Curiosities of the Sky Part 3

The Banners of the Sun

As all the world knows, the sun, a blinding globe pouring forth an inconceivable quant.i.ty of light and heat, whose daily pa.s.sage through the sky is caused by the earth's rotation on its axis, const.i.tutes the most important phenomenon of terrestial existence. Viewed with a dark gla.s.s to take off the glare, or with a telescope, its rim is seen to be a sharp and smooth circle, and nothing but dark sky is visible around it. Except for the interference of the moon, we should probably never have known that there is any more of the sun than our eyes ordinarily see.

But when an eclipse of the sun occurs, caused by the interposition of the opaque globe of the moon, we see its immediate surroundings, which in some respects are more wonderful than the glowing central orb.

These surroundings, although not in the sense in which we apply the term to the gaseous envelope of the earth, may be called the sun's atmosphere. They consist of two very different parts -- first, the red ''prominences,'' which resemble tongues of flame ascending thousands of miles above the sun's surface; and, second, the ''corona,'' which extends to distances of millions of miles from the sun, and shines with a soft, glowing light. The two combined, when well seen, make a spectacle without parallel among the marvels of the sky. Although many attempts have been made to render the corona visible when there is no eclipse, all have failed, and it is to the moon alone that we owe its revelation. To cover the sun's disk with a circular screen will not answer the purpose because of the illumination of the air all about the observer. When the moon hides the sun, on the other hand, the sunlight is withdrawn from a great cylinder of air extending to the top of the atmosphere and spreading many miles around the observer.

There is then no glare to interfere with the spectacle, and the corona appears in all its surprising beauty. The prominences, however, although they were discovered during an eclipse, can now, with the aid of the spectroscope, be seen at any time. But the prominences are rarely large enough to be noticed by the naked eye, while the streamers of the corona, stretching far away in s.p.a.ce, like ghostly banners blown out from the black circle of the obscuring moon, attract every eye, and to this weird apparition much of the fear inspired by eclipses has been due. But if the corona has been a cause of terror in the past it has become a source of growing knowledge in our time.

The story of the first scientific observation of the corona and the prominences is thrillingly interesting, and in fact dramatic. The observation was made during the eclipse of 1842, which fortunately was visible all over Central and Southern Europe so that scores of astronomers saw it. The interest centers in what happened at Pavia in Northern Italy, where the English astronomer Francis Baily had set up his telescope. The eclipse had begun and Bailey was busy at his telescope when, to quote his own words in the account which he wrote for the Memoirs of the Royal Astronomical Society:

I was astounded by a tremendous burst of applause from the streets below, and at the same moment was electrified by the sight of one of the most brilliant and splendid phenomena that can well be imagined; for at that instant the dark body of the moon was suddenly surrounded with a corona, or kind of bright glory, similar in shape and magnitude to that which painters draw round the heads of saints...

Pavia contains many thousand inhabitants, the major part of whom were at this early hour walking about the streets and squares or looking out of windows in order to witness this long-talked-of phenomenon; and when the total obscuration took place, which was instantaneous, there was a universal shout from every observer which ''made the welkin ring,'' and for the moment withdrew my attention from the object with which I was immediately occupied. I had, indeed, expected the appearance of a luminous circle round the moon during the time of total obscurity; but I did not expect, from any of the accounts of preceding eclipses that I had read, to witness so magnificent an exhibition as that which took place...

Splendid and astonishing, however, as this remarkable phenomenon really was, and although it could not fail to call forth the admiration and applause of every beholder, yet I must confess that there was at the same time something in its singular and wonderful appearance that was appalling...

But the most remarkable circ.u.mstance attending the phenomenon was the appearance of three large protuberances apparently emanating from the circ.u.mference of the moon, but evidently forming a portion of the corona. They had the appearance of mountains of a prodigious elevation; their color was red tinged with lilac or purple; perhaps the color of the peach-blossom would more nearly represent it. They somewhat resembled the tops of the snowy Alpine mountains when colored by the rising or the setting sun. They resembled the Alpine mountains in another respect, inasmuch as their light was perfectly steady, and had none of that flickering or sparkling motion so visible in other parts of the corona...

The whole of these protuberances were visible even to the last moment of total obscuration, and when the first ray of light was admitted from the sun they vanished, with the corona, altogether, and daylight was instantly restored.

I have quoted nearly all of this remarkable description not alone for its intrinsic interest, but because it is the best depiction that can be found of the general phenomena of a total solar eclipse. Still, not every such eclipse offers an equally magnificent spectacle. The eclipses of 1900 and 1905, for instance, which were seen by the writer, the first in South Carolina and the second in Spain, fell far short of that described by Bailey in splendor and impressiveness. Of course, something must be allowed for the effect of surprise; Bailey had not expected to see what was so suddenly disclosed to him. But both in 1900 and 1905 the amount of scattered light in the sky was sufficient in itself to make the corona appear faint, and there were no very conspicuous prominences visible. Yet on both occasions there was manifest among the spectators that mingling of admiration and awe of which Bailey speaks. The South Carolinians gave a cheer and the ladies waved their handkerchiefs when the corona, ineffably delicate of form and texture, melted into sight and then in two minutes melted away again. The Spaniards, crowded on the citadel hill of Burgos, with their king and his royal retinue in their midst, broke out with a great clapping of hands as the awaited spectacle unfolded itself in the sky; and on both occasions, before the applause began, after an awed silence a low murmur ran through the crowds. At Burgos it is said many made the sign of the cross.

