Chapter I: Progress of Solar Physics

Solar Physics is the science of the sun as an individual body. It is not concerned with the sun as the ruler of the planetary system, or as a member of the sidereal system. The questions which it seeks to answer relate exclusively to the “thing in itself.” And these questions, through the effectiveness of modern methods, have become _answerable_. Few of them, it is true, have yet been _answered_, and all can never be set at rest, since each reply marks only the starting-point for a fresh interrogatory. This must be so; the prospect inevitably widens with the attainment of a higher point of view. Nor is it likely that the ascent will soon terminate. It is indeed towards a summit cloud-wrapt and self-withdrawn. The essential point, however, is that stagnation has given way to progress, surmise to inquiry, and barren wonder to stimulating curiosity.

Solar research made a threefold start about the middle of the last century. First came Schwabe’s discovery of a decennial sun-spot period, followed up by Sabine’s announcement of a coincident terrestrial-magnetic period. Then, in 1860, Kirchhoff published his momentous chemical interpretation of the Fraunhofer lines, showing the presence of familiar metals as glowing vapours in the sun’s atmosphere. Finally, on 18th July of the same year, the “red prominences” were photographically referred to their true location, and the whole marvellous eclipse-garniture was at once annexed to the domain of solar physics. The investigations corresponding to these three beginnings were pursued at very unequal rates of advance, and with considerable disparity of success. The chief triumphs were those of the prismatic method. From the spectroscope single-handed, the old order of ideas received its death-blow. Glaring incongruities notwithstanding, it might have survived a couple of decades longer had it not been for the reading of the strange Fraunhofer inscription. But the subversive effect of the attack delivered in 1860 was too palpable to be ignored. At last, unmistakably, the Herschelian theory of the sun was in ruins, and it only remained to clear away the rubbish of the structure preparatory to erecting a modern edifice on new foundations.

Its corner-stone was the principle of the conservation of energy. This obtained its first solar application from Helmholtz in 1853. His gravitational hypothesis explained the enormous outflow of heat from the focal hearth of the planetary household with a directness and simplicity that compelled conviction of its truth. Energy of position is, in this view, the store drawn upon by radiation; and it is a store so vast that millenniums of thermal expenditure will make no perceptible encroachment upon it. As the great globe cools, it contracts; and each one of its constituent particles falls, day by day, infinitesimally nearer to the centre, heat being thus mechanically evolved. Potential energy is in this way converted into actual energy, and we are warmed and lighted because the sun shrinks, and is raised by shrinkage to a surpassing pitch of incandescence. His constitution must then be such as to meet these requirements. For an ideal body, endowed at pleasure with fanciful properties, a machine has to be substituted, definitely adapted to the fulfilment of a recognised function. This change in the point of view is characteristic of astrophysical aims. It makes all the difference between antique and modern science. A great deal is involved in it. The demands of the novel situation are multitudinous, and can be met, not by speculative efforts, but only by toilsome experimental comparisons. These will need time and much patience, and can never be wholly satisfactory in view of the contrast between terrestrial and solar conditions. Yet the efforts towards their assimilation, unremittingly prompted by the new astronomy, lead to a continual growth of knowledge, and are unlikely to be relaxed until the torch has finally dropped from human hands.

The absence, then, of a satisfactory all-round theory of the sun need not be taken as an implication of failure. On the contrary, progress is necessarily attended by incompleteness. Facts, when research is most active and successful, accumulate too rapidly to be at once collocated. Finality means stagnation. Compare the map of the world drawn by Hecataeus with that by Herodotus. The earlier is by far the more finished production. Neat and trim, with its circumfluent Ocean Stream, it pictured the earth mainly from ideas of what it ought to be. The later chart, on the other hand, as the upshot of wider experience, admitted ignorance by abolishing limits and leaving room for the unknown. A true theory must always be somewhat expansible. It must be capable of accommodating new facts. Otherwise their intrusion will speedily rive it asunder.

Only the broad lines of solar theory can then at present be laid down. Details must be filled in gradually with the progress of research. The preliminary ideas, however, already acquired are unlikely to be subverted; we can represent to ourselves a sun which is a reality, and no figment of the brain.

Our luminary is neither solid nor liquid. It is mainly, perhaps entirely, gaseous; but its gaseity is of the “critical” kind, due to the combination of intense heat with enormous pressure. The thermal supplies needed to meet its vast emissive expenditure must be continually and rapidly brought from the central parts to the surface; and this can only be accomplished by the actual transport of the heated materials, conductive processes being much too slow to meet the exigencies of the situation. We thus recognise in the sun a globe riddled with convection-currents, of which the shining cloud-shell of the photosphere constitutes the limit. At the photospheric level the uprushing torrents deliver their cargo of radiative energy, and from the photospheric level the corresponding subsidence of cooled matter starts for the unimaginable furnace below. This course of exchange, however, must be greatly complicated by the rotation of the plastic mass in which it progresses. A true vertical circulation is rendered by it impossible; the ascending and descending currents must be variously and incalculably deflected. Incalculably, since the state of the sun’s interior lies, in some respects, beyond the range even of conjecture.

