Popular Science Monthly/Volume 11/July 1877/The Zodiacal Light
|THE ZODIACAL LIGHT.|
By Professor C. E. BRAME, A.M.
THE purpose of this contribution is to draw attention to a phenomenon which has received too little notice, and has been strangely neglected by astronomers, but which, in fact, if the conclusions of the author of the work under review are correct, is to the inhabitants of the earth one which emanates from a very near and remarkable cosmical body.
The third volume of "The United States Japan Expedition" records a series of observations on the zodiacal light, which were made by Rev. George Jones, A.M., chaplain in the United States Navy, from April 2, 1853, to April 22, 1855, during which time he accompanied that expedition sent out by the United States Government, under the command of Commodore M. C. Perry. It also contains the observations and theories of other astronomers, particularly those made by the celebrated Dominicus Cassini, a distinguished savant of the seventeenth century, attached to the Royal Observatory at Paris, who was the greatest cosmologist of that age.
The deductions of our author are—1. That the zodiacal light is emitted by a nebulous ring, with the earth for its centre; that is, there is a ring of nebulous matter around the earth; 2. That 60° is the full width of the stronger light, and 90° its full width including the diffused light; 3. That it is on both sides of the ecliptic; 4. That it may be seen in some latitudes under favorable circumstances, forming a continuous arch across the heavens; 5. That it must rotate on a common centre, and seems full of commotion within itself; 6. From observations made then and afterward at Quito, in South America, that it is not entirely dependent on light reflected from the sun, but must shine partly by its own light.
These conclusions are given to the public with much diffidence, and with strong arguments to support them, based on a large accumulation of facts ascertained by the most careful observation.
Our chaplain became intensely interested in this investigation, and has recorded three hundred and twenty-eight observations on stereotyped star-charts, published with the volume. Sometimes, when unable from sickness to go upon deck, he was carried in his cot, that he might have an opportunity of noticing this intensely interesting phenomenon. As the voyage was one which circumnavigated the globe, he had a most favorable opportunity to prosecute his examination; and his recorded statement embraces a most minute and faithful account of his observations in nearly every longitude, and over a tract extending from 41° 49' north to 53° 48' south latitude.
After the return of the expedition he sought the earliest occasion to go to Quito, in South America, which is directly on the equator, where he saw the light extending in one continuous arch across the heavens. During his residence there he published several articles in Silliman's Journal on "The Zodiacal Light," which gave additional interest to the subject.
If anything since then of importance has appeared in any scientific journal on this subject, we have not met with it. Having for over twenty years considered it, and being anxious that more information and more confirmed conclusions should be arrived at concerning it, we invite the attention of the learned and observing to the consideration of this subject. We believe that there is no physical truth which stands isolated, but every new discovery in physical science opens the way for other truths and discoveries. If naturalists can spend days and weeks in examining and commenting on a new discovery in botany or zoölogy; if foreign countries are visited, forests and mountains traversed, to bring to light some hitherto unknown plant; if it is a triumph for a naturalist to discover a new species of microscopic animalcula—surely it is not time wasted to direct attention to a cosmical body so interesting as the zodiacal light.
Astronomical science is full of attractive interest to every lover of Nature. It is not astonishing that the study of the heavens is the oldest natural science. At an early period of the world's history the human eye and intellect were directed, with absorbing interest, to the azure arch above us, amid whose vast expanse orbs of grandeur are unceasingly running their wonderful courses. The stars, beaming with inextinguishable brilliancy, are known to be oceans of flames and centres of worlds, though apparently but points of light. The planets were known to the ancients to be more identified with our world than other stellar bodies. Traversing space with inestimable velocity and performing their revolutions with unvaried regularity, they have long been known as part of our solar system. It seems strange that a cosmical body so near the earth as the zodiacal light should have received comparatively so little attention.
As many who read this article have never seen this light, it is necessary that it be described.
