Popular Science Monthly/Volume 7/July 1875/The Sun's Work

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THE SUN'S WORK.

THAT the Sun causes a saving of fire and candle was known to all antiquity from the day fire and candle were first invented; and that was nearly all they knew about him. Nothing more was known for ages. It was only yesterday that he set up the business of sketching portraits and no matter what. He did it so cheaply and so correctly as to rob poor miniature-painters of their bread; and then came another halt, though only a short one, in our knowledge of what the Sun can do. But now, the more we know about him, the more grounds do we find for surmising that he is a marvelous servant—perhaps master—of all work.

Among the cartes de visite with which the sun presents us, are now to be included his own, in various moods of temper and expression. Thanks to photography and spectral analysis, the solar phenomena are daily fixed on paper and submitted to the inspection of an inquiring public. They thus escape from the narrow and not very accessible domain of observatories, and enter the grand current of publicity. Both in America and in England, numerous specimens of astronomical photography are offered for sale. First as to merit stand the admirable photographs of the moon published by Mr. Lewis Rutherfurd; and those of the Sun's disk, which present the spots, the facules, and the brilliant marblings of his surface with as much clearness and as striking an effect as the very best telescope; and also those of the solar spectrum, whose stripes have been self-registered with a fidelity which leaves no room for cavil. The low price of the "Annuaire of the Bureau des Longitudes"—where M. Faye has published the essay from which this paper has derived its facts—does not allow it to give actual photographs; it is obliged to be content with carefully executed engravings from originals supplied by the Observatory of Wilna.

Cosmic meteorology, that is the meteorology of the universe considered as a whole, is a new and somewhat ambitious term which is striving to obtain a footing in science; and, as the Sun evidently plays one of the leading parts in it, we naturally ask what is his line of action and what he does. And our inquiries are advancing not only under the guidance of more extended views, but by the help of bold and patient research already resolutely carried out. Astronomers long concentrated (with great success) their talents within the limits of our solar system, ignoring the rest of the universe, as if it were necessarily excluded from the domain of positive knowledge. The first step taken out of those narrow bounds was Fourier's conception of the important influence of the temperature of interstellar space, due to the heat given out by the stars.

Fourier's notion has recently been amplified by the account which has been taken of the chemical radiations that reach us from the same source. On beholding tiny stars imprint their image on the photographer's sensitive plate in less time than the Moon herself, we cannot help believing that the actinic power of these myriads of distant strangers—outsiders to our system though they be—must exert some kind of influence upon our world.

Do not meteorites and shooting-stars put us in daily relation with cosmic materials, which reach us straight from the depths of space, or which have not formed part of our system for more than a few centuries? Moreover (and it is another view of the question), within our system itself we observe phenomena which we cannot attribute to the ordinary action of solar heat, and which, nevertheless, are evidently connected with the sun. From which we draw the obvious conclusion that the Sun has active powers of which we are ignorant; that the same is the case, perhaps, with the planets themselves; in short, that we have yet to investigate a multitude of relations between the earth and other heavenly bodies, of whatever kind. We naturally hope to prove the mutual influence of the stars, more particularly in respect to terrestrial magnetism and the electricity of our globe—as a striking instance of which may be cited M. Becquerel's happy hypothesis of the solar origin of atmospherical electricity. The only fear is, lest, once embarked in this line of speculation, we should be tempted to carry it too far.

There is less danger of error in studying certain actions of the sun which, though still mysterious, are constant and undeniable. By fixing our attention on the sun himself, the source of all energy here below, we run less risk of being led astray. For example: do we find, in known solar phenomena, any reason to believe that the Sun has other means of action besides his attraction, his heat, and his light? The answer, "Yes," is ready. The Sun possesses and exercises a plainly-marked repulsive power, of which meteorology has hitherto taken no account, although our atmosphere must experience its effects. Of the existence of this special force no doubt is entertained by astronomers who have studied the astounding configuration of comets. Olbers believed in a repulsive action; Bessel in a polar force; Bond, recently, on the occasion of Donati's comet, so deeply studied in America, deduced from it a simply repulsive force, and M. Roche, of Montpellier, adopted the same idea.