It was not long before Bailey's idea that the prominences were a part of the corona was abandoned, and it was perceived that the two phenomena were to a great extent independent. At the eclipse of 1868, which the astronomers, aroused by the wonderful scene of 1842, and eager to test the powers of the newly invented spectroscope, flocked to India to witness, Janssen conceived the idea of employing the spectroscope to render the prominences visible when there was no eclipse. He succeeded the very next day, and these phenomena have been studied in that way ever since.

There are recognized two kinds of prominences -- the ''erruptive'' and the ''quiescent.'' The latter, which are cloud-like in form, may be seen almost anywhere along the edge of the sun; but the former, which often shoot up as if hurled from mighty volcanoes, appear to be a.s.sociated with sun-spots, and appear only above the zones where spots abound. Either of them, when seen in projection against the brilliant solar disk, appears white, not red, as against a background of sky.

The quiescent prominences, whose elevation is often from forty thousand to sixty thousand miles, consist, as the spectroscope shows, mainly of hydrogen and helium. The latter, it will be remembered, is an element which was known to be in the sun many years before the discovery that it also exists in small quant.i.ties on the earth. A fact which may have a significance which we cannot at present see is that the emanation from radium gradually and spontaneously changes into helium, an alchemistical feat of nature that has opened many curious vistas to speculative thinkers. The eruptive prominences, which do not spread horizontally like the others, but ascend with marvelous velocity to elevations of half a million miles or more, are apparently composed largely of metallic vapors -- i.e. metals which are usually solid on the earth, but which at solar temperatures are kept in a volatilized state. The velocity of their ascent occasionally amounts to three hundred or four hundred miles per second. It is known from mathematical considerations that the gravitation of the sun would not be able to bring back any body that started from its surface with a velocity exceeding three hundred and eighty-three miles per second; so it is evident that some of the matter hurled forth in eruptive prominences may escape from solar control and go speeding out into s.p.a.ce, cooling and condensing into solid ma.s.ses. There seems to be no reason why some of the projectiles from the sun might not reach the planets. Here, then, we have on a relatively small scale, explosions recalling those which it has been imagined may be the originating cause of some of the sudden phenomena of the stellar heavens.

Of the sun-spots it is not our intention here specifically to speak, but they evidently have an intimate connection with eruptive prominences, as well as some relation, not yet fully understood, with the corona. Of the real cause of sun-spots we know virtually nothing, but recent studies by Professor Hale and others have revealed a strange state of things in the clouds of metallic vapors floating above them and their surroundings. Evidences of a cyclonic tendency have been found, and Professor Hale has proved that sun-spots are strong magnetic fields, and consist of columns of ionized vapors rotating in opposite directions in the two hemispheres. A fact which may have the greatest significance is that t.i.tanium and vanadium have been found both in sun-spots and in the remarkable variable Mira Ceti, a star which every eleven months, or thereabout, flames up with great brilliancy and then sinks back to invisibility with the naked eye. It has been suggested that sun-spots are indications of the beginning of a process in the sun which will be intensified until it falls into the state of such a star as Mira. Stars very far advanced in evolution, without showing variability, also exhibit similar spectra; so that there is much reason for regarding sunspots as emblems of advancing age.

The a.s.sociation of the corona with sun-spots is less evident than that of the eruptive prominences; still such an a.s.sociation exists, for the form and extent of the corona vary with the sun-spot period of which we shall presently speak. The const.i.tution of the corona remains to be discovered. It is evidently in part gaseous, but it also probably contains matter in the form of dust and small meteors. It includes one substance altogether mysterious -- ''coronium.'' There are reasons for thinking that this may be the lightest of all the elements, and Professor Young, its discoverer, said that it was ''absolutely unique in nature; utterly distinct from any other known form of matter, terrestial, solar, or cosmical.'' The enormous extent of the corona is one of its riddles. Since the development of the curious subject of the ''pressure of light'' it has been proposed to account for the sustentation of the corona by supposing that it is borne upon the billows of light continually poured out from the sun. Experiment has proved, what mathematical considerations had previously pointed out as probable, that the waves of light exert a pressure or driving force, which becomes evident in its effects if the body acted upon is sufficiently small. In that case the light pressure will prevail over the attraction of gravitation, and propel the attenuated matter away from the sun in the teeth of its attraction. The earth itself would be driven away if, instead of consisting of a solid globe of immense aggregate ma.s.s, it were a cloud of microscopic particles. The reason is that the pressure varies in proportion to the surface of the body acted upon, while the gravitational attraction is proportional to the volume, or the total amount of matter in the body. But the surface of any body depends upon the square of its diameter, while the volume depends upon the cube of the diameter. If, for instance, the diameter is represented by 4, the surface will be proportional to 4 4, or 16, and the volume to 4 4 4, or 64; but if the diameter is taken as 2, the surface will be 2 2, or 4, and the volume 2 2 2, or 8. Now, the ratio of 4 to 8 is twice as great as that of 16 to 64. If the diameter is still further decreased, the ratio of the surface to the volume will proportionally grow larger; in other words, the pressure will gain upon the attraction, and whatever their original ratio may have been, a time will come, if the diminution of size continues, when the pressure will become more effective than the attraction, and the body will be driven away. Supposing the particles of the corona to be below the critical size for the attraction of a ma.s.s like that of the sun to control them, they would be driven off into the surrounding s.p.a.ce and appear around the sun like the clouds of dust around a mill.

