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| No. 449. New Series. | SATURDAY, AUGUST 7, 1852. | Price 1½d. |
THE ZODIACAL LIGHT.
There is a certain degree of satisfaction to the inquiring mind in knowing that, even in these days of aptness for discovering and explaining