It is defined, in the work under review, to be "a brightness that appears in the western sky after sunset, and in the east before sunrise; following nearly or quite the line of the ecliptic in the heavens, and stretching upward to various elevations according to the season of the year." There is a slight objection to this definition, by which inexperienced observers may be led astray. It is spoken of as appearing after sunset, by which some would be led to suppose that it is visible immediately after sunset; whereas it is never to be seen until after the night has fully set in, and the sun's rays are some distance below the horizon. Its varied elevation, indeed its appearance, is also dependent upon the latitude of the observer as well as the season, so much so, that in very high latitudes it is but seldom seen to advantage. It has also been seen at almost every hour of the night, but is usually more distinctly seen in the temperate zones, when observed between dark and nine o'clock, as after that hour its light frequently becomes dim and diffuse.
"It appears to best advantage when the ecliptic makes its highest angle with the spectator's horizon, at which time, in moderate latitudes, it reaches to the zenith or beyond it, having near the horizon a striking brilliancy, and thence fading upward, mostly by imperceptible degrees, till at its vertex it can be made out only by a careful and experienced eye. As the seasons advance, when the ecliptic is declining gradually toward the horizon, the zodiacal light fades away till it is perhaps entirely lost to view, or can be seen only by those who have followed it in its changes, night after night, and are thus able, by familiar acquaintance, to detect and trace its dim markings in the sky."
Humboldt, in "The Cosmos," vol, i., remarks:
"Those who have been for many years in the zone of palms must retain a pleasing impression of the mild radiance with which the zodiacal light, shooting pyramidically upward, illumines a part of the uniform length of tropical nights. I have seen it shine with an intensity of light equal to the milky-way in Sagittarius, and that not only in the rare and dry atmosphere of the summit of the Andes, at an elevation of from 13,000 to 15,000 feet, but even on the boundless grassy plains, the llanos of Venezuela, and on the sea-shore beneath the ever-clear sky of Cumana."
Prof. Olmsted, in his "Astronomy," describes it as follows:
The crepuscles, or streaks of light from the sun, must not be mistaken for the zodiacal light; the former are sometimes visible between twilight and dark, the latter not until the shades of night have fully set in. Neither must the milky-way be mistaken for it by inexperienced observers, as the zodiacal light has a warm yellowish tinge unlike the cold white light of the milky-way.
Having endeavored to open the way for a consideration of the subject, we will now proceed to give a history of former observations.
There is no mention of any appearance of this light by very early writers. There is mention of Arcturus and the bands of Orion in the book of Job, and the constellations of the zodiac were assigned names at a very early date. The zodiacal belt was in use among the ancient Egyptians and Hindoos. If this light had been visible, it is highly probable some ancient writer would have spoken of it. Our author observes:
"It is scarcely probable that a phenomenon so striking in southern latitudes should have escaped the attention of early astronomers in those countries, but we meet with nothing in their works (referring to it) of a fully definite and reliable character."
He is, however, of the opinion that it may have been overlooked or mistaken for the crepuscle at early dawn or twilight.
It has been supposed that Pliny, who wrote in the first century of the Christian era, alludes to it under the name of trabes or the δοκονς of the Greeks, but Humboldt thinks that Pliny refers to another matter.
Ammonius, in his life of Charlemagne, a. d. 807, mentions an appearance somewhat like the zodiacal light, but there was no reliable notice of it before it was described in Childrey's "Britannia Baconica" in 1661, which gives a brief description of its appearance and shape. The reference to it may be found in that work, at page 183.
It was reserved for Cassini to direct attention to its examination for the first time with earnest inquiry and interest. His first notice of it was on the evening of the 18th of March, 1683. He was watching in the west for other things, but was struck with the appearance of this luminous streak reaching far up in the sky. Like most discoverers, Cassini immediately formed a theory in regard to it after making but ten observations. This hypothesis, based on very insufficient data, has continued to the present time to influence the opinions of astronomers, and has retarded interest in observing its phenomena. Like most theories of the heavenly bodies in their first origin, it was doubtless erroneous, and, like the Ptolemaic theory of the solar system, it has materially interfered with the establishment of a correct knowledge of what is the truth.
Cassini discovered very soon that as time advanced through March and April, the upper or northern edge of this light inclined more and more from the ecliptic, and stretched farther to the northward; and knowing that the sun's equator, as shown by his spots, was also then stretching off from the ecliptic in a similar way, he came to the conclusion that the substance giving this light was closely connected with the sun's equator, and was consequently changing its position with regard to that equator.