We shall return to this subject in connection with the Zodiacal Light, the Aurora, and Comets.

On the other hand, there are parts of the corona which suggest by their forms the play of electric or magnetic forces. This is beautifully shown in some of the photographs that have been made of the corona during recent eclipses. Take, for instance, that of the eclipse of 1900. The sheaves of light emanating from the poles look precisely like the ''lines of force'' surrounding the poles of a magnet. It will be noticed in this photograph that the corona appears to consist of two portions: one comprising the polar rays just spoken of, and the other consisting of the broader, longer, and less-defined ma.s.ses of light extending out from the equatorial and middle-lat.i.tude zones. Yet even in this more diffuse part of the phenomenon one can detect the presence of submerged curves bearing more or less resemblance to those about the poles. Just what part electricity or electro-magnetism plays in the mechanism of the solar radiation it is impossible to say, but on the a.s.sumption that it is a very important part is based the hypothesis that there exists a direct solar influence not only upon the magnetism, but upon the weather of the earth. This hypothesis has been under discussion for half a century, and still we do not know just how much truth it represents. It is certain that the outbreak of great disturbances on the sun, accompanied by the formation of sun-spots and the upshooting of eruptive prominences (phenomena which we should naturally expect to be attended by action), have been instantly followed by corresponding ''magnetic storms'' on the earth and brilliant displays of the auroral lights. There have been occasions when the influence has manifested itself in the most startling ways, a great solar outburst being followed by a mysterious gripping of the cable and telegraph systems of the world, as if an invisible and irresistible hand had seized them. Messages are abruptly cut off, sparks leap from the telegraph instruments, and the entire earth seems to have been thrown into a magnetic flurry. These occurrences affect the mind with a deep impression of the dependence of our planet on the sun, such as we do not derive from the more familiar action of the sunlight on the growth of plants and other phenomena of life depending on solar influences.

Perhaps the theory of solar magnetic influence upon the weather is best known in connection with the ''sun-spot cycle.'' This, at any rate, is, as already remarked, closely a.s.sociated with the corona. Its existence was discovered in 1843 by the German astronomer Schwabe. It is a period of variable length, averaging about eleven years, during which the number of spots visible on the sun first increases to a maximum, then diminishes to a minimum, and finally increases again to a maximum. For unknown reasons the period is sometimes two or three years longer than the average and sometimes as much shorter.

Nevertheless, the phenomena always recur in the same order. Starting, for instance, with a time when the observer can find few or no spots, they gradually increase in number and size until a maximum, in both senses, is reached, during which the spots are often of enormous size and exceedingly active. After two or three years they begin to diminish in number, magnitude, and activity until they almost or quite disappear. A strange fact is that when a new period opens, the spots appear first in high northern and southern lat.i.tudes, far from the solar equator, and as the period advances they not only increase in number and size, but break out nearer and nearer to the equator, the last spots of a vanishing period sometimes lingering in the equatorial region after the advance-guard of its successor has made its appearance in the high lat.i.tudes. Spots are never seen on the equator nor near the poles. It was not very long after the discovery of the sun-spot cycle that the curious observation was made that a striking coincidence existed between the period of the sun-spots and another period affecting the general magnetic condition of the earth. When a curved line representing the varying number of sun-spots was compared with another curve showing the variations in the magnetic state of the earth the two were seen to be in almost exact accord, a rise in one curve corresponding to a rise in the other, and a fall to a fall.

Continued observation has proved that this is a real coincidence and not an accidental one, so that the connection, although as yet unexplained, is accepted as established. But does the influence extend further, and directly affect the weather and the seasons as well as the magnetic elements of the earth? A final answer to this question cannot yet be given, for the evidence is contradictory, and the interpretations put upon it depend largely on the predilections of the judges.

But, in a broad sense, the sun-spots and the phenomena connected with them must have a relation to terrestial meteorology, for they prove the sun to be a variable star. Reference was made, a few lines above, to the resemblance of the spectra of sun-spots to those of certain stars which seem to be failing through age. This in itself is extremely suggestive; but if this resemblance had never been discovered, we should have been justified in regarding the sun as variable in its output of energy; and not only variable, but probably increasingly so. The very inequalities in the sun-spot cycle are suspicious. When the sun is most spotted its total light may be reduced by one-thousandth part, although it is by no means certain that its outgiving of thermal radiations is then reduced. A loss of one-thousandth of its luminosity would correspond to a decrease of .0025 of a stellar magnitude, considering the sun as a star viewed from distant s.p.a.ce. So slight a change would not be perceptible; but it is not alone sun-spots which obscure the solar surface, its entire globe is enveloped with an obscuring veil. When studied with a powerful telescope the sun's surface is seen to be thickly mottled with relatively obscure specks, so numerous that it has been estimated that they cut off from one-tenth to one-twentieth of the light that we should receive from it if the whole surface were as brilliant as its brightest parts. The condition of other stars warrants the conclusion that this obscuring envelope is the product of a process of refrigeration which will gradually make the sun more and more variable until its history ends in extinction. Looking backward, we see a time when the sun must have been more brilliant than it is now. At that time it probably shone with the blinding white splendor of such stars as Sirius, Spica, and Vega; now it resembles the relatively dull Procyon; in time it will turn ruddy and fall into the closing cycle represented by Antares. Considering that once it must have been more radiantly powerful than at present, one is tempted to wonder if that could have been the time when tropical life flourished within the earth's polar circles, sustained by a vivific energy in the sun which it has now lost.