The very remarkable circumstance has been emphasised by recent inquiries that the photosphere fixes a boundary between two solar regions scarcely less strongly contrasted—to speak illustratively—than the terraqueous globe and its encompassing atmosphere. The sun has several distinct envelopes, but none, apparently, in the condition of atmospheric equilibrium. There is first a shallow, veil-like covering by which the disc is reddened and darkened. Next comes the “reversing layer,” a bed of mixed incandescent vapours, some hundreds of miles in thickness, the absorptive action of which mainly produces the dusky lines in the Fraunhofer spectrum. It is overlaid, to a depth of four or five thousand miles, by the chromosphere, a gaseous ocean incarnadined by the crimson blaze of hydrogen. The irregularities of its outline develop, locally and temporarily, into “prominences,” often of gigantic size, but belonging to the chromosphere as essentially as mounting waves and tossed spray do to the ocean. Finally, we reach the far-spreading corona, a mere lustrous phantom, approaching the absolute zero of density, yet of astounding decorative effect during total eclipses. Between the corona and the chromosphere there seems to be absolutely no material continuity, although structural relationships have been traced.

These appendages are distinguished by two peculiarities, rendered more obvious at each step forward in research. The first is that they contain an extremely small quantity of matter. The second, that the effect of the sun’s gravity upon them is, in some way, neutralised. We have only to consider that at an elevation of three and a half miles air is reduced to one-half its sea-level density, while the corresponding height at the surface of the sun—where gravity is twenty-eight times more powerful than it is on the earth—is but one-eighth of a mile. Into the compass of a shell just one furlong thick, accordingly, half the substance of the reversing layer, chromosphere, and corona should be compressed, if the sway of gravity over them were undisputed. Its comparative impotence is attested not only by their vast extent and excessively slow rate of luminous degradation, but still more emphatically by the almost total absence from them of spectroscopic symptoms of internal compression.

The various “claims” into which the wide field of solar physics has inevitably come to be divided are marked by curious differences of productiveness. Some are thickly sown with “pockets” of bright ore; others have hitherto yielded little beyond the “sparkle of golden splendour” on the surface. Thus the geometrical relations of sun-spots are not now more surely known than in the days of Derham and Cassini. A consensus of opinion that lasted a full century has given way to notorious disagreement. The elementary question as to whether spot-umbræ are elevations or excavations, is once more actively debated. True, the overthrow of an artificial unanimity often preludes a forward movement; yet it might have been expected that the immense mass of photographic records accumulated during thirty years would have amply sufficed to settle this matter once for all. It must indeed be admitted that direct sun-pictures, notwithstanding the exquisite perfection to which the art of taking them has been brought, and the striking nature of the details they often exhibit, have contributed only in a minor degree to the promotion of definite knowledge. Super-eminent among them are the long series due to M. Janssen’s skill; yet after twenty years the _réseau photosphérique_, a phenomenon of “blurring” manifested by their means, continues enigmatic as to its nature, and open to doubt even as to its solar origin.

The swiftest advances in solar physics have been along the various routes opened by light-analysis. Four of these are broadly separated by differences of aim and method. The inquiries they have made practicable relate to the chemistry of the sun itself, the daylight study of prominences and faculæ through a selected element of their emissions, to radial movements in the sun, and to spectroscopic disclosures during eclipses. The Fraunhofer spectrum has been studied year by year with minuter accuracy, and similar refinements in the treatment of the arc-spectra compared with it have assured real, and annulled fictitious correspondences, largely, as may readily be imagined, through photographic agency. Spectra, to be exactly collated, must be durably imprinted. The fine measurements now executed upon them would be an impossible task for the eye. Professor Rowland’s invention of concave gratings in 1883 has also contributed very notably to the development of solar chemistry. They simultaneously focus impinging rays and disperse by diffracting them. No lens needs to be interposed, and thus the disturbing effects of selective absorption and unequal deviation are avoided. Improvements in the technical processes of photography—the substitution of gelatine for collodion as the vehicle of the decomposable salt; the intensification of sensitiveness in plates; modifications in their colour-susceptibility—have been equally essential. Mechanical contrivance has not been behindhand. Without faultless screws, for instance, there could be no perfectly ruled gratings. The chemistry of the sun has indeed drawn upon many and unexpected resources for its promotion.

