Popular Science Monthly/Volume 24/December 1883/A Belt of Sun-Spots
|A BELT OF SUN-SPOTS.|
EVERYBODY who watched the sun with a telescope last summer must have wondered at the great belt of spots lying across the southern part of the disk during the last half of July. Several of the spots and groups were of extraordinary size, and their arrangement was very singular. When the belt extended completely across the sun, there was visible at one time almost every characteristic form that sun-spots present. There was the yawning black chasm with sharply defined yet ragged edges, vast enough to swallow up the whole earth, with room to spare, and surrounded by a regular penumbral border as evenly shaded as an artist could have made it; there was the double or triple spot whose black centers, though widely separated from one another, were tangled, as it were, in one twisted and torn veil of penumbra, or connected by long, shadowy bands; there was the monstrous spot of grotesque form surrounded by a crowd of smaller spots of even more fantastic shape, and enveloped in a broad, irregular penumbra as bizarre and wonderful as the mighty sun-chasms inclosed in it; there was the great spot, often of singular outline, accompanied outside its shadowy borders by one or more swarms of minute black specks pitting the white photosphere in the most extraordinary fashion; there was the huge group, visible even to the unassisted eye, and consisting of half a dozen or more large spots intermingled with smaller ones whose number seemed to defy counting, and enveloped in a penumbral cloak of becoming amplitude; there, near the edges of the disk, were the crinkling lines and heaped-up masses of faculæ, the mountainous hydrogen-flames which marked the places where the intensest solar action was going on—in short, there was a panorama in which every variety of sun-spot seemed to be passing in a gigantic procession across the disk. And what a procession it was!—long enough, nearly, to reach from the earth to the moon and back again three times!
But the most extraordinary feature of this great solar display was the linear arrangement of the spots making a belt, or band, that half encircled the sun; there was also a noticeable regularity in the distances separating the groups composing this singular belt, and this peculiarity increased the likeness to a procession which must have impressed every observer who beheld the gradual march of the sun-spot army across the
solar disk. It was like watching a parade of masqueraders; each company of spots had its own characteristic and conspicuous make-up, and each kept its place in the line at a nearly invariable distance from the group in front of it and the one that followed.
The separate spots and groups did not, however, present an unvarying appearance. There was change as well as variety in this paralleled pageant on the sun. Changes were continually going on in the shape and even the size of the spots, and in the configuration of the different members of the groups—minor evolutions in the ever-advancing column. New spots of small size made their appearance in the neighborhood of larger ones; and in one instance, at least, a perfect swarm of little spots broke out near one of the largest components of the belt, as if the surface of the sun had been suddenly punctured by huge needles.
A very good idea of the appearance of the band of spots, and of their progressive motion from east to west with the revolution of the sun, as well as of the principal changes that took place in their form and arrangement, can be obtained from the series of sketches accompanying this article. The originals of these sketches I made at the time the spots were visible, and they represent with approximate accuracy the appearance of the spots with a magnifying power of sixty-five diameters. They do not, however, by any means show all the details visible with such a power. With higher magnifying powers the crowd of details in some of the larger groups was so great and confusing as to defy the power of the pencil to represent them. Some remarkable phenomena were also observed with the spectroscope during
this sun-spot display. When the huge group, seen near the left-hand edge of the sun in Fig. 2, was just coming around the edge, its approach was announced by an outburst of gas which M. Thollon observed as a small but extremely brilliant protuberance, that exhibited very marked displacement of the C-line toward the violet end of the spectrum. In a communication to the French Academy of Sciences, M. Thollon says that an hour before his observation on the C-line he had observed in the same region a slighter displacement not only of the lines of hydrogen and of the b-group but also of the coronal line 1,474. He observed on several days other remarkable spectroscopic phenomena, and noticed that nearly the whole southern half of the sun's disk gave manifest signs of violent agitation. In view of these facts, it seems surprising that little apparent effect was produced upon the earth by these solar outbursts. Two or three times in 1882 the earth responded instantly with magnetic storms and brilliant auroral displays to the solar activity, but this year the great sun-spots and their accompanying phenomena have shown comparatively little power to affect terrestrial magnetism.
Fig. 1 shows the sun as it appeared on the 16th of July, when the advancing procession of spots had reached two thirds of the way across the disk.
Fig. 2 represents the sun on the 20th of July, when the spot belt extended completely across the disk.
Fig. 3 shows the appearance of the sun on the 25th of July, when more than half of the procession had disappeared around the western edge, and the great group bringing up the rear was near the meridian.
In the latter part of August and early in September a row of spots, principally in the southern hemisphere, was again seen upon the sun, but it was shorter, more crooked, and composed of fewer spots and groups, than the great belt of July.