The corona, as we have said, varies with the sun-spot cycle. When the spots are abundant and active the corona rises strong above the spotted zones, forming immense beams or streamers, which on one occasion, at least, had an observed length of ten million miles. At the time of a spot minimum the corona is less brilliant and has a different outline. It is then that the curved polar rays are most conspicuous. Thus the vast banners of the sun, shaken out in the eclipse, are signals to tell of its varying state, but it will probably be long before we can read correctly their messages.

The Zodiacal Light Mystery

There is a singular phenomenon in the sky -- one of the most puzzling of all -- which has long arrested the attention of astronomers, defying their efforts at explanation, but which probably not one in a hundred, and possibly not one in a thousand, of the readers of this book has ever seen. Yet its name is often spoken, and it is a conspicuous object if one knows when and where to look for it, and when well seen it exhibits a mystical beauty which at the same time charms and awes the beholder. It is called ''The Zodiacal Light,''

because it lies within the broad circle of the Zodiac, marking the sun's apparent annual path through the stars. What it is n.o.body has yet been able to find out with certainty, and books on astronomy usually speak of it with singular reserve. But it has given rise to many remarkable theories, and a true explanation of it would probably throw light on a great many other celestial mysteries. The Milky Way is a more wonderful object to look upon, but its nature can be comprehended, while there is a sort of uncanniness about the Zodiacal Light which immediately impresses one upon seeing it, for its part in the great scheme of extra-terrestrial affairs is not evident.

If you are out-of-doors soon after sunset -- say, on an evening late in the month of February -- you may perceive, just after the angry flush of the dying winter's day has faded from the sky, a pale ghostly presence rising above the place where the sun went down. The writer remembers from boyhood the first time it was pointed out to him and the unearthly impression that it made, so that he afterward avoided being out alone at night, fearful of seeing the spectral thing again.

The phenomenon brightens slowly with the fading of the twilight, and soon distinctly a.s.sumes the shape of an elongated pyramid of pearly light, leaning toward the south if the place of observation is in the northern hemisphere. It does not impress the observer at all in the same manner as the Milky Way; that looks far off and is clearly among the stars, but the Zodiacal Light seems closer at hand, as if it were something more intimately concerning the earth. To all it immediately suggests a connection, also, with the sunken sun. If the night is clear and the moon absent (and if you are in the country, for city lights ruin the spectacles of the sky), you will be able to watch the apparition for a long time. You will observe that the light is brightest near the horizon, gradually fading as the pyramidal beam mounts higher, but in favorable circ.u.mstances it may be traced nearly to the meridian south of the zenith, where its apex at last vanishes in the starlight. It continues visible during the evenings of March and part of April, after which, ordinarily, it is seen no more, or if seen is relatively faint and unimpressive. But when autumn comes it appears again, this time not like a wraith hovering above the westward tomb of the day-G.o.d, but rather like a spirit of the morning announcing his reincarnation in the east.

The reason why the Zodiacal Light is best seen in our lat.i.tudes at the periods just mentioned is because at those times the Zodiac is more nearly perpendicular to the horizon, first in the west and then in the east; and, since the phenomenon is confined within the borders of the Zodiac, it cannot be favorably placed for observation when the zodiacal plane is but slightly inclined to the horizon. Its faint light requires the contrast of a background of dark sky in order to be readily perceptible. But within the tropics, where the Zodiac is always at a favorable angle, the mysterious light is more constantly visible. Nearly all observant travelers in the equatorial regions have taken particular note of this phenomenon, for being so much more conspicuous there than in the temperate zones it at once catches the eye and holds the attention as a novelty. Humboldt mentions it many times in his works, for his genius was always attracted by things out of the ordinary and difficult of explanation, and he made many careful observations on its shape, its brilliancy, and its variations; for there can be no doubt that it does vary, and sometimes to an astonishing degree. It is said that it once remained practically invisible in Europe for several years in succession. During a trip to South Africa in 1909 an English astronomer, Mr E. W. Maunder, found a remarkable difference between the appearance of the Zodiacal Light on his going and coming voyages. In fact, when crossing the equator going south he did not see it at all; but on returning he had, on March 6th, when one degree south of the equator, a memorable view of it.

It was a bright, clear night, and the Zodiacal Light was extraordinarily brilliant -- brighter than he had ever seen it before. The Milky Way was not to be compared with it. The brightest part extended 75 from the sun. There was a faint and much narrower extension which they could just make out beyond the Pleiades along the ecliptic, but the greater part of the Zodiacal Light showed as a broad truncated column, and it did not appear nearly as conical as he had before seen it.