Spectroscopic work at the sun’s edge was carried on steadily for twenty-three years after its initiation in 1868. Its outcome was the collection of a mass of valuable information regarding the chromosphere and its jutting eminences. Their forms, movements, and duration were registered, the law of their distribution was ascertained, the mode of their conformity to the spot-cycle inferred. So rich a harvest was, in fact, gathered at once that the soil began to show signs of exhaustion; the prospect seemed dim of detecting any further essential novelties in this direction; the routine task of daily promenading the slit of the spectroscope round the limb lost its zest. Then in 1891 a novel commencement was made, and made in duplicate by Professor Hale at Chicago and M. Deslandres in Paris. They transferred the business from the eye to the sensitive plate, definitively and with splendid success. The photography of prominences, although tried on a correct principle by Professor Young in 1870, remained in an abortive experimental stage until recourse was had to the device of isolating the K-line of calcium, and depicting them in this single element of their light. It proved applicable to faculæ as well, and in one minute a complete picture of the disc and its appurtenances, as shown in the violet ray profusely emitted by them, can be secured with the spectroheliograph whenever the sun shines on either side of the Atlantic. The very name—“spectroheliograph”—of the instrument invented for the purpose comprises a history of changing methods—of the supersession by photography of eye-and-hand delineation, and of the replacement in turn of direct photographic portrayal by impressions of spectral images. And there has been a corresponding modification of ideas. New conceptions are gaining ground, not through the broaching of startling theories, but under the steady guidance of undeniable, and often surprising facts.

Doppler’s principle was applied by Sir Norman Lockyer about 1870 to _meteorological_ investigations (as they might be called) in the sun. They disclosed the not infrequent occurrence there of portentous cyclonic agitations. Distortions and displacements of the hydrogen lines attested the rushing of incandescent whirlwinds at speeds up to 250 miles a second. Much has yet to be learned regarding these extraordinary phenomena, their relationships having scarcely received the detailed and particular attention that they deserve. Professor Young’s use of the same method in 1876 to measure the solar rate of rotation served both as a test of its validity, which it established beyond cavil, and as a prelude to important refinements in the treatment of that intricate subject. Notwithstanding its anomalous retardation north and south from the equator, M. Dunér obtained in 1887–9 spectroscopic evidence of axial movement up to fifteen degrees from either pole, and thereby brought a widened range of its complexities under observational control. Line-displacements, too, similarly produced, have become the standard criterion for discriminating between the genuine solar, and the merely telluric constituents of the Fraunhofer spectrum; and it need scarcely be pointed out that to set them decisively apart is a pre-requisite to solar chemical progress.

Each favourable eclipse since 1842 has furnished to science its quota of new facts and inspiring suggestions. In 1851, the solar status of the “sierra” and “red protuberances,” demonstrated in 1860, was recognised by all except a few obstinate sceptics. In 1868, the hydrogen and helium spectrum of these wonderful objects came into view; in 1869, the green coronal ray was detected. Then on 22nd December 1870, Young’s tangential slit was momentarily lit up by the “flash spectrum” of the reversing layer, which, after twenty-six years, was photographically captured by Shackleton, and so became the subject of definite and critical investigation. This was greatly promoted by the multiplied records of it obtained along the line of totality which crossed India, 22nd January 1898. The cyclical variation of coronal types, indicated by the substitution of luminous “wings” for the more familiar “glory,” during the Rocky Mountains eclipse of 29th July 1878, was verified by a splendid series of coronal photographs taken in Egypt in 1882, at the Caroline Islands and Grenada in 1883 and 1886, in California and at Cayenne during the January and December totalities of 1889, at Novaya Zemlya in 1896, in the Deccan in 1898, in Sumatra and Mauritius in 1901. Sir William Huggins’s device for photographing the corona in daylight, invented under the stimulus of the Nile-eclipse disclosures, unfortunately remains in abeyance. Its realisation is a prime desideratum in solar physics.

The progress of science in this branch might with substantial accuracy be described in the condensed statement that a fabulous luminary has made way for a working machine—a machine, it is true, of infinite complexity, yet in touch with, although transcending, the common order of things. Just here reside the extreme interest and value of such inquiries. They deal with what is concrete; they can be pushed on by experiment, but by experiment always straining to widen its resources. Limits are accordingly pushed back little by little—limits of temperature, of rarefaction, of ethereal stress as manifested by electric and magnetic intensity. The end of the process is not within view. The way, arduous though it be, lies open, and is securely travelled by those who, relying on the unity and continuity of nature, confidently hope to attain by it to the knowledge of higher truths.