Part 24
During the total eclipse of 1868 the American observers found that the spectrum of the corona is continuous, but crossed by certain bright lines. If we accept the absence of dark lines as established by the evidence (which is doubtful), this result seems at first sight very difficult to explain. Referring to the principles of spectroscopic analysis stated on pp. 338-339, it will be seen that we should be led to infer that the corona consists of incandescent matter surrounded by certain glowing gases. It is difficult to suppose that this is the real explanation of the phenomenon.
Mr. Lockyer suggests that, if the corona shone by reflecting the solar light, the continuous spectrum might be accounted for by supposing the light from the glowing vapors around the sun to supply the part wanting where the solar dark lines are, and that some of these vapors shining yet more brightly would exhibit their bright lines upon the continuous background of the spectrum. This view, as applied by Mr. Lockyer to the theory that the corona is a terrestrial phenomenon, is untenable, for the reasons already adduced. But, independently of those reasons, there are others which render such a solution of the difficulty unavailable.
Now, remembering that we have two established facts for our guidance—(1) the fact that the corona can not be a solar atmosphere, and (2) the fact that it must be a solar appendage—I think a way may be found toward a satisfactory explanation.
Let it be premised that the bright lines of the coronal spectrum correspond in position to those seen in the spectrum of the aurora, and that the same lines are seen in the spectrum of the Zodiacal Light, and in that of the phosphorescent light occasionally seen over the heavens at night.
Since we have every reason to believe that the light of the aurora is due to electrical discharges taking place in the upper regions of the air, we are invited to the belief that the coronal light may be due to similar discharges taking place between the particles (of whatever nature) constituting the corona.
Now, though the appearance of an aurora is due to some special terrestrial action (however excited), yet the material substances between which the discharges take place must be assumed to be at all times present in the upper regions of air. In all probability, they are the particles of those meteors which the earth is continually encountering. And since we know that meteor-systems must be aggregated in far greater numbers near the sun than near the earth, we may regard the coronal light as due to electrical discharges excited by the sun’s action, and taking place between the members of such systems. Besides this light, however, there must necessarily be a large proportion of light reflected from these meteoric bodies. In this way the peculiar character of the coronal spectrum may be readily accounted for. We know, from the auroral spectrum, that the principal bright lines due to the electrical discharges would be precisely where we see bright lines in the coronal spectrum. But, besides these, there would be fainter bright lines corresponding to the various elements which exist in the meteoric masses. These elements, we know, are the same as those in the substance of the sun. Thus the bright lines would correspond in position with the dark lines of the solar spectrum. Hence, as light reflected by the meteors would give the ordinary solar spectrum, there would result from the combination a continuous spectrum, on which the bright lines first mentioned would be seen, as during the American eclipse.
What the polariscope has told us respecting the corona is in accordance with this view.
In the same way the quality of the Zodiacal Light admits of being perfectly accounted for, without resorting to the hypothesis that this phenomenon is a terrestrial one.
The explanation thus put forward has at least the advantage of being founded on well-established relations. We know that the auroral light is associated with the earth’s magnetism, and that meteoric bodies are continually falling upon the earth’s atmosphere. We know, also, that the sun exerts magnetic influences a thousand-fold more intense than those of the earth, and that in his neighborhood there must be many million times more meteoric systems.
But we have other and independent reasons, which must not be overlooked, for considering the corona to be of some such nature as I have suggested. Leverrier has shown that there probably exists in the neighborhood of the sun a family of bodies whose united mass suffices appreciably to affect the motions of the planet Mercury. It would not be safe to neglect considerations thus vouched for.
Mr. Baxendell also has shown that certain periodic variations in the earth’s magnetism point to the existence of such a family of bodies; and he has been able to assign to them a position according well with that determined by Leverrier.
Now, whatever opinion we form as to the exact character of the system of bodies pointed to by the researches of Leverrier and Baxendell—whether we suppose that system to form a zone around the sun, or that (as I believe) the system is merely due to the aggregation of meteoric perihelia in the sun’s neighborhood—we may be quite certain of this, that during a total solar eclipse the system could not fail to become visible. Hence there is a double objection to the view put forward by Mr. Lockyer and others. In the first place, it fails to account for the appearance presented by the corona; in the second place, it fails to render an account of the implied non-appearance of the system which, according to the researches of Leverrier and Baxendell, circles around the sun.