When out of the brief twilight of intertropical lands, where the sun drops vertically to the horizon and night rushes on like a wave of darkness, the Zodiacal Light shoots to the very zenith, its color is described as a golden tint, entirely different from the silvery sheen of the Milky Way. If I may venture again to refer to personal experiences and impressions, I will recall a view of the Zodiacal Light from the summit of the cone of Mt Etna in the autumn of the year 1896 (more briefly described in Astronomy with the Naked Eye). There are few lofty mountains so favorably placed as Etna for observations of this kind. It was once resorted to by Prof. George E. Hale, in an attempt to see the solar corona without an eclipse. Rising directly from sea-level to an elevation of nearly eleven thousand feet, the observer on its summit at night finds himself, as it were, lost in the midst of the sky. But for the black flanks of the great cone on which he stands he might fancy himself to be in a balloon. On the occasion to which I refer the world beneath was virtually invisible in the moonless night. The blaze of the constellations overhead was astonishingly brilliant, yet amid all their magnificence my attention was immediately drawn to a great tapering light that sprang from the place on the horizon where the sun would rise later, and that seemed to be blown out over the stars like a long, luminous veil. It was the finest view of the Zodiacal light that I had ever enjoyed -- thrilling in its strangeness -- but I was almost disheartened by the indifference of my guide, to whom it was only a light and nothing more. If he had no science, he had less poetry -- rather a remarkable thing, I thought, for a child of his clime. The Light appeared to me to be distinctly brighter than the visible part of the Milky Way which included the brilliant stretches in Auriga and Perseus, and its color, if one may speak of color in connection with such an object, seemed richer than that of the galactic band; but I did not think of it as yellow, although Humboldt has described it as resembling a golden curtain drawn over the stars, and Du Chaillu in Equatorial Africa found it of a bright yellow color. It may vary in color as in conspicuousness. The fascination of that extraordinary sight has never faded from my memory. I turned to regard it again and again, although I had never seen the stellar heavens so brilliant, and it was one of the last things I looked for when the morning glow began softly to mount in the east, and Sicily and the Mediterranean slowly emerged from the profound shadow beneath us.

The Zodiacal Light seems never to have attracted from astronomers in general the amount of careful attention that it deserves; perhaps because so little can really be made of it as far as explanation is concerned. I have referred to the restraint that scientific writers apparently feel in speaking of it. The grounds for speculation that it affords may be too scanty to lead to long discussions, yet it piques curiosity, and as we shall see in a moment has finally led to a most interesting theory. Once it was the subject of an elaborate series of studies which carried the observer all round the world. That was in 1845--46, during the United States Exploring Expedition that visited the then little known j.a.pan. The chaplain of the fleet, the Rev. Mr Jones, went out prepared to study the mysterious light in all its phases. He saw it from many lat.i.tudes on both sides of the equator, and the imagination cannot but follow him with keen interest in his world-circling tour, keeping his eyes every night fixed upon the phantasm overhead, whose position shifted with that of the hidden sun.

He demonstrated that the flow extends at times completely across the celestial dome, although it is relatively faint directly behind the earth. On his return the government published a large volume of his observations, in which he undertook to show that the phenomenon was due to the reflection of sunlight from a ring of meteoric bodies encircling the earth. But, after all, this elaborate investigation settled nothing.

Prof. E. E. Barnard has more recently devoted much attention to the Zodiacal Light, as well as to a strange attendant phenomenon called the ''Gegenschein,'' or Counterglow, because it always appears at that point in the sky which is exactly opposite the sun. The Gegenschein is an extremely elusive phenomenon, suitable only for eyes that have been specially trained to see it. Professor Newcomb has cautiously remarked that

it is said that in that point of the heavens directly opposite the sun there is an elliptical patch of light... This phenomenon is so difficult to account for that its existence is sometimes doubted; yet the testimony in its favor is difficult to set aside.

It certainly cannot be set aside at all since the observations of Barnard. I recall an attempt to see it under his guidance during a visit to Mount Hamilton, when he was occupied there with the Lick telescope. Of course, both the Gegenschein and the Zodiacal Light are too diffuse to be studied with telescopes, which, so to speak, magnify them out of existence. They can only be successfully studied with the naked eye, since every faintest glimmer that they afford must be utilized. This is especially true of the Gegenschein. At Mount Hamilton, Mr Barnard pointed out to me its location with reference to certain stars, but with all my gazing I could not be sure that I saw it. To him, on the contrary, it was obvious; he had studied it for months, and was able to indicate its shape, its boundaries, its diameter, and the declination of its center with regard to the ecliptic. There is not, of course, the shadow of a doubt of the existence of the Gegenschein, and yet I question if one person in a million has ever seen or ever will see it. The Zodiacal Light, on the other hand, is plain enough, provided that the time and the circ.u.mstances of the observation are properly chosen.

In the attempts to explain the Zodiacal Light, the favorite hypothesis has been that it is an appendage of the sun -- perhaps simply an extension of the corona in the plane of the ecliptic, which is not very far from coinciding with that of the sun's equator. This idea is quite a natural one, because of the evident relation of the light to the position of the sun. The vast extension of the equatorial wings of the corona in 1878 gave apparent support to this hypothesis; if the substance of the corona could extend ten million miles from the sun, why might it not extend even one hundred million, gradually fading out beyond the orbit of the earth? A variation of this hypothesis a.s.sumes that the reflection is due to swarms of meteors circling about the sun, in the plane of its equator, all the way from its immediate neighborhood to a distance exceeding that of the earth. But in neither form is the hypothesis satisfactory; there is nothing in the appearance of the corona to indicate that it extends even as far as the planet Mercury, while as to meteors, the orbits of the known swarms do not accord with the hypothesis, and we have no reason to believe that clouds of others exist traveling in the part of s.p.a.ce where they would have to be in order to answer the requirements of the theory. The extension of the corona in 1878 did not resemble in its texture the Zodiacal Light.

