Popular Science Monthly/Volume 30/November 1886/Recent Advances in Solar Astronomy
|RECENT ADVANCES IN SOLAR ASTRONOMY.|
WHILE during the past four years there has been no great or startling discovery in solar astronomy, there has been beyond question important progress at many points. Increased precision of numerical data has been attained, new methods of observation have been devised and put in practice, theories have been brought to trial with varying results of condemnation or approval, and mathematical and physical investigations have been initiated which give some promise of solving the mysterious problems of the sun's surface-drift, and the periodicity of the spots. We propose in this paper briefly to summarize these advances.
The transit of Venus on December 6, 1882, was widely, and, on the whole, successfully observed. The Americans alone used photography to any great extent, and at the nine different stations (four of them in the southern hemisphere) nearly fifteen hundred photographs were obtained, of which over a thousand are good for measurement. The German heliometer parties were also successful; and a great body of contact and micrometric observations and some photographs were obtained by French, English, and Belgian parties. The publication of the photographic and heliometric results is waited for with much interest, but, for some reason, has been greatly delayed. The general impression, however, is that the results will not prove as consistent and accurate as had been hoped, the probable error remaining still pretty large, and indicating that transits can not compete in accuracy with some of the other methods of determining the solar parallax.
Since 1882 the Washington experiments of Professor Newcomb upon the velocity of light have been completed and published, along with a new and independent determination by Michelson, at Cleveland. The anomalies in Newcomb's earlier observations were traced to their source and removed, and now the results of both observers stand in very close and gratifying accordance. Newcomb's is 299,860 kilometres, Michelson's 299,853.
To go with this in fixing the solar parallax, we have the new determination of the constant of aberration, by Nyrén, of Pulkova, based on all the Pulkova observations up to 1883. This value, 20"⋅492, combined with the above velocity of light, and with Clark's value for the earth's equatorial radius (6378⋅2 kilometres), gives for the solar parallax 8"⋅794—almost absolutely accordant with that deduced from the heliometer observations of Mars, in 1877. The observations of the eclipses of Jupiter's satellites, by Professor Pickering's photometric method, now in progress at Cambridge and Paris, will also give an extremely valuable result when the twelve-year cycle is completed. It has fixed the precise number of seconds required for light to traverse the mean distance between the earth and the sun.
The most remarkable result which has been arrived at, with reference to the solar radiation since 1882, is the fact, ascertained by Langley, that we do not receive from the sun any of the low-pitched, slowly pulsing rays, such as are emitted from surfaces at or below the temperature of boiling water. The solar spectrum appears to be cut off squarely at the lower end, and this cutting off we know can not have been effected in the earth's atmosphere, because we receive from the moon just the very kind of rays that are missing from the solar spectrum, and that in considerable quantity as compared with the rays of higher refrangibility. Langley finds these rays also abundant in the radiation from the electric arc, so that we can hardly suppose them originally absent from the solar energy. Unless there is some unsuspected error in the observations, it looks as if we must admit that they have been suppressed either in the atmosphere of the sun itself, or in interplanetary space.
Arrangements are now made by the English Solar-Physics Committee by which it is expected to secure at least one solar photograph, on a scale of eight inches to the sun's diameter, for every day of the year. These photographs are to be taken at Greenwich, at Dehra-Doon, in India; on the Island of Mauritius; and at some station in Australia: their comparison, measurement, and reduction are undertaken by the astronomer royal, at Greenwich. Much solar photographic work is also done at Potsdam and Meudon, but as yet nothing of the kind has been undertaken in the United States. Janssen has recently obtained some sun-spot photographs on a very large scale, but, so far as we know, they do not reveal anything new.
With the great twenty-three-inch telescope at Princeton, and on a few occasions, when the seeing has been fine enough to permit the use of powers of from six hundred and upward, the writer has found that, in many cases at least, the apparently club-like, almost bulbous, ends of the penumbral filaments are really fine, sharp-pointed hooks, reminding one of the curling tips of flames, or grass-blades bending over. Ordinarily they are seen as club-like, simply because of their brightness, and the irradiation and diffraction effects of moderate-sized object-glasses.