[Illustration: Fig. 34.—Scale of Planets
_Jupiter and Saturn are shown in their true axial positions, Uranus and Neptune in the axial positions inferred from the motions of their satellites_]
We know that the sun is the sole source whence light and heat are plentifully supplied to the worlds which circle around him. The question immediately suggests itself—Whence does the sun derive those amazing stores of force from whence he is continually supplying his dependent worlds? We know that, were the sun a mass of burning matter, he would be consumed in a few thousand years. We know that, were he simply a heated body, radiating light and heat continually into space, he would in like manner have exhausted all his energies in a few thousand years—a mere day in the history of his system. Whence, then, comes the enormous supply of force which he has afforded for millions on millions of years, and which also our reason tells us he will continue to afford while the worlds which circle around him have need of it—in other words, for countless ages to come?
Now, there are two ways in which the solar energies might be maintained. The mere contraction of the solar substance, Helmholtz tells us, would suffice to supply such enormous quantities of heat that, if the heat actually given out by the sun were due to this cause alone, there would not, in many thousands of years, be any perceptible diminution of the sun’s diameter. But, secondly, the continual downfall of meteors upon the sun would cause an emission of heat in quantities vast enough for the wants of all the worlds circling round him; while his increase of mass from this cause would not be rendered perceptible in thousands of years, either by any change in his apparent size or by changes in the motions of his family of worlds.
It seems far from unlikely that both these processes are in operation at the same time. Certainly the latter is, for we know, from the motions of the meteoric bodies which reach the earth, that myriads of these bodies must continually fall upon the sun. And if the corona and Zodiacal Light really be due to the existence of flights of meteoric systems circling around the sun, or to the existence in his neighborhood of the perihelia of many meteoric systems, then there must be a supply of light and heat from this source very nearly if not quite sufficient to account for the whole solar emission.
It is well worthy of notice, too, that the association between meteors and comets has an important bearing on this question. We know that the most remarkable characteristic of comets is the enormous diffusion of their substance. Now, in this diffusion there resides an enormous fund of force. The contraction of a large comet to dimensions corresponding to a very moderate mean density would be accompanied by the emission of a vast supply of heat. And the question is worth inquiring into, whether we can indeed assume that the meteors which reach our atmosphere are solid bodies, and not rather of cometic diffusion; since it is difficult otherwise to account for the light and heat which they emit. Friction through the rarer upper strata of our atmosphere will certainly not account for these phenomena; nor, I think, will the compression of the atmosphere in front of the meteors; on the other hand, the sudden contraction of a diffused vapor would be accompanied by precisely such results. But, be this as it may, it is certain that a large portion of the substance of every comet is in a singularly diffused state. And since the meteoric systems circling in countless millions round the sun are, in all probability, associated in the most intimate manner with comets, we may recognize in this diffusion, as well as in the mere downfall of meteors, the source of an enormous supply of light and heat.
And lastly, turning from our sun to the other suns which shine in uncounted myriads throughout space, we see the same processes at work upon them all. Each star-sun has its coronal and its zodiacal disks, formed by meteoric and cometic systems; for otherwise each would quickly cease to be a sun. Each star-sun emits, no doubt, the same magnetic influences which give to the Zodiacal Light and to the solar corona their peculiar characteristics. And thus the worlds which circle round those orbs may resemble our own in all those relations which we refer to terrestrial magnetism, as well as in the circumstance that on them also there must be, as on our own earth, a continual downfall of minute meteors. In those worlds, perchance, the magnetic compass directs the traveler over desert wastes or trackless oceans; in their skies, the aurora displays its brilliant streamers; while, amid the constellations which deck their heavens, meteors sweep suddenly into view, and comets extend their vast length athwart the celestial vault, a terror to millions, but a subject of study and research to the thoughtful.
FOOTNOTES:
[23] Professor Kirkwood has published a most interesting series of inquiries, going far to prove that the real secret or the planetary influences lies in the fact that the sun’s surface is not uniform, and that on a certain solar longitude the planetary influences are more effective than elsewhere.
[24] To these may be added the following law:
4. Light reflected from any opaque body gives the same spectrum as it would have given before reflection.
5. But if the opaque body be surrounded by vapors, the dark lines corresponding to these vapors make their appearance in the spectrum with a distinctness proportioned to the extent to which the light has penetrated those vapors before being reflected to us.
6. If the reflecting body be itself luminous, the spectrum belonging to it is superadded to the spectrum belonging to the reflected light.