He argued, further, that as the sun has an atmosphere, and is therefore capable of emitting dense vapors, and is continually sending out matter of exceeding fineness which we call light, consequently this luminary might also, by its motion on its axis, send out a substance intermediate in character between the two; which substance, either self-luminous or by reflection, might give us the zodiacal light. To support this theory, he gives to this body a lenticular shape, about twice the thickness of the sun as seen in March, but only of the sun's thickness when seen in June. Whether he meant to have this lenticular-shaped medium to be attached to the sun, and revolving with it at the same time, is not apparent. He devoted a part of his time for about eleven years, in a very desultory manner, to observing this light.
Cassini's labors led other observers to direct their attention to the zodiacal light. Fabio de Duillier, who was his colleague for a while at Paris, is worthy of particular notice, as having conceived the idea that it consisted of particles of matter distinct from the sun, and arranged in shape like a solid zodiac; which body of uneven surfaces, and rotating round the sun, he supposed, gave us the zodiacal light.
In 1731 Mairan gave considerable attention to this subject in a work on "The Aurora Borealis." He advanced the theory that this light is reflected from the sun's atmosphere, stretched out into a flattened spheroid or lenticular-shaped body, revolving with the sun—an idea which Laplace has forever set at rest by demonstrating that the sun's atmosphere can extend no farther than to the orbit of a planet whose periodical revolution is performed in the same time as the sun's rotary motion about its axis, or in twenty-five days and a half; that is, only so far as nine-twentieths of Mercury's distance from the sun.
Since the time of Mairan, until 1853, but little attention appears to have been given to this subject. In 1833, however, Biot, in order to account for the meteor-shower of that year, attempted to show that the shower was owing to the earth's passing at that time near the node of the zodiacal light. But calculations were made by J. C. Housseau in order to see whether the nodes of the sun and the zodiacal light do actually correspond. After careful observations and calculations, in which he was assisted by nine diligent observers, Housseau thinks he has shown that these nodes are different, and that therefore "the supposition of the existence of this light in the plane of the sun's equator does not satisfy the observations made." The closing sentence of his very interesting article gives the first intimation that the zodiacal light has a near connection with our globe. Housseau says:
"One is struck, without doubt, with the near approach, which our elements show, between the line of nodes of the zodiacal light and that of the nodes of the terrestrial equator upon the ecliptic. This circumstance, as far as it is verified, may help to explain the causes of this luminous phenomenon—causes which are, it may be, more local than has hitherto been supposed."
This notice is the only one, of any importance whatever, that was taken of the zodiacal light for many years, with the exception of some experiments made to ascertain whether there is any heat connected with it—experiments which we think resulted in establishing the fact that it is near enough to emit heat sufficient to affect a thermo-electric pile of twenty-five pairs, causing the needle of the galvanometer to indicate 12° when the pile was directed toward the base of the zodiacal light.
We come now to a consideration of the observations taken during the cruise of the Japan Expedition, and we hope our readers are not weary of the subject; for, if it can be demonstrated that we have a ring of nebulous matter around our globe at a distance of 150,000 to 200,000 miles, there are many inquiries growing out of that fact full of interest, and yet to be answered by experiment and induction. It will be a matter, then, to be considered whether it is self-luminous—that is, of the nature of the sun's photosphere—or whether it shines only by reflected light, like the rings of Saturn; whether it is increasing or diminishing in magnitude and extent. If it is shown to be self-luminous, we will then wish to know whether it may not be widening and extending itself, at the same time increasing in its capacity to emit both light and heat, until it envelops the entire globe, modifies climate, melts the icebergs and snows of northern latitudes, and dispels darkness from our globe. We may inquire, too, whether it may not be used as a grand instrument in the hands of the Creator for the future development of his purposes in regard to our globe. It may also be an inquiry whether it does not already influence the magnetic state of the earth, and play an important part in the causes producing the aurora borealis. These and many more similar questions will arise from the establishment of the fact that the zodiacal light emanates from a ring of nebulous matter so near our earth. Let us, then, give our author a close examination and a patient hearing, that we may be enabled to form a correct judgment as to the truth of his deductions.
We have not space to present all the reasoning by which our author shows that the zodiacal light cannot be produced by a ring of nebulous matter revolving round the sun, either within or beyond the orbit of the earth.
He starts with the proposition that such a ring must be either—1. Within the earth's orbit; 2. Surrounding the earth; or, 3. Without the orbit of the earth.
It cannot be within the earth's orbit; if so, we could never have it at any great altitude at any period of the night, and we could never have the zodiacal light at midnight on both horizons simultaneously.
We omit the diagram constructed for the consideration of the theory which supposes this ring to be around the sun and beyond the orbit of the earth, by which he shows that this supposition will not account for the lateral changes observed in the zodiacal light. He says:
"The query arises, 'Can such lateral changes, so uniformly observed, as the evening or morning advances, agree at all with the idea of a nebulous ring giving us this light at a distance from the spectator of 160,000,000 or 180,000,000 miles?' A ring of the character supposed, it seems to me, could give us a zodiacal light only of one uniform shape, namely, with the opposite borders receding from each other for a short distance from the apex, and then running parallel, one to the other, the whole way down to the base. Nor could the hourly changes of time produce any other changes in these boundaries than a rising or sinking of the apex of the light; the boundaries, say at nine o'clock p. m., extending a little higher in the sky than at eight, and so continued, with a parallelism of the opposite sides, down to the horizon. How different this from the true facts of the case!
"The evident and most decided connection between these boundary-lines and the spectator's place, as regards the ecliptic, is also a matter of the greatest significance in drawing our conclusions in regard to the origin of this light. Now, supposing the base of the zodiacal light to be at a distance of 200,000,000 miles, how is it possible that the fact that the spectator is a short distance north or south of the ecliptic can govern the reflection from the nebulous ring at that immense distance, and place it on his side of the ecliptic? If he is on the north side of the ecliptic, not only is the main body of the light down to its base on that side, but the lateral changes of the boundaries, as the hours pass, are altogether or chiefly on that side; and so equally with the south. If he is on the ecliptic or near it, the zodiacal light stretches equally, or nearly so, on each side of that line. Also, if he changes rapidly during the night to or from the ecliptic (as was the case on shipboard) the boundaries of this light also change, being regulated by his motion (from one place to another). That the apex of the zodiacal light, from such a ring around the sun, might be so affected by the spectator's position is not unreasonable, but that the boundary-lines should be so affected seems to be utterly inadmissible.
"It is worthy of remark, also, how even and uniform, from apex to base, the change in the boundary-lines is as the hours change; as if the substance giving the zodiacal light were not only near, but also at one uniform distance from the spectator; the portions of it at the apex and base of the light all equally affected by his changes on the earth."
He shows by Bouguer's law of reflected light that the zodiacal light, if from a ring of nebulous matter beyond the earth's orbit, would emit a light of the same intensity the whole way from the base to the apex. But another law of optics, that the strength of light is inversely as the squares of the distances of the object affording the light, would here make its application; and this ring, at our zenith, being by that supposition about 140,000,000 miles nearer to us, than at the base, we should then have the zodiacal light far more intense at the apex than at the base; all of which is entirely opposite to the facts of the case.
He offers, now, as a last conclusion, the hypothesis of a nebulous ring with the earth for a centre. He makes certain deductions from the examination of the preceding theories.
"The hypothesis is that the zodiacal light is a ring around the earth."The thought is a somewhat startling one, yet startling only from its novelty; for it is entirely in accordance with what we know of our sister planet (Saturn), and also with the whole of Laplace's celebrated theory of the formation of globes."
We will not take up time and space with quoting his application of that theory in explaining some of the phenomena of this light.
Avoiding the consideration of these topics, we will proceed to apply the result of Bouguer's experiments on reflected light to this case.
In the annexed diagram he takes an observation made on the 4th of September, 1854, as an example, for the reason that it is a simple one, and one also in which the spectator is near the plane of the ecliptic. It was made in latitude 22° 18' north, longitude 114° 10' east. The sun rose at 5h 48m. The stronger light was at 3h 30m to 4h 30m, the diffuse light at 3h 45m. Sun's longitude 161° 35'. The horizons at 4h 30m, 3h 30m, 2h 30m, and 1h 30m, and at midnight, are given, together with the line of the spectator's vertices, as well as his positions O, o, etc., at 4h 30m and 3h 30m, A, B, C, F, are the boundaries of the zodiacal light at 4h 30m, and E, F, G, at 3h 30m; the apices C and G are carried a little above the more condensed portions of the ring; but the reader is at liberty to suppose them to be at any other part, as he may think best. The direction of the sun is given; and S', S", S'", S"", S'"", are supposed to be rays of light proceeding from that luminary. In this diagram, the sun's rays being S', S", etc., BO, FO, etc. will be the reflected rays; and the several angles between these lines of incidence and reflection, together with the number of rays reflected
Nebulous Ring, with the Earth for its Centre.
|[The relative proportions of the earth and the ring, and also its distance, are of course not given in this diagram with any effort at certainty; the upward extent of the ring is probably far greater than can be here represented. The diagram is, however, sufficiently correct for our present purposes of elucidation.]
to the eye, out of every 1,000 incident rays, according to Bouguer, are in the following table:
|ANGLE||Rays reflected from
|Rays reflected from|
Plate-glass not quick-
|S''''', at midnight||90°||18||25|
"We find in the above table a strong argument for such a ring around the earth. The figures, taking either of the two columns, for water or for glass, correspond in a very striking degree with the varying intensity of the zodiacal light from the base upward, as we have it on any clear morning or evening when the ecliptic is at a high angle with the horizon, and when, consequently, the nebulous figure is not brought angularly to our eye. They also correspond to what is, indeed, almost synonymous with what has been stated—namely, to the fact that at 4h 30m the zodiacal light at the horizon is far greater at its base than it is at 3h 30m; at 3h 30m than it is at 2h 30m, etc., back to midnight. Any person who has ever looked attentively at this light when making a high angle with the horizon will see at once the coincidence between the proportions of the figures in the above table, showing the number of reflected rays, and what has been always presented to the eye. If the reader will also carry these lines of incident and reflected light beyond the midnight horizon-line, to any point there of the nebulous ring, he will see how we may easily get what is referred to in my charts under the German name of Gegenschein, i. e., dim light seen, when the circumstances are favorable for it, in those portions of the sky opposed to the sun. This hypothesis shows also very clearly how I could have the zodiacal light above both horizons at the midnight hours, as I was often able to do, and it harmonizes fully with the strength of the light as then presented to the eye. While there are some things still left unexplained, I have yet not been able to see anything in this hypothesis antagonistic to the facts of the zodiacal light. On the contrary, almost all of them are explained by it; and they all, as I can perceive, fully harmonize with it through the whole of the manifold change which the light underwent, either from the changes of the ecliptic toward any fixed point, or from my numerous and great changes of latitude during our cruise.
"If we could have a zodiacal light of an undoubted character produced by the full moon, not only would the question before us be set at rest, but the ring would be shown to be within the orbit of the moon, and how near we came to a case of the kind on the evening of February 14, 1854, the reader will decide for himself. . . .
"This ring must, according to the laws of matter, rotate on its centre; and it must be full of commotion within itself. The existence of pulsations seems scarcely to admit of a doubt, recorded as they have been by observers in such distant quarters of the globe."
In conclusion, we would say that two simultaneous observations, made in equatorial regions—for instance, one by an observer at Quito, and another by an observer on the island of Sumatra, in which both observations presented the zodiacal light stretching as an arch across the heavens from east to west—would, it seems to us, demonstrate the fact that it is a ring around the earth. Now, if observations taken in almost every space of 15° of longitude have been made and its existence demonstrated, does not that amount to about the same thing?
We leave the subject, inviting information and discussion from all who are informed in regard to this long-neglected phenomenon.
- ↑ Third volume, United States Japan Expedition. "Observations on the Zodiacal Light from April 2, 1853, to April 22, 1855; made chiefly on board the United States Steam-frigate Mississippi, during her Cruise in the Eastern Seas and her Voyage homeward; with Conclusions from the Data thus obtained." By Rev. George Jones, A.M., Chaplain, U.S. Navy.