Some recent investigations upon the rotation of fluid masses, by Jukowsky, of Moscow, as applied to solar conditions by his colleague Belopolsky, seem to warrant a hope that the phenomena of surface-drift in longitude, and even the periodicity of the spots, may soon find a rational explanation as necessary results of the slow contraction of a non-homogeneous and mainly gaseous globe. The subject is difficult and obscure; but if it can be proved, as seems likely, that, on mechanical principles, the time of rotation of the central portions of such a whirling mass must be shorter than that of the exterior, then there will be, of necessity, an interchange of matter between the inside and outside of the sphere, a slow surface-drift from equator toward the poles, a more rapid internal current along and near the axis, from the poles toward the equator, a continual "boiling up" of internal matter on each side of the equator, and, finally, just such an eastward drift near the equator as is actually observed. Moreover, the form of the mass, and the intensity of the drift and consequent "boiling up" from underneath, might, and probably would, be subject to great periodical variations. Belopolsky's paper is given in the "Astronomische Nachrichten," No.2722, and there is an English notice of it in "Nature" for May 20, 1886.
This theory falls in well with the facts established by Spoerer respecting the motion of the sun-spot zones, and the general, though slow, poleward movement of sun-spots.
Per contra, we have to note that Mr.Lockyer, in his recent lectures on solar physics, reported in "Nature," appears to be ready to accept the old theory that the spots, and their accompanying rings of prominences, are "splashes," due to the fall of meteoric matter upon the sun. He maintains that the spots appear first, and after them the faculæ and prominences; unless the writer is much mistaken, however, the reverse occurs sometimes, and even frequently—first faculæ and then spots among the faculæ.
The question of sun-spots and the weather is still debated with about the same vigor as ever; but, on the whole, there seems to be no reason to modify the opinions expressed in the text. While it is not at all unlikely that careful and continued investigation will result in establishing some real influence of sun-spots upon terrestrial meteorology, it is now also practically certain that this influence, if it exists at all, is extremely insignificant, and so masked and veiled as to be very difficult to determine. There is no ground or reason for the current speculations of certain newspaper writers who ascribe almost every great storm in the eastern part of the United States to some sun-spot or other.
The strange connection between solar disturbances and magnetic disturbances on the earth has, however, become more certain, if possible, than ever before, and is no longer anywhere disputed. In November, 1882, there was a very remarkable instance of an intense magnetic storm and polar aurora, simultaneous over all the earth, and coincident with the sudden outbreak of an enormous group of sunspots.
Mr.Lockyer announces, as the result of a long series of observations upon sun-spot spectra, that there is a striking difference between the spot-spectra at the time of maximum and minimum sun-spot frequency; the lines that are most conspicuous by widening and darkening are by no means the same in the two cases. The most remarkable change is in the lines of iron, which are usually conspicuous, but almost vanish from the spot-spectrum at the sun-spot maximum.
The writer also has ascertained a curious and probably an important fact with reference to the structure of the spot-spectrum. Under extremely high dispersion it is found that the spectrum of the nucleus of a spot is not continuous, but is made up of countless fine, dark lines, for the most part touching or slightly overlapping, but leaving here and there unoccupied intervals which look like (and may be) bright lines. Each dark line is spindle-shaped—i.e., thicker in the middle where the spectrum is darkest and tapers to a fine, faint, hairlike mark at each end; most of them can be traced across the penumbra-spectrum, and even out upon the general surface of the sun. The average distance between the lines is about half that between the two components of b3, so that within the b group the total number of dark lines is some 300, and there are seven or eight of the bright lines. This structure is most easily seen in the part of the spectrum between E and F; above F the lines are crowded so closely that it is difficult to resolve them, and below E they appear to grow wider, more diffuse, and fainter. It seems to indicate that the principal absorption which darkens the center of a sun-spot is not such as would be caused by minute solid or liquid particles—by smoke or cloud—which would give a continuous spectrum; but it is a true gaseous absorption, producing a veritable dark-line spectrum, in which the lines are countless and contiguous.