7. Glowing vapors surrounding an incandescent source of light may cause bright lines or dark lines to appear in the spectrum, according as they are more or less heated; or, they may emit just so much light as to make up for what they absorb, in which case there will remain no trace of their presence.
8. The electric spark presents a bright-line spectrum, compounded of the spectra belonging to the vapors of those substances between which, and of those through which, the discharge takes place. According to the nature of these vapors and of the discharge itself, the relative intensity of the component parts of the spectrum will be variable.
Lastly, the appearance of the spectrum belonging to any element will vary according to the circumstances of pressure and temperature under which the element may emit light.
[25] It is also shown most conclusively, by a photograph of the eclipse of August, 1868, taken an instant before the totality. Here we see the glare trenching upon the moon’s disk (elsewhere black), as it should theoretically. So soon as totality commenced, the glare had reached the moon’s limb, whence it must immediately have passed quickly away.
[26] In fact, if we take the mode of reasoning by which Mr. Lockyer has endeavored to get over certain physical difficulties presently to be mentioned, we shall be able to point definitely to the place where his argument fails. He says, conceive a tiny moon placed so as to appear coincident with the centre of the sun’s disk. There will be atmospheric glare as well as direct sunlight. Now, conceive this small moon to expand until it all but covers the sun. Still there will be glare and a certain small proportion of direct sunlight. So far his reasoning is most just. But when he allows his expanding moon to cover the sun, and to extend beyond the solar disk as in total eclipse, the atmospheric glare can no longer be assumed to exist all round the expanding moon: at the moment when the moon just hides the sun, the glare begins to leave the moon, a gradually expanding black ring being formed round that body. It is only necessary to consider where the glare comes from to see that this must be so.
I have taken no account of diffraction here, because it has been abundantly proved that no corona of appreciable width could be formed around the moon during total eclipse by the diffraction of the rays of light as they pass near the moon’s limb.
MERCURY.—WILLIAM F. DENNING
Mercury is the nearest known planet to the sun. It is true that a body, provisionally named Vulcan, has been presumed to exist in the space inferior to the orbit of Mercury; but absolute proof is lacking, and every year the idea is losing strength in the absence of any confirmation of a reliable kind. Not one of the regular and best observers of the sun has recently detected any such body during its transits (which would be likely to occur pretty frequently), and there is other evidence of a negative character; so that the ghost of Vulcan may be said to have been laid, and we may regard it as proven that no major planet revolves in the interval of 36,000,000 miles separating Mercury from the sun.
Copernicus, amid the fogs of the Vistula, looked for Mercury in vain, and complained in his last hours that he had never seen it. Tycho Brahe, in the Island of Hueen, appears to have been far more successful. The planet is extremely fugitive in his appearances, but is not nearly so difficult to find as many suppose. Whenever the horizon is very clear, and the planet well placed, a small sparkling object, looking more like a scintillating star than a planetary body, will be detected at a low altitude and may be followed to the horizon.
Mercury revolves round the sun in 87 days, 23 hours, 15 minutes, and 44 seconds in an eccentric orbit, so that his distance from that luminary varies from 43,350,000 to 28,570,000 miles. When in superior conjunction the apparent diameter of the planet is 4″.5; at inferior conjunction it is 12″.9, and at elongation 7″. His real diameter is 3,000 miles.
Being situated so near to the sun, it is obvious that to an observer on the earth he must always remain in the same general region of the firmament as that body. His orbital motion enables him to successively assume positions to the east and west of the sun, and these are known as his elongations, which vary in distance from 18° to 28°. He becomes visible at these periods either in the morning or evening twilight, and under the best circumstances may remain above the horizon two hours in the absence of the sun. The best times to observe the planet are at his E. elongations during the first half of the year, or at his W. elongations in the last half; for his position at such times being N. of the sun’s place, he remains a long while in view.
Occasionally he presents quite a conspicuous aspect on the horizon, as in February, 1868, when I thought his lustre vied with that of Jupiter, and in November, 1882, when he shone brighter than Sirius. The planet is generally most conspicuous _a few mornings after his W. elongations and a few evenings before his E. elongations_.
In the course of his orbital round, Mercury exhibits all the phases of the moon. Near his elongations the disk is about half illuminated, and similar in form to that of our satellite when in the first or third quarter. But the phase is not to be distinctly made out unless circumstances are propitious. Galileo’s telescope failed to reveal it, and Hevelius, many years afterward, found it difficult. This is explained by the small diameter of the planet and the rarity with which his disk appears sharply defined. The phase is sometimes noted to be less than theory indicates; for the planet has been seen crescented when he should have presented the form of a semicircle. Several observers have also remarked that his surface displays a rosy tint, and that the terminator is more deeply shaded and indefinite than that of Venus.