At first sight, it seems very strange to find the same body producing at once two opposite actions, an attraction and a repulsion. Nevertheless, if these two forces act according to different laws, they may coexist without being confounded in one single result, and may produce perfectly distinct effects. It is thus that the Newtonian attraction, which subsists between the molecules of every individual body, is by no means confounded or incorporated with the electrical or magnetic phenomena of which that same body may be the seat, or with the repulsive actions due to heat.

Now, although the most delicate observation of the celestial movements (planets and satellites) has hitherto revealed attraction alone, it is impossible at the present day to deny that the striking phenomena displayed by comets betray the existence of a quite different force, capable of driving to a distance, with incredible velocity, the most loosely attached and most attenuated particles of the matter composing them.

When a comet, arriving from the depths of the firmament, approaches the Sun, describing round him an immense ellipse almost parabolic in form, it appears to us as a spherical nebulosity more or less condensed toward its centre—that is, in the shape assumed by a body whose particles have freely taken their places under the sole influence of their mutual attractions. The sun's attraction (which at that great distance is virtually equal for all those particles), does no more than draw the comet toward it, as a whole, and in a lump as it were, without affecting its shape. But, when the distance diminishes, the parts of this sphere nearest the sun are drawn with greater force than the more distant parts diametrically opposite, and the primitive spherical figure can no longer subsist. The comet tends to grow longer and longer in the direction of the ideal line which connects it with the sun, absolutely in the same way as our globe, in its liquid portion, is drawn out into two opposite swellings familiarly known as ocean-tides. Nay, more: if the bond of mutual attraction which holds the particles of the comet together is not sufficiently powerful, it will give way; under the sun's attractive action, the comet will be decomposed, scattering its materials along its orbit, gradually transforming itself into a sort of very elongated ring of dust, like those which Schiaperelli's discovery shows to be the cause of shooting-stars when the earth happens to traverse them.

This is all that can result from the sun's attraction. But matters do not end here; and comets which have resisted for ages the destructive agency of attraction, now present quite different phenomena, which, consequently, can no longer be attributed to that force. We see them drawn out and lengthened in the direction of the abovementioned ideal line; but the matter which gushed forth (so to speak) on the sunshiny side is quickly arrested and driven back, while on the opposite side the matter rushes out, without meeting an obstacle, with an impetuosity of which no earthly phenomenon can give an idea. In a few days there is thus produced a tail diametrically opposite to the sun; and this tail may stretch before our eyes to a length of ten, twenty, thirty, or even sixty millions of leagues.

So great, in truth, is the complexity of cosmical phenomena, even when connected with unknown forces, that we might despair of success were we not offered a resource of which astronomical science has often availed itself—namely, the comparison of phenomena which occur at the same periodical intervals. Long before the discovery of universal gravitation, it was easy to see that the tides depended on the moon, since the periodicity is identical for the oscillations of the sea and the movements of our satellite. In like manner, the most complicated phenomena of meteorology, if they manifest a periodical character and their course agrees with the period of other strange phenomena, betray thereby their connection, in some way or other, with the latter.

Let us take, as an illustration, the variations of the dip of the magnetic needle. Every day, that needle, suspended freely, deviates in the morning from the position of equilibrium, and every day returns to it in the evening, after an excursion of variable extent. These regular movements evidently depend on the presence of the Sun above the horizon of any given spot; they also depend on its geographical situation, for they increase with the latitude on one hemisphere, and change their direction in passing from one hemisphere to the other. They are not due to a magnetic action proper to the sun; for, even supposing it to exist, the Sun, in consequence of his enormous distance, would be incapable of exercising a directing influence on a magnetic needle; but they result from some unknown action exerted on the electricity of the globe, and on the currents resulting from it—currents which themselves react on the direction of the needle, and often seriously disturb its movements.

How are we to give a precise account of this mode of action? In the midst of so many unknown details, how are we to lay hold of those which really require our attention? Observation, only, aided by this special form of empiricism—pointed out by M. Faye in his "Notice," and here attempted to be described—can help us in the matter. The diurnal variations of the needle have been noted for nearly a century past; it is remarked that they are not constant from one year to another; that they present maxima and minima, epochs of greatest and least activity; that these maxima occur every eleventh year. The phenomenon is periodical, and its period is eleven years.