Now, it has so often happened in the history of science that an important discovery in one branch has thrown unexpected but most welcome light upon some pending problem in some other branch, that a strong argument might be based upon that fact alone against the too exclusive devotion of many investigators to the narrow lines of their own particular specialty; and the Zodiacal Light affords a case in point, when it is considered in connection with recent discoveries in chemistry and physics. From the fact that atoms are compound bodies made up of corpuscles at least a thousand times smaller than the smallest known atom -- a fact which astounded most men of science when it was announced a few years ago -- a new hypothesis has been developed concerning the nature of the Zodiacal Light (as well as other astronomical riddles), and this hypothesis comes not from an astronomer, but from a chemist and physicist, the Swede, Svante Arrhenius. In considering an outline of this new hypothesis we need neither accept nor reject it; it is a case rather for suspension of judgment.

To begin with, it carries us back to the ''pressure of light''

mentioned in the preceding chapter. The manner in which this pressure is believed generally to act was there sufficiently explained, and it only remains to see how it is theoretically extended to the particles of matter supposed to const.i.tute the Zodiacal Light. We know that corpuscles, or ''fragments of atoms'' negatively electrified, are discharged from hot bodies. Streams of these ''ions'' pour from many flames and from molten metals; and the impact of the cathode and ultra-violet rays causes them to gush even from cold bodies. In the vast laboratory of the sun it is but reasonable to suppose that similar processes are taking place. ''As a very hot metal emits these corpuscles,'' says Prof. J. J. Thomson, ''it does not seem an improbable hypothesis that they are emitted by that very hot body, the sun.'' Let it be a.s.sumed, then, that the sun does emit them; what happens next? Negatively charged corpuscles, it is known, serve as nuclei to which particles of matter in the ordinary state are attracted, and it is probable that those emitted from the sun immediately pick up loads in this manner and so grow in bulk. If they grow large enough the gravitation of the sun draws them back, and they produce a negative charge in the solar atmosphere. But it is probable that many of the particles do not attain the critical size which, according to the principles before explained, would enable the gravitation of the sun to retain them in opposition to the pressure of the waves of light, and with these particles the light pressure is dominant. Clouds of them may be supposed to be continually swept away from the sun into surrounding s.p.a.ce, moving mostly in or near the plane of the solar equator, where the greatest activity, as indicated by sunspots and related phenomena, is taking place. As they pa.s.s outward into s.p.a.ce many of them encounter the earth. If the earth, like the moon, had no atmosphere the particles would impinge directly on its surface, giving it a negative electric charge. But the presence of the atmosphere changes all that, for the first of the flying particles that encounter it impart to it their negative electricity, and then, since like electric charges repel like, the storm of particles following will be sheered off from the earth, and will stream around it in a maze of hyperbolic paths. Those that continue on into s.p.a.ce beyond the earth may be expected to continue picking up wandering particles of matter until their bulk has become so great that the solar attraction prevails again over the light pressure acting upon them, and they turn again sunward. Pa.s.sing the earth on their return they will increase the amount of dust-clouds careering round it; and these will be further increased by the action of the ultra-violet rays of the sunlight causing particles to shoot radially away from the earth when the negative charge of the upper atmosphere has reached a certain amount, which particles, although starting sunward, will be swept back to the earth with the oncoming streams. As the final result of all this acc.u.mulation of flying and gyrating particles in the earth's neighborhood, we are told that the latter must be transformed into the semblance of a gigantic solid-headed comet provided with streaming tails, the longest of them stretching away from the direction of the sun, while another shorter one extends toward the sun. This shorter tail is due to the particles that we have just spoken of as being driven sunward from the earth by the action of ultra-violet light. No doubt this whole subject is too technical for popular statement; but at any rate the general reader can understand the picturesque side of the theory, for its advocates a.s.sure us that if we were on the moon we would doubtless be able to see the comet-like tails of the earth, and then we could appreciate the part that they play in producing the phenomenon of the Zodiacal Light.

That the Light as we see it could be produced by the reflection of sunlight from swarms of particles careering round the earth in the manner supposed by Arrhenius' hypothesis is evident enough; and it will be observed that the new theory, after all, is only another variant of the older one which attributes the Zodiacal Light to an extension of the solar corona. But it differs from the older theory in offering an explanation of the manner in which the extension is effected, and it differentiates between the corona proper and the streams of negative particles shot away from the sun. In its details the hypothesis of Arrhenius also affords an explanation of many peculiarities of the Zodiacal Light, such as that it is confined to the neighborhood of the ecliptic, and that it is stronger on the side of the earth which is just turning away from a position under the sun than on the other side; but it would carry us beyond our limits to go into these particulars. The Gegenschein, according to this theory, is a part of the same phenomenon as the Zodiacal Light, for by the laws of perspective it is evident that the reflection from the streams of particles situated at a point directly opposite to the sun would be at a maximum, and this is the place which the Gegenschein occupies. Apart from its geometrical relations to the position of the sun, the variability of the Zodiacal Light appears to affirm its solar dependence, and this too would be accounted for by Arrhenius'

hypothesis better than by the old theory of coronal extension. The amount of corpuscular discharge from the sun must naturally be governed by the state of relative activity or inactivity of the latter, and this could not but be reflected in the varying splendor of the Zodiacal Light. But much more extended study than has yet been given to the subject will be required before we can feel that we know with reasonable certainty what this mysterious phenomenon really is.

