Astronomy: The Science of the Heavenly Bodies

by David P. Todd

The true corona is at least a triple phenomenon and a very complex one. The photographs reveal it much as the eye sees it, with all its complexity of interlacing streamers projected into a flat, or plane, surrounding the disk of the dark moon which hides the true sun completely. But we must keep in mind the fact that the sun is a globe, not a disk, and that the streamers of the corona radiate more or less from all parts of the surface of the solar sphere, much as quills from a porcupine.

From the sun's magnetic poles branch out the polar rays, nearly straight throughout their visible extent. Gradually as the coronal rays originate at points around the solar disk farther and farther removed from the poles, they are more and more curved. Very probably they extend into the equatorial regions, but it is not easy to trace them there because they are projected upon and confused with the filaments having their origin remote from the poles. Then there is the inner equatorial corona, apparently connected intimately with truly solar phenomena, quite as the polar rays are. The third element in the composite is the outer ecliptic corona, for the most part made up of long streamers. This is most fully developed at the time of the fewest spots on the sun. It is traceable much farther against the black sky with the naked eye than by photography. Without any doubt it is a solar appendage and possibly it may merge into the zodiacal light.

Naturally this superb spectacle must have been an amazing sight to the beholders of antiquity who were fortunate enough to see it. Historical references are rare: perhaps the earliest was by Plutarch about A. D. 100, who wrote of it, "A radiance shone round the rim, and would not suffer darkness to become deep and intense." Philostratus a century later mentions the death of the emperor Domitian at Ephesus as "announced" by a total eclipse.

Kepler thought the corona was evidence of a lunar atmosphere; indeed, it was not until the middle of the 19th century that its lack of relation to the moon was finally demonstrated. Later observers, Wyberd in 1652 and Ulloa, got the impression that the corona turned round the disk catherine-wheel fashion, "like an ignited wheel in fireworks, turning on its center." But no later observer has reported anything of the sort. Quite the contrary, there it stands against the black sky in motionless magnificence a colorless pearly mass of wisps and streamers for the most part nebulous and ill-defined, fading out very irregularly into the black sky beyond, but with a complex interlacing of filaments, sometimes very sharply defined near the solar poles. It defies the skill of artist and draughtsman to sketch it before it is gone.

Photograph it? Yes, but there are troubles. Of course the camera work is superior to sketches by hand. As Langley used to say, "The camera has no nerves, and what it sets down we may rely on." Foremost among the photographic difficulties is the wide variation in intensity of the coronal light in different regions of the corona. If a plate is exposed long enough to get the outer corona, the exceeding brightness of the inner corona overexposes and burns out that part of the plate or film. If the exposure is short, we get certain regions of the inner corona excellently, but the outer regions are a blank because they can be caught only by a long exposure.

So the only way is to take a series of pictures with a wide range of exposures, and then by careful and artistic handwork, combine them all into a single drawing. Wesley of London has succeeded eminently in work of this character, and his drawings of the sun's corona, visible at total eclipses from 1871 onward, in possession of the Royal Astronomical Society, are the finest in existence. They give a vastly better idea of the corona, as the eye sees it, than any single photograph possibly can.

The early observers apparently never thought of the corona as being connected with the sun. It was a halo merely, and so drawn. Its real structure was neither known, depicted, or investigated. Sketches were structureless, as any aureola formed by stray sunlight grazing the moon might naturally be. That the rays are curved and far from radial round the sun was shown for the first time in the sketches of 1842, and in 1860 Sir Francis Galton observed that the long arms or streamers "do not radiate strictly from the center."

The inner corona had first been recorded photographically on a daguerreotype plate during the eclipse of 1851, but the lens belonged to a heliometer, and was of course uncorrected for the photographic rays. The wet collodion plates of the eclipse of 1860, by De la Rue, proved that not only the prominences but the corona were truly solar, because his series of technically perfect pictures revealed the steady and unchanged character of these phenomena while the moon's disk was passing over them as totality progressed. And at the eclipse of 1869, Young put the solar theory of the corona beyond the shadow of any further doubt by examination of its light with the spectroscope and discovering a green line in the spectrum due to incandescent vapor of a substance not then identified with anything terrestrial, and therefore called coronium.

The total brilliance of the corona was very differently estimated by the earlier observers, though pretty carefully measured at later eclipses. The standard full moon is used for reference, and at one eclipse the corona falls short of, while at another it will exceed the full moon in brightness. Variations in brilliancy are quite marked: at one eclipse it was nearly four times as bright as the full moon. Much evidence has already accumulated on this question; but whether the observed variations are real, or due mainly to the varying relative sizes of sun and moon at different eclipses, is not yet known. The coronal light is largely bluish in tint, and this is the region of the spectrum most powerfully absorbed by our atmosphere. Eclipses are observed by different expeditions located at stations where the eclipsed sun stands at very different altitudes above the horizon; besides this the localities of observation are at varied elevations above sea level; so that the varying amount of absorption of the coronal light renders the problem one of much difficulty.

The long ecliptic streamers of the corona were first seen by Newcomb and Langley during the totality of 1878. On one side of the sun there was a stupendous extension of at least twelve solar diameters, or nearly 11 millions of miles. Langley observed from the summit of Pike's Peak, over 14,000 feet high, and was sure that he was witnessing a "real phenomenon heretofore undescribed." The vast advantage of elevation was apparent also from the fact that he held the corona for more than four minutes after true totality had ended. These streamers are characteristic of the epoch of minimum spots on the sun, as Ranyard first suggested. It was found that this type of corona had been recorded also in 1867; and it has reappeared in 1889, 1900 and 1911, and will doubtless be visible again in 1922.