Does there exist in the Sun (for it would evidently be useless to seek elsewhere) a phenomenon with a similar period? If such is the case, we shall perhaps have laid our hand on a simple relation of cause and effect. At all events, it will be a valuable hint and a sort of first indication of the track we ought to follow. Now, the spots on the Sun observe a precisely analogous period. Every eleven years they offer a strongly-marked maximum of frequency, followed, after an interval of several years, by a minimum, during which the Sun appears every day without a single accident or blemish on his brilliant surface. We are led, therefore, to investigate the case more closely.

More closely, in fact, we ought to look; for the coincidence may not be strictly exact. In that case, the present agreement of the two phenomena would be purely accidental; at the close of several periods it would disappear, and we should have been the dupes of a mere illusion. But M. Faye quotes a comparative table of the periods of the solar spots and of terrestrial magnetism, drawn up by M. Wolf, of Zurich, from which it appears that even the slight anomalies that occur, in respect to the average period of one of these phenomena, are faithfully reproduced by the other. This remarkable coincidence was almost simultaneously pointed out by General Sabine, Monsieur R. Wolf, of Zurich, and Monsieur Gautier, of Geneva.

Thus the spots on the Sun—those amplified whirlwinds which, by digging hollows in his surface here and there, introduce into his brilliant shell masses (more or less considerable) of the cooler hydrogen which envelops it—exercise on magnetism a daily action which is perfectly sensible to us. The problem of these mysterious variations, thus circumscribed, becomes henceforth more accessible.

True, the problem is not solved by that solitary circumstance. The advent of the spots determines two influences: first, they sensibly reduce the extent of the active surface of the Sun, and consequently of his radiations; secondly, they cause in the chromosphere, and far above it, gigantic hydrogenous eruptions, whose effects we are unable to appreciate. But that is precisely the point on which we have to concentrate our means of investigation; it is exactly there that Science may hope to seize the word of the enigma.

And, since there is an enigma, we are all the more strongly urged to solve it, because the daily variations of the magnetic needle are not alone in being affected by the variations of the photosphere. The same mode of reasoning connects them with the appearance of auroræ boreales. Here we are on familiar ground, discussing a phenomenon visible to and admired by all. Already Arago had remarked a sort of connection between the apparitions of the northern lights and disturbances of the magnetic needle. The concomitance was singular. And now we find that those auroræ present, exactly like the variations of the magnetic dip, a period agreeing with that of the sun-spots. A table drawn up by Mr. Loomis, without suspecting their remarkable coincidence, establishes it beyond a doubt.

Nor is this all. Once started on the road, empiric science follows the clew of its deductions. After the auroræ boreales come the cirri, the mare's-tail clouds, of a peculiar form, which float very high in the atmosphere, entirely formed of extremely minute spicules of ice. These have an intimate connection with auroræ boreales, and seem to be in some sort the atmospheric substratum or stage of all their manifestations. It is now endeavored to ascertain whether there does not also exist some relation between the frequency of those clouds and that of the solar spots. In short, there is now, in meteorology, an emulation of discoveries based on these analogies of periods or on the influence of the solar rotation. And we are bound to call attention to this novel tendency of astronomical research, which Donati, a few days before his death, characterized as the advent of a cosmic meteorology—that is, as already stated, of a meteorology in which account should be taken of the multiple reactions of the stars on each other, without limiting those reactions to the habitual forces of attraction and heat.

To this cosmic meteorology evidently belongs Fourier's notion of a combined action which the universe (leaving the Sun out of the question) exercises upon us by its calorific radiations. If the Sun were to go out, the temperature of the solar system would not sink indefinitely—or rather it would not fall to the absolute zero (273° Centigrade)—but would stop at a certain point, which Fourier estimated at 62° Centigrade below the freezing-point of water. The importance of this temperature must not be estimated by the abnormal figures which measure it: for it appears to be a condition of our very existence, by imposing a limit to the lowerings of temperature produced by the long nocturnal radiation of polar regions. It must be added that Messers. Huggins and Stone have recently justified this bold conception by measuring the heat radiation of several bright stars which they find superior to that of the Moon herself—who ought, one would at first sight believe, to reflect so fair a share of solar heat.

This, then, is the Sun's work. He controls the compass; he marshals the northern lights; he permits or forbids ice-crystal clouds to hover high in the atmosphere; besides performing other offices which we may not at present even suspect. And thus a deeper study of his nature and action tends to modify notably the face of Science, to enlarge our views, and to demonstrate more clearly by what multitudinous links terrestrial existences are connected with the entire universe.—All the Year Round.