By the hypothesis of Arrhenius every planet that has an atmosphere must have a Zodiacal Light attending it, but the phenomenon is too faint for us to be able to see it in the case, for instance, of Venus, whose atmosphere is very abundant. The moon has no corresponding ''comet's tail'' because, as already explained, of the lack of a lunar atmosphere to repel the streams by becoming itself electrified; but if there were a lunar Zodiacal Light, no doubt we could see it because of the relative nearness of our satellite.

Marvels of the Aurora

One of the most vivid recollections of my early boyhood is that of seeing my father return hastily into the house one evening and call out to the family: ''Come outside and look at the sky!'' Ours was a country house situated on a commanding site, and as we all emerged from the doorway we were dumbfounded to see the heavens filled with pale flames which ran licking and quivering over the stars. Instantly there sprang into my terrified mind the recollection of an awful description of ''the Day of Judgment'' (the Dies Irae), which I had heard with much perturbation of spirit in the Dutch Reformed church from the lips of a tall, dark-browed, dreadfully-in-earnest preacher of the old-fashioned type. My heart literally sank at sight of the spectacle, for it recalled the preacher's very words; it was just as he had said it would be, and it needed the a.s.sured bearing of my elders finally to convince me that

That Day of Wrath, O dreadful day, When Heaven and Earth shall pa.s.s away, As David and the Sibyl say

had not actually come upon us. And even the older members of the household were not untouched with misgivings when menacing spots of crimson appeared, breaking out now here, now there, in the shuddering sky. Toward the north the spectacle was appalling. A huge arch spanned an unnaturally dark segment resting on the horizon, and above this arch sprang up beams and streamers in a state of incessant agitation, sometimes shooting up to the zenith with a velocity that took one's breath, and sometimes suddenly falling into long ranks, and marching, marching, marching, like an endless phalanx of fiery specters, and moving, as I remember, always from east to west. The absolute silence with which these mysterious evolutions were performed and the quavering reflections which were thrown upon the ground increased the awfulness of the exhibition. Occasionally enormous curtains of lambent flame rolled and unrolled with a majestic motion, or were shaken to and fro as if by a mighty, noiseless wind. At times, too, a sudden billowing rush would be made toward the zenith, and for a minute the sky overhead would glow so brightly that the stars seemed to have been consumed. The spectacle continued with varying intensity for hours.

This exhibition occurred in Central New York, a lat.i.tude in which the Aurora Borealis is seldom seen with so much splendor. I remember another similar one seen from the city of New York in November, 1882.

On this last occasion some observers saw a great upright beam of light which majestically moved across the heavens, stalking like an apparition in the midst of the auroral pageant, of whose general movements it seemed to be independent, maintaining always its upright posture, and following a magnetic parallel from east to west. This mysterious beam was seen by no less than twenty-six observers in different parts of the country, and a comparison of their observations led to a curious calculation indicating that the apparition was about one hundred and thirty-three miles tall and moved at the speed of ten miles per second!

But, as everybody knows, it is in the Arctic regions that the Aurora, or the ''Northern Lights,'' can best be seen. There, in the long polar night, when for months together the sun does not rise, the strange coruscations in the sky often afford a kind of spectral daylight in unison with the weird scenery of the world of ice. The pages in the narratives of Arctic exploration that are devoted to descriptions of the wonderful effects of the Northern Lights are second to none that man has ever penned in their fascination. The lights, as I have already intimated, display astonishing colors, particularly shades of red and green, as they flit from place to place in the sky. The discovery that the magnetic needle is affected by the Aurora, quivering and darting about in a state of extraordinary excitement when the lights are playing in the sky, only added to the mystery of the phenomenon until its electro-magnetic nature had been established.

This became evident as soon as it was known that the focus of the displays was the magnetic pole; and when the far South was visited the Aurora Australis was found, having its center at the South Magnetic Pole. Then, if not before, it was clear that the earth was a great globular magnet, having its poles of opposite magnetism, and that the auroral lights, whatever their precise cause might be, were manifestations of the magnetic activity of our planet. After the invention of magnetic telegraphy it was found that whenever a great Aurora occurred the telegraph lines were interrupted in their operation, and the ocean cables ceased to work. Such a phenomenon is called a ''magnetic storm.''

The interest excited by the Aurora in scientific circles was greatly stimulated when, in the last half of the nineteenth century, it was discovered that it is a phenomenon intimately a.s.sociated with disturbances on the sun. The ancient ''Zurich Chronicles,'' extending from the year 1000 to the year 1800, in which both sun-spots visible to the naked eye and great displays of the auroral lights were recorded, first set Rudolf Wolf on the track of this discovery. The first notable proof of the suspected connection was furnished with dramatic emphasis by an occurrence which happened on September 1, 1859. Near noon on that day two intensely brilliant points suddenly broke out in a group of sun-spots which were under observation by Mr R. C. Carrington at his observatory at Redhill, England. The points remained visible for not more than five minutes, during which interval they moved thirty-five thousand miles across the solar disk. Mr R.

Hodgson happened to see the same phenomenon at his observatory at Highgate, and thus all possibility of deception was removed. But neither of the startled observers could have antic.i.p.ated what was to follow, and, indeed, it was an occurrence which has never been precisely duplicated. I quote the eloquent account given by Miss Clerke in her History of Astronomy During the Nineteenth Century.