There is one point of view from which the sun-spot belt just scribed appears particularly interesting, and that relates to the supposed resemblance between the larger planets, and more especially Jupiter, and the sun. Everybody knows that Jupiter has a conspicuous dark-colored belt on each side of his equator, for those belts are one of the commonest objects of celestial sight-seeing. Saturn too has belts similarly situated, although they are less conspicuous than those of Jupiter. All the trustworthy evidence we have points to the conclusion that these huge planets are yet in a state which has more points of resemblance to the condition of a sun than to that of a cool and solid globe. There can be little doubt that Jupiter is surrounded by a cloud-laden atmosphere of great depth, and that his geological development, so to speak, is in a stage much earlier than any whose former existence is recorded in the present rock strata of the earth. In other words, Jupiter probably has not yet a continuous solid crust, even if the formation of such a crust has been begun. But, accepting the nebular hypothesis, we must conclude that Jupiter is gradually cooling and contracting, and that eventually he will have as solid a surface as the earth's. He seems, then, to be in a transition state between a luminous sun and an opaque world, and, if so, his present condition may throw light upon the future condition of the sun, just as the moon throws light upon the future condition of our own earth. For this reason it may be interesting for the reader to compare
with the figures representing the belt of sun-spots seen last summer a picture of Jupiter and his belts, shown in Fig. 4. It is, of course a long step from the string of separate spots in one case to the unbroken bands in the other, and yet it is easily seen that some resemblance exists, which becomes all the more striking if we believe that Jupiter was once a true sun, which has parted with most of its light and heat, and is approaching the condition of a crusted globe. It would only be necessary to increase the number of sun-spots in order to make a continuous belt around the sun, and, when one such belt was formed, it is likely that there would be another to match it on the other side of the equator, for, as is well known, the regions in which the greatest number of sun-spots appear lie on each side of the solar equator, and any general cause which increased the absolute number of sun-spots would proportionally increase the number seen in the two regions of their greatest frequency.
There are other points of resemblance between the sun and Jupiter which add strength to the suggestion that the sun may now be just entering upon a stage which is the precursor of the gradual loss of its light and heat, and of its approach to the present condition of Jupiter. Careful observation has shown that different portions of the sun rotate in different times, the equatorial region moving faster than any other part, and curiously enough the same peculiarity is seen in Jupiter. This fact came out very clearly through the study of the great red spot which made its appearance in the southern hemisphere of the planet in the summer of 1878, and which has only just now faded out of sight. It was found that the red spot lagged behind the equatorial spots, so that the latter made a complete circuit of the planet, with respect to the red spot, in about forty-four and a half days.
It must not be overlooked, however, that belts of sun-spots, no matter how numerous the spots composing them might be, would bear only a superficial resemblance to the belts of Jupiter, for the latter have a cloud-like appearance, while sun-spots are clearly huge chasms in the photosphere. In fact, a continuous band of sun-spots, as such, could not exist. But in view of the close resemblance between the situation of Jupiter's belts with respect to his equator, and that of the zones of sun-spots with respect to the sun's equator, it is easy to conceive that similar causes may be concerned in the production of both phenomena, the effects varying with the difference in condition of the two bodies. One of these causes, which would probably be operative in both cases, is the rotation of the body acting upon its fluid envelope. Even on the earth we have a zone of winds and violent revolving storms produced in the atmosphere on each side of the equator. On Jupiter, in corresponding latitudes, we see the great belts and spots, whose broken and ever-changing aspect indicates the action of tempestuous forces in the deep and dense atmosphere of that planet of a magnitude incomparably greater than anything of the kind upon the earth. On the sun, still in corresponding latitudes, we have the spot-zones wherein rage solar tornadoes and hurricanes, as far exceeding the storms upon Jupiter as the latter exceed those upon the earth. "We see, then, that in three members of the solar system—the Earth, Jupiter, and the Sun—representing stages of development separated by vast intervals of time, certain regions north and south of their equators are the scene of violent disturbances in their fluid shells or envelopes. But it will not do to liken these phenomena upon the three different globes too closely to one another, for they unquestionably differ not merely in magnitude but in kind and in mode of operation, and this is specially true as to the earth and the sun. We may speak of a sun-spot as a solar cyclone, but we must not forget that it is very different from our West Indian cyclones or East Indian typhoons. The point is that in each case—that of a solidified globe like the earth, surrounded by a comparatively rare atmosphere; that of a partially cooled globe, like Jupiter, enveloped in a dense atmosphere of great depth; and that of a completely gaseous globe like the sun, possessing a sort of shell of partly condensed gases—certain regions near the equator are those in which the greatest disturbance is visible, and in every case, probably, the force of rotation is a powerful factor in the production of these zones of commotion. This shows a sort of survival of the action of certain causes under changed conditions, as a globe proceeds in the process of cooling and condensation from the condition of a sun to that of an unsolidified planet, and so on to the condition of a crusted or solid earth. So, then, we may with some show of reason suggest that the half-belted appearance of the sun last summer was in a certain sense prophetic of its future condition, and that in time its spot-zones will be succeeded by continuous belts resembling those of Jupiter. But no human eye will ever behold the sun thus robbed of his majesty, with his glorious light extinguished by bands of gloomy vapors; for, long before he could reach such a condition, life would cease in the solar system, from want of his vivifying radiations.
The picture of Jupiter here given possesses some interest in itself, as it is a representation of the planet as it appeared in September, 1879, when the celebrated red spot was a very striking object. The spot is seen at the left hand edge of the disk, just above the great southern belt which is narrowed, or indented, in a very singular way, opposite the spot. The red spot is no longer visible, and as it was, perhaps, the most remarkable marking, except the belts themselves, ever seen upon Jupiter, pictures of it will possess great interest in the future.