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 nobody 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 assumes 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 circumstances 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-god, 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 latitudes 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 Japan. 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 latitudes 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 circumstances of the observation are properly chosen.