This unique phenomenon seemed as if specially designed to accentuate the inference of a sympathetic relation between the earth and the sun. From August 28 to September 4, 1859, a magnetic storm of unparalleled intensity, extent, and duration was in progress over the entire globe. Telegraphic communication was everywhere interrupted -- except, indeed, that it was in some cases found practicable to work the lines without batteries by the agency of the earth-currents alone; sparks issued from the wires; gorgeous auroras draped the skies in solemn crimson over both hemispheres, and even in the tropics; the magnetic needle lost all trace of continuity in its movements and darted to and fro as if stricken with inexplicable panic. The coincidence was even closer. At the very instant of the solar outburst witnessed by Carrington and Hodgson the photographic apparatus at Kew registered a marked disturbance of all the three magnetic elements; while shortly after the ensuing midnight the electric agitation culminated, thrilling the whole earth with subtle vibrations, and lighting up the atmosphere from pole to pole with coruscating splendors which perhaps dimly recall the times when our ancient planet itself shone as a star.

If this amazing occurrence stood alone, and as I have already said it has never been exactly duplicated, doubt might be felt concerning some of the inferences drawn from it; but in varying forms it has been repeated many times, so that now hardly anyone questions the reality of the a.s.sumed connection between solar outbursts and magnetic storms accompanied by auroral displays on the earth. It is true that the late Lord Kelvin raised difficulties in the way of the hypothesis of a direct magnetic action of the sun upon the earth, because it seemed to him that an inadmissible quant.i.ty of energy was demanded to account for such action. But no calculation like that which he made is final, since all calculations depend upon the validity of the data; and no authority is unshakable in science, because no man can possess omniscience. It was Lord Kelvin who, but a few years before the thing was actually accomplished, declared that aerial navigation was an impracticable dream, and demonstrated its impracticability by calculation. However the connection may be brought about, it is as certain as evidence can make it that solar outbursts are coincident with terrestial magnetic disturbances, and coincident in such a way as to make the inference of a causal connection irresistible. The sun is only a little more than a hundred times its own diameter away from the earth. Why, then, with the subtle connection between them afforded by the ether which conveys to us the blinding solar light and the life-sustaining solar heat, should it be so difficult to believe that the sun's enormous electric energies find a way to us also? No doubt the impulse coming from the sun acts upon the earth after the manner of a touch upon a trigger, releasing energies which are already stored up in our planet.

But besides the evidence afforded by such occurrences as have been related of an intimate connection between solar outbreaks and terrestial magnetic flurries, attended by magnificent auroral displays, there is another line of proof pointing in the same direction. Thus, it is known that the sun-spot period, as remarked in a preceding chapter, coincides in a most remarkable manner with the periodic fluctuations in the magnetic state of the earth. This coincidence runs into the most astonishing details. For instance, when the sun-spot period shortens, the auroral period shortens to precisely the same extent; as the short sun-spot periods usually bring the most intense outbreaks of solar activity, so the corresponding short auroral periods are attended by the most violent magnetic storms; a secular period of about two hundred and twenty-two years affecting sun-spots is said to have its auroral duplicate; a shorter period of fifty-five and a half years, which some observers believe that they have discovered appears also to be common to the two phenomena; and yet another ''superposed'' period of about thirty-five years, which some investigators aver exists, affects sun-spots and aurora alike. In short, the coincidences are so numerous and significant that one would have to throw the doctrine of probability to the winds in order to be able to reject the conclusion to which they so plainly lead.

But still the question recurs: How is the influence transmitted? Here Arrhenius comes once more with his hypothesis of negative corpuscles, or ions, driven away from the sun by light-pressure -- a hypothesis which seems to explain so many things -- and offers it also as an explanation of the way in which the sun creates the Aurora. He would give the Aurora the same lineage with the Zodiacal Light. To understand the application of this theory we must first recall the fact that the earth is a great magnet having its two opposite poles of magnetism, one near the Arctic and the other near the Antarctic Circle. Like all magnets, the earth is surrounded with ''lines of force,'' which, after the manner of the curved rays we saw in the photograph of a solar eclipse, start from a pole, rising at first nearly vertically, then bend gradually over, pa.s.sing high above the equator, and finally descending in converging sheaves to the opposite pole. Now the axis of the earth is so placed in s.p.a.ce that it lies at nearly a right angle to the direction of the sun, and as the streams of negatively charged particles come pouring on from the sun (see the last preceding chapter), they arrive in the greatest numbers over the earth's equatorial regions. There they encounter the lines of magnetic force at the place where the latter have their greatest elevation above the earth, and where their direction is horizontal to the earth's surface. Obeying a law which has been demonstrated in the laboratory, the particles then follow the lines of force toward the poles. While they are above the equatorial regions they do not become luminescent, because at the great elevation that they there occupy there is virtually no atmosphere; but as they pa.s.s on toward the north and the south they begin to descend with the lines of force, curving down to meet at the poles; and, encountering a part of the atmosphere comparable in density with what remains in an exhausted Crookes tube, they produce a glow of cathode rays. This glow is conceived to represent the Aurora, which may consequently be likened to a gigantic exhibition of vacuum-tube lights. Anybody who recalls his student days in the college laboratory and who has witnessed a display of Northern Lights will at once recognize the resemblance between them in colors, forms, and behavior. This resemblance had often been noted before Arrhenius elaborated his hypothesis.