Since the notes to the second edition were written, great advances have been made in the study and mapping of the spectrum. While the maps of Kirchhoff and Ångström will always remain standards from the historical point of view, they are by no means adequate to represent what is seen by our present instruments, and a number of new ones have been recently constructed which must entirely supersede them for all detailed work. The most important of these are the maps of Thollon and Rowland. The former, for which its author received the Lalande prize of the French Academy of Sciences last January (1886), was constructed from visual observations with a great spectroscope having a train of his powerful compound bisulphide of carbon prisms. This map covers the whole length of the visible spectrum, and embodies the results of some two years' continuous labor; it was presented (as a drawing) to the Academy last year, but its engraving and publication are not yet completed, so that it will not be accessible for some time to come.
Professor Rowland's map is photographic, and extends from wavelength 5790, half-way between D and E, through the whole upper portion of the spectrum, and far beyond the visual limits. Its scale is from three to four times as large as that of Ångström. Five of the seven sheets are already published and in the hands of subscribers. The original negatives were made by means of a four-by-six-inch concave diffraction-grating, having about 90,000 lines, and a focal length of about thirty feet.
Professor C.P.Smyth, of Edinburgh, has also published a map of the whole visible spectrum, made with a very large diffraction spectroscope, having four-inch collimator and telescope, and a three-and-a-half by five-inch flat grating by Rowland. This map is constructed on a scale, not of wave-lengths, as usual, but of wave-numbers—i.e., the scale expresses for any given ray the number of its waves in the length of one "British inch." The dispersion is about the same as in Rowland's map. Important and very useful maps, on a slightly smaller scale, were published a year or two earlier by Fievez, of Brussels, and Vogel, of Potsdam.
In 1883 Egoroff, a Russian physicist, succeeded in showing that the great A and B groups of the solar spectrum are due to the oxygen in our atmosphere.
Cornu, by a very ingenious arrangement, in which he makes a small image of the sun, four or five millimetres in diameter, oscillate across the slit of a powerful spectroscope three or four times a second, has succeeded in bringing out conspicuously, and at a glance, the difference between the true solar and the telluric lines in the spectrum. The solar lines oscillate slightly as the eastern and western, the advancing and receding, limbs of the sun come alternately to the slit, while the telluric lines stand fast.
Mr.Lockyer has called in question the existence of the so-called "reversing layer" of the chromosphere, being disposed to hold that certain of the lines which we identify as belonging to the spectrum of any given substance, say iron, are due to absorption in upper and cooler regions of the solar atmosphere, while others are produced low down. In support of this idea, he adduces the observation that, at the Egyptian eclipse of 1882, certain of the so-called "iron-lines," between b and F, were much longer, though no brighter, than other "iron-lines" close by them, and remained much longer visible as the moon advanced to cover the chromosphere. There is not room to discuss the matter here. Those who believe that the Fraunhofer lines are mainly produced by that portion of the solar atmosphere which bathes and sustains the photospheric clouds, or lies immediately above them, would not quarrel with the idea that the upper regions also co-operate to a certain extent; but we see no proof from observation, as yet, that lines which are produced by the absorption of the upper regions of the solar atmosphere are not also found in the lower. The question undoubtedly is interesting and important: Does each region of the solar atmosphere have its own spectrum, peculiar and distinct from those of other regions above and below; all of them co-operating by simple summation to form the spectrum as we see it—or, on the other hand, as has been usually admitted, does the spectrum of the lowest stratum contain everything, while the spectra of the higher regions differ from it merely by defect? Eclipse observations may possibly decide it. Of course, if Mr.Lockyer is right, the fact would be a very effective argument for the theory of "compound elements," which theory, notwithstanding the failure of its "basic-line" defense, seems to be, decidedly gaining ground in scientific opinion.
