems to have very generally passed unnoticed. Inquiry among my neighbors failed to discover any one who had seen it.
Numerous observations in many parts of Europe and this country leave little room for question that the corona is produced in the upper atmosphere, and that it was continuously present above the cloudy or dusty lower air over a large part if not over the whole of the earth throughout 1884 and 1885.
The explanation of the optical process by which such a corona may be produced offers no particular difficulty. It is a relatively simple effect of diffraction, an effect of the same nature as that seen in the colored rings surrounding a light looked at through a glass that is faintly frosted over, as may be noticed almost any cold winter evening when looking out of a window. A brief statement of the process may be made, following the explanation given by Kiessling, to whom the author is much indebted in the preparation of this article.
Let us first consider the action of a beam of parallel rays of mono-chromatic light—that is, of strictly one-colored light, whose waves all agree in their period of vibration—as it passes an excessively fine thread stretched at right angles to its path, and falls on a screen beyond. The waves will be turned aside from and bent around both sides of the thread, as if diverging there from new centers of radiation. This is diffraction. A gross figure of the process is here given (Fig. 1) on a plane at right angles to the thread, T H. The point A
Fig. 1.
on the screen will be illuminated, although it is behind the thread, for the waves that reach A from either side of the thread agree in phase. Take a point, B or C, such that the distance B H exceeds B T by half a wave-length. Then the diffracted waves which agreed in phase at the thread will be just opposed at B, the crest of one will fall with the trough of the other; they are thus extinguished by interference, and darkness will result. Take another point, D, on the screen, such that D H differs from D T by a whole wave-length. Now the diffracted waves will agree in phase at D, and this point will be illuminated, like A. The screen will therefore be marked by a bright band behind the thread, and by dark and bright bands, blending together and parallel to it on either side. Their breadth will vary directly as the wave-length, and inversely as the diameter of the thread. The redder the ray and the finer the thread, the broader the bands.
Next consider the case of a single small particle of diameter greater than the wave-length in the path of the monochromatic beam. The same figure now may represent a plane parallel to the rays, passing through the particle in any direction. The parallel bands become concentric rings with a bright central spot behind the particle. The diameter of the rings varies, as above stated for the bands. The bluer the light and the larger the particle, the narrower the rings.
The next step makes an approach to the actual case by supposing a great number of one-sized particles floating in the space traversed by the waves, and considers their effect as perceived by an observer at A (Fig. 2). The unaltered light is seen in the direction of the rays
Fig. 2.
A R. Interference of the waves diffracted from B causes a dark circle on the surface VT, of diameter A H; from C, a circle A J; from D, a circle A N. Hence all the particles situated on the surface of a cone whose axis is A R, and apical angle is F A G, give no light to A, and the luminous center R seems to be surrounded by a dark ring at an angular distance R A F. This may be called a subjective ring in distinction from the objective rings, A H, A J, A N. In the same way, the particles situated on the cone P A S will contribute to the formation of a bright subjective ring of radial angle R A P. The center R will appear to be surrounded by dark and bright rings.
Now we must introduce the supposition of many-colored or poly-chromatic light—the white light of many wave-lengths that comes from the sun. Such light, passing a fine thread, forms a series of prismatic bands on a screen; passing a single particle, it forms a series of concentric prismatic rings with the blue inside; for the first blue ring will fall a little inside of the first yellow, and the first yellow inside of the first red—and so on with the others, until at a distance from the center the outer rings overlap irregularly. The subjective rings seen when white light passes through a transparent medium containing many one-sized particles will, for the same reason, appear many-colored, with the blue inside and the red outside; the central area will be white, with a reddish margin.
Finally, the actual case is reached when the suspended particles are of different sizes. The colors of the central area now overlap so irregularly that they unite to form a whitish or silvery disk; but the outer red margin of the central area formed by the smallest particles is still uncounterbalanced. The silvery disk will be reddish about the circumference; and the colors thus deduced by theory are so closely like those observed in Bishop's ring around the sun that it may be safely considered a diffraction corona. The outer rings are too faint to be seen in daytime.
