Flowers of the Sky

by Richard A. Proctor

Again, it seems that such falls occur periodically, or rather that at regular intervals great meteoric streams pour upon the sun's surface. For instance, the periodic increase and decrease in the number of sun-spots is accompanied (so far as we can judge by the observations made at Edinburgh and Greenwich) by an accession and diminution of the solar heat; and if the change is attributed to the passage of a meteoric stream athwart the sun, we should have to assign to such a stream a period of rather more than eleven years. This, from what we know about the association between meteors and comets, would correspond simply to the existence of a comet whose path intersects the sun's globe, and which is followed by a train of millions of large meteoric masses, many of which are consumed at each passage of the rich portion of the train athwart the globe of the sun. This comet must of necessity be inconspicuous, since it has hitherto escaped detection. In fact, its head and nucleus must long since have been entirely destroyed. Only the meteoric train, far more widely scattered, remains, simply because at each passage past the sun, though many are captured, far greater numbers get safely past.

I am careful to remind the reader that though I have, for convenience, used the indicative mood in describing these matters, I am in reality presenting merely a theory. It may be that the solar spots and the accessions of heat are produced in some other way. But I must admit I find strong reasons for regarding as probable the general theory, that the alternations of solar activity (not the solar activity itself be it noted) are excited from without. And since we know, as a matter of fact, that meteors exist in enormous numbers within the solar system, and that they aggregate with rapidly increasing density in the sun's neighbourhood, we must believe that they fall upon the sun in enormous numbers. We also perceive that the supply cannot be uniform, but must vary greatly from time to time; while what we know about the periodicity of meteoric showers on our own earth suggests the belief, we may almost say the certainty, that there must be periodic downfalls of very heavy meteoric showers upon the sun's surface. We have, then, strong probability in favour of the belief that events may occur which, if they occurred, might be expected, with a high degree of probability, to produce effects resembling those actually observed,—viz., the production of a heat more intense than usual, accompanied by signs of great disturbance like the sun-spots. It does, therefore, seem at least not improbable that these accessions of heat and these signs of great disturbance really are brought about in the way supposed.

A further argument in favour of the meteoric origin of solar alternations of heat is to be found in the fact that, on one occasion at least, a solar phenomenon, corresponding precisely to what we should expect to see, if great meteoric masses fell upon the sun, has been followed by precisely the same signs of terrestrial disturbance which accompany and follow the formation of great solar spots. I refer to the remarkable occurrence witnessed by Carrington and Hodgson (at different observatories) in September, 1859, when two intensely bright points of light were seen travelling beside each other at the rate of about 120 miles per second along a short arc of the sun's surface,—an arc only equal in length to some four-and-a-half times the diameter of our earth.

On that occasion the emission of solar heat may or may not have been increased in an appreciable degree for several minutes. My own belief is that it must have been; but we certainly have no means of proving that it was. What we do know certainly is, that on that day all the phenomena which usually accompany the existence of many and large sun-spots showed themselves with exaggerated intensity. The magnetic needle was greatly disturbed, auroras displayed their coloured streamers in both hemispheres, telegraphic communication was interrupted, and everything tended to show that a disturbance of the same general character as that which produces sun-spots, but much more active while it lasted, had affected the sun. It seems, then, altogether reasonable to infer that sun-spots are due to the same cause as the disturbance which then occurred. So that if we conclude, with most astronomers competent to form an opinion, that the disturbance witnessed by Carrington and Hodgson was due to the downfall of two very large meteoric masses upon the sun, it would follow that sun-spots are due to more wide-spread meteoric showers, not consisting of masses so large.

The reader will long since have guessed, no doubt, to what all this tends. If the periodical variations of the sun's surface are due to meteoric and cometic systems whose orbits intersect the sun's globe, their periods being short (that is, lasting but a few years), it may well be that more important meteoric and cometic systems intersecting the sun's globe exist, which have much longer periods. When next one of these makes its passage athwart the sun, far more important solar disturbances may take place than those which occur when the regularly recurring systems salute the sun. Two or three times in the history of science comets have approached very close to the surface of the sun, as in 1680, and again in 1843, but without actually impinging upon it. Very slight changes in the motions of those comets, owing to the disturbing influences of the planets, would cause their very nuclei to strike the sun, and their meteoric trains to pour afterwards in a full stream upon him for many days, or even for many months and years in succession.

Now I do not think our sun would necessarily suffer very much from any of these known comets. They may long since have parted with the greater quantity of their substance. But it is quite possible that even one of those well-known comets of the solar system might cause very serious outbursts of solar heat and light; and it is certainly not only possible but extremely probable that other comets, such as have visited the solar system on paths fortunately not bringing them near to the sun, would have worked much mischief had their paths been differently situated.

We know that Newton held this opinion. He considered the real danger from comets to reside, not in the possibility that one might strike our earth, but in the possibility that one, falling upon the sun, might excite that orb to a degree of heat so intense that all life on this earth would be destroyed. It is true that, in Newton's time, physical laws were not so well understood as at present, and a considerable portion of Newton's reasoning was consequently inexact. But nothing which is now known opposes itself to the belief which Newton adopted on this subject. On the contrary, whereas Newton only recognised the danger arising from the consumption of a comet as fuel for the sun, we now recognise a far more serious danger, from the force of meteoric impact, and the heat excited as the thermal equivalent of the destroyed velocities. Of this part of the danger Newton had no clear conception, the relations between mechanical energy and heat not having been established until quite recent times.

It appears to me, however, that the danger in the case of our own sun—or may we not say our danger?—arises only from the possibility that some one of the comets which visit us from the star-depths may make straight for the sun; and this danger is exceedingly small. Almost certainly a comet which, leaving the domain of another sun, falls under the attractive influence of our own, would approach him on a path passing many millions of miles from his surface. The chances against a more direct approach are so great that they may be regarded as, to all intents and purposes, overwhelming. A comet might visit us from the star-depth on a destructive course, just as a single black ball might be drawn at the first trial from a bag containing a million white balls and only that single black one. But the danger is exceedingly small.

We see, indeed, that other suns have suffered in this way, assuming cometic downfall to be the true cause of stellar outbursts. There are so many millions of suns, however, in the region of space to which telescopic survey extends that the occurrence of ten or twelve such outbursts in the course of four or five centuries need not be regarded as implying any serious danger. Moreover, all the suns which have thus suffered lie within a particular region of the heavens,—viz., in the Milky Way, and in that half of the Milky Way which is most irregular, one may almost say ragged, in structure. (With one exception—the star in the Northern Crown, which, nevertheless, lies on a faint outlying streamer of the Milky Way not discernible to ordinary vision.) If then our sun belongs to this region of space, the danger for him and for us is somewhat greater than my previous argument would indicate. For, in that case, we must compare the number of outbursts, not with the total number of stars within telescopic range, but with the number of those stars which lie within this particular region of space. On the other hand, if our sun does not lie within that region of space, the danger for him and for us is very much less; for instead of a certain small number of accidents among his fellow suns, there have been no such accidents, only accidents affecting other suns which must be differently classed.

The case may be compared to the estimation of the dangers, let us say, of travelling by ocean steamships on a particular route. If we take the total number of accidents, for instance, to steamships travelling between England and the United States, we should estimate the risk of the journey as very small, the number of passengers who have lost their lives being very small compared with the number who have made the journey. But even this small risk is diminished if we estimate the danger for a passenger by Cunard steamships, simply because no passenger has yet lost his life through accident to one of these Cunard vessels.