As to the bright-line spectrum of the chromosphere, no great discoveries have been made; a number of lines, probably fifteen or twenty, have been added by the writer to the two hundred and seventy-three long ago catalogued as constantly or occasionally appearing. Most of the new lines are in the violet and ultra-violet. Not one of them is below C.
Trouvelot has observed (or thinks he has) dark prominences—i.e., jets and clouds of hydrogen cooler than the luminous prominences, and so looking black when projected on a background of the hotter gas. Tacchini and Respighi have kept up a careful and systematic record of chromospheric phenomena in a statistical way.
During the past four years the most important investigations upon the solar radiation have been those of Langley. The results of the Mount Whitney Expedition of 1881, with those of certain supplementary investigations, were published in 1884. They fully confirm his earlier conclusion, that the previously received value of the solar constant should be largely increased, and from his data he fixes it at about thirty calories per square metre per minute instead of twenty-five. Of course, this involves a corresponding increase of twenty per cent in all the figures which are given to illustrate the immensity of the solar radiation in various ways.
It is worth noting also that Langley, following many French authorities, prefers to employ a smaller heat-unit than the calory used by the writer—the gramme-degree instead of the kilogramme-degree. His solar constant is the number of these small calories received per minute upon a square centimetre, and therefore on his scale of notation stands as three instead of thirty. We mention it, because this discordance in the definition of the calory has led to some confusion among those not entirely familiar with the subject. If we were to follow strictly the so-called c.g.s. system, the solar constant would be represented by a number still sixty times smaller—viz., 0⋅050 (gramme-degrees per centimetre per second).
Professor Langley has also, with the bolometer, carried the investigation of the invisible portion of the sun's heat-spectrum far beyond any of his predecessors, and has discovered and measured the wavelength of a large number of absorption-bands in this region; his results are confirmed by those of Becquerel and Abney, the latter operating by means of photography, and the former by means of the effect of the invisible infra-red radiations in quenching the phosphorescence of a suitably prepared screen.
Langley finds that the solar spectrum seems to terminate abruptly at a wave-length of about thirty thousand on Ångström's scale: he does not find in the sun's heat any of the long-waved, slowly-pulsating radiations, such as are emitted by bodies at or below the temperature of boiling water. We might think that they had been absorbed in the earth's atmosphere, were it not that he finds just these rays relatively abundant in the spectrum of lunar heat. He also finds them in the heat-spectrum of the electric arc, so that it is difficult to suppose that they do not originally exist in the solar spectrum. Unless there is some hidden fallacy or error in some of the observations, we are almost driven to admit that they have been absorbed in interplanetary space. But probably it will be best to await further confirmation of the experimental results before accepting so remarkable a conclusion.
At the eclipse of 1883, observed on Caroline Island, in the Pacific Ocean, by French and American parties, Professor Hastings made observations for the purpose of testing a theory he had framed that the outlying regions of the corona are merely a diffraction effect produced by the edge of the moon; the diffraction being not that due to the regular periodicity of light-vibrations, ordinarily discussed, but due to the probable continually occurring discontinuity or change of phase in the vibrations. It seems probable, from a not perfectly complete investigation, that such discontinuity might scatter light far beyond the limits of ordinary diffraction. He found during the eclipse, by an apparatus constructed expressly for the purpose, that the bright corona-line (1474 K) was always visible to a much greater distance from the sun on the side least deeply covered by the moon than on the other, as unquestionably ought to be the case if his theory were correct.
But the same thing would result from the diffusion of light by the air; and the French observers, and nearly all others who have discussed the matter, feel satisfied that this is the true explanation of what he saw. He himself now, we understand, thinks it not impossible that a thin cloud may have passed over the sun just at the critical moment, and so have vitiated his observation.
The discussion which has followed his publication seems to have only strengthened the older view, that the corona is a true solar appendage, an intensely luminous though inconceivably attenuated cloud of gas, fog, and dust, surrounding the sun, formed and shaped by solar forces.