The atmosphere of Mercury is probably far less dense than that of Venus. The latter being furthest from the sun might be expected to shine relatively more faintly than the former, but the reverse is the case. Mercury has a dingy aspect in comparison with the bright white lustre of Venus. On May 12, 1890, when the two planets were visible as evening stars, and separated from each other by a distance of only 2°, I examined them in a 10-inch reflector, power 145. The disk of Venus looked like newly polished silver, while that of Mercury appeared of a dull leaden hue. A similar observation was made by Mr. Nasmyth on September 28, 1878. The explanation appears to be that the atmosphere of Mercury is of great rarity, and incapable of reflection in the same high degree as the dense atmosphere of Venus.
As a naked-eye object, Mercury must necessarily be looked for when near the horizon; but there is no such need in regard to telescopic observation, which ought to be only attempted when the planet surmounts the dense lower vapors and is placed at a sufficient elevation to give the instrument a fair chance of producing a steady image. The presence of sunshine need not seriously impair the definition, or make the disk too faint for detail.
I have occasionally seen Mercury, about two or three hours after his rising, with outlines of extreme sharpness and quite comparable with the excellent views obtained of Venus at the time of sunrise or sunset. Those who possess equatorials should pick up the planet in the afternoon and follow him until after sunset, when the horizontal vapors will interfere. Others who work with ordinary altazimuth stands will find it best to examine the planet at his western elongations during the last half of the year, when he may be found soon after rising by the naked eye or with an opera-glass, and retained in the telescope for several hours after sunrise if necessary.
Mercury was displayed under several advantages in the morning twilight of November, 1882, and I made a series of observations with a 10-inch reflector, power 212. Several dark markings were perceived, and a conspicuous white spot. The general appearance of the disk was similar to that of Mars, and I forwarded a summary of my results to Professor Schiaparelli of Milan, who favored me with the following interesting reply:
“I have myself been occupied with this planet during the past year (1882). You are right in saying that Mercury is much easier to observe than Venus, and that his aspect resembles Mars more than any other of the planets of the Solar System. It has some spots which become partially obscured and sometimes completely so; it has also some brilliant white spots in a variable position.”
Professor Schiaparelli used an 8½-inch refractor in this work, and was able under some favorable conditions to apply a power of 400. The outcome of his researches, encouraged since 1882 by the addition of an 18-inch refractor to the appliances of his observatory, was announced in the curious fact that the rotation of Mercury is performed in the same time that the planet revolves round the sun! If this conclusion is just, Mercury constantly presents one and the same hemisphere to the sun, and the behavior of the moon relatively to the earth has found an analogy.
Spots or markings of any kind have rarely been distinguished on Mercury. On June 11, 1867, Prince recorded a bright spot, with faint lines diverging from it northeast and south. The spot was a little south of the centre. Birmingham on March 13, 1870, glimpsed a large white spot near the planet’s east limb, and Vögel, at Bothkamp, observed spots on April 14 and 22, 1871. These instances are quoted by Webb, and they, in combination with the markings seen by Schiaparelli at Milan and by the author at Bristol in 1882, sufficiently attest that this object deserves more attentive study.
One of the most interesting phenomena, albeit a somewhat rare event, in connection with Mercury, is that of a transit across the sun. The planet then appears as a black circular spot. Observers have noticed one or two very small luminous points on the black disk, and an annulus has been visible round it. These features are probably optical effects.
THE PLANET VENUS.—CAMILLE FLAMMARION
Revolving round the sun in 224 days, Venus has its motion combined with ours in such a manner that it passes its inferior conjunction, between the sun and us, every 584 days; but the plane in which it revolves is inclined 3° 23′ to that in which the earth itself moves. When Venus attains its greatest elongations from the sun it shines in the west in the evening, then in the morning in the east, with a splendid brightness which eclipses that of all the stars. It is, without comparison, the most magnificent star of our sky. Its light is so vivid that it casts a shadow. Sometimes, even, it pierces the azure of the sky, in spite of the presence of the sun above the horizon, and _shines in full daylight_.
The maximum visibility of Venus is produced by its greatest phase, by its greatest elongation from the sun, and by the clearness of our atmosphere.