Colored coronal rings may be seen around a light when looking at it through a glass strewed lightly over with spores of lycopodium; they are so nearly of the same size that a number of concentric rings appear. Kiessling describes some interesting experiments with thin artificial clouds of condensed vapors, through which the sun is seen surrounded with coronal rings. The moon is often surrounded with similar rings of small diameter, formed by diffraction, probably on small floating particles of ice, even when the sky seems clear. These are easily distinguished from halos. The latter are of definite and much larger diameter, and, when seen around the moon, are generally whitish; if formed around the sun, they are visibly colored with the red inside; and they are due to refraction and reflection on minute ice-crystals.
All this is safe enough; it is the origin of the diffracting particles and the long endurance of their effect that give trouble. Indeed, the experimental and mathematical knowledge of optics, based on the undulatory theory of light, has advanced so far that the physicist is now better able to suggest processes by which effects may be produced than the meteorologist is able to apply them. The physicist can safely say that a sufficient supply of extremely fine liquid or solid dust scattered through the atmosphere would produce just such a solar corona as Bishop's ring. It is for the meteorologist to inquire whether a supply of dust sufficient in quantity and quality, appearing at the right time and enduring long enough, can he accounted for.
Kiessling, of Hamburg, already referred to, has done the best work on the corona as well as on the great sunsets with which it is evidently connected. His pamphlet, entitled "Die Dämmerungserscheinungen im Jahre 1883, und ihre physikalische Erklärung" (Hamburg, 1885), gives the most satisfactory account and explanation of the twilights that I have seen; and its value is largely increased by the experimental illustrations that the author has devised in imitation of the strange natural phenomena that he accounts for so well. A later paper, "Ueber die geographische Verbreitung des Bishop'schen Sonnenringes," in the May number of the little meteorological journal, "Das Wetter," and a short paper by Forel on "Le Cercle de Bishop," in the Geneva "Archives des Sciences" for June, are the most recent articles of consequence on the corona, and give important evidence as to the origin of its diffraction particles by showing its relation to the famous sunsets. The new corona was first noticed in Honolulu on the 5th of September, 1883, by the Rev. Sereno F. Bishop, who called attention to it by descriptions published at the time, and in letters to "Nature." Although seen so early in September in the Sandwich Islands, it was not recognized in this country till November 24th, when Professor Le Conte saw it at Berkeley, California; nor in Europe till the days directly following; but ever since then it has been continuously visible till now, in proper conditions of weather as already described. After rarely being seen in the summer, it has reappeared in the clearer days of the winter. Being always relatively inconspicuous, the date of its first visibility can not generally be determined with accuracy—alas for the neglect of so rare an opportunity of valuable observation!—but the agreement of the growth of the area in which it was noted and the spread of the great sunsets is placed beyond a doubt; and with them its origin must be referred to the explosive eruption of Krakatoa. Kiessling considers this relation of cause and effects to be firmly established, and even quotes approvingly the name given by Arcimis in Madrid, "corona solar krakatoense," although the name of "Bishop's ring" is undoubtedly the one that will come into general use.
The evident difficulties in the way of accepting the volcanic origin of the diffracting particles are the great quantity of material that would seem to be needed, the excessive fineness of its texture, and its long suspension in the thin upper air; but I believe that these difficulties are by no means fatal to the volcanic theory. The quantity needed is not absolutely so great, after all. Tyndall suggested that the minute, almost molecular particles, to which the blue color of the sky is usually referred, could all be contained in a snuff-box; and, while this need not be taken as in any way an accurate estimate of the mass of matter involved, it may nevertheless serve to measure the very low order of its quantity. Many snuff-boxfuls were thrown out of Krakatoa. Moreover, the dust-particles may be very sparsely scattered; the miles of air through which they are spread compensating for the wide space between them. The fineness of the solid dust is a legitimate result of what is now known of the constitution of lavas. Microscopic examination of igneous rocks has shown lithologists how well a volcanic explosion can produce diffracting dust; high magnifying power, applied to rocks that are presumably old buried lavas which failed to reach the surface, reveals the presence of the minutest cavities containing liquids or gases or both, so small and so closely packed that myriads would be contained in a cubic inch: under the decreasing pressures found as lavas rise through a vent to escape at the surface outlet, the occluded gases and vapors would escape, and in so doing would shatter the lavas to the finest imaginable dust. It is probably by this intimate process, as well as by ordinary forms of mechanical violence, that Krakatoa was, figuratively, blown to atoms. The greater and coarser part of the dust darkened the sky for a day or two and soon fell on the surrounding lands and seas; a finer remnant was carried high into the air by the outrushing gases, and then spread far and wide over the earth to produce the marvelous sunsets; does the finest residue still hang aloft and give us Bishop's ring? How can it be suspended so long?
Kiessling's experiments have led him to believe that the coronal diffraction does not take place immediately around the volcanic dust-particles, but rather around the minute globules of water or ice condensed on these particles as nuclei. Recent researches have shown that water-vapor may remain in the gaseous state below the temperature proper to its condensation, provided there is no solid or liquid matter present on which the condensation can begin; the change from the gaseous to the liquid state seems to desire the presence of some point of beginning, such as is furnished by ordinary dust, or by the far finer, ultra-microscopical particles always present in the air. It is in part for this reason that great cities in damp countries must be hopelessly foggy; however perfect the combustion in their numerous furnaces, unburned ash in very fine division must fly up the chimneys as long as wood and coal are used, and the finer the ash the better for the fog, when the coolness for condensation arrives. Now, in connection with this, there is a very peculiar point to be considered, concerning the distribution of water-vapor in the atmosphere. Water-vapor is a light, elastic, condensible gas, and its elastic lightness is always tending to throw it to an altitude where the cold of its expansion would require a part of it to condense into the liquid or solid state. It can, as yet, hardly be said that some minute point of beginning is absolutely necessary for all such condensation, but it may be safely asserted that the presence of dust aids and increases the rapidity of the process; and it is this office that the finest and highest of the Krakatoa dust is thought to have performed. And here a peculiar cycle of operations, first suggested by Wollaston years ago, and generally neglected since then, may be reconsidered. So long as the water-substance is in the vaporous condition, it acts as a gas, and tends to expand upward; part of it would thereby be condensed, generally in the solid state, and on losing the gaseous condition the frozen particles would at once tend to fall toward the earth, impeded only by the presence of the thin air; but, after a certain length of falling, they would reach air warm and dry enough to allow them to re-evaporate, whereupon their vapor would again expand upward, and the cycle of operations begins anew. Wollaston suggested that the gases of the air might be thus affected by the extreme cold of upper space, and that a limit of the atmosphere might so be determined. There is, however, no experimental evidence yet adduced to prove that oxygen and nitrogen would behave in such a way, and the limitation of the atmosphere must be due to other causes; but the upward extension of water-vapor might be thus controlled. May we not, therefore, imagine that the vapor of the upper atmosphere, re-enforced liberally by steam from Krakatoa and other volcanoes in eruption at the same time, found its opportunity for condensation much improved for several months by the lava-dust from the same sources; and thus explain the brilliant sunsets and the strength of color in Bishop's ring during the winter of 1883-'84? But gradually the dust settles down, very slowly on account of its large ratio of surface to weight; and the vapor also decreases by slow downward diffusion; then the brilliancy of the display is lost, and the moderate residual of vapor, condensing as well as it can alone, produces only a fainter-colored ring and sunset glows that are visible only under especially favorable circumstances.
Be all this as it may, it is well to bear in mind that some such explanation must be found and accepted, for the facts of diffracting particles and their relation to Krakatoa are too well proved to be doubted, unless evidence not yet forthcoming shall appear in great strength.
The ring is doomed to disappear, and hence deserves a close watching. For, as Forel has pointed out, the outburst of Krakatoa must have had its rivals in ancient if not in modern times, and rings like Bishop's must in all probability have resulted from former dusty explosions. But these had all faded long before Bishop's ring appeared, and we must, therefore, conclude that it will fade away also. It should be carefully watched, especially from high-level stations, and those who make a persevering record of it should not fail to inform Professor Kiessling, of Hamburg, about what they see.