The fact that comets, themselves mere airy nothings, have several times (the last instance was in 1882) passed absolutely through the corona without experiencing any sensible disturbance of path or structure, has, however, been always felt by many as an almost insuperable difficulty with this accepted theory, and more than anything else led Professor Hastings to propose his new hypothesis. But, on careful consideration, we shall find that our conceptions of the possible attenuation of shining matter near the sun will bear all the needed stretching without involving any absurdity. Recalling the phenomena of the electrical discharge in Crookes's tubes, it is clear that a "cloud," with perhaps only a single molecule to the cubic foot (but thousands of miles in thickness), would answer every luminous condition of the phenomena. And all the rifts and streamers, and all the peculiar structure and curved details of form, cry out against the diffraction hypothesis.
At present the most interesting debate upon the subject centers around the attempts of Dr.Huggins (first in 1883) to obtain photographs of the corona in full sunlight. He succeeded in getting a number of plates showing around the sun certain faint, elusive halo-forms which certainly look very coronal. Plans were made, and were carried out, in 1884, for using a similar apparatus upon the Riffelberg in Switzerland, and since then at the Cape of Good Hope. So far nothing has been obtained, however, much in advance of Dr.Huggins's own first results. But since September, 1883, until very recently, the air, as every one knows, has been full of a fine haze, probably composed in the main of dust and vapor from Krakatoa, which has greatly interfered with all such operations. It is now fast clearing away, and, if Dr.Huggins's views are correct, it is reasonable to expect that a much greater measure of success will be reached next winter at the Cape, and perhaps during the present summer in England and Switzerland.
About the same time that Dr.Huggins was photographing in England, Professor Wright, of New Haven, was experimenting on the same subject in a different way. He reflected the sun's rays into a darkened room by a heliostat, cut out all but the blue and violet rays by a suitable absorbing-cell, and then formed an image of the sun and its surroundings upon a sensitive fluorescent screen, stopping out the sun's disk itself. He obtained on the screen, on more than one occasion, what he then believed and still believes to be a true image of the corona. But the aërial haze soon intervened to put an end to all such operations; for of course it is evident that success, whether by photography or by fluorescence, is possible only under conditions of unexceptionable atmospheric purity.
Both Professor Wright and Dr.Huggins base their hopes upon the belief, which seems to be warranted by the spectrum-photographs obtained during the Egyptian eclipse of May, 1882, that the light of the corona and of the upper regions of the sun's "atmosphere" is peculiarly rich in violet and ultra-violet rays—that the corona is far more brilliant to the photographic plate and to the fluorescent screen than to the eye.
Probably it must be admitted that at present the predominant opinion among astronomers and photographers is against the practicability of reaching the corona without an eclipse, by any such methods; at the same time, to the writer at least, the case appears by no means hopeless, and success is certainly most devoutly to be desired.
P.S.—The reports from the recent eclipse of August 29th, observed by British and American parties on the Island of Grenada, in the Southern West Indies, have just come to hand, and are strongly unfavorable to the reality of the coronal appearances obtained by Huggins and Wright in their attempts to render the corona visible without an eclipse.
Plates furnished by Mr.Huggins, and precisely similar to those which he has employed in his photographic experiments, were exposed by Captain Darwin during the totality (as well as before and after it), in an apparatus like Mr.Huggins's, with a time of exposure the same that he has been using, and were treated and developed according to his directions. The plates exposed during the totality show no corona at all, the exposure proving insufficient to bring it out. Of course, this makes it extremely probable that what looks like the corona upon plates exposed to the uneclipsed sun is merely a fallacious ghost, due, as his opponents have always claimed, to something in his apparatus or process, or else to the scattered light in our atmosphere.
It is true, as Mr. Common points out, that the air was by no means satisfactorily clear during the eclipse, and the result, therefore, is not absolutely conclusive; but it must be conceded, and Mr. Huggins himself admits it, that the probability is now heavily against him.
Captain Darwin obtained good pictures of the corona with ordinary plates exposed for a longer time in the usual apparatus.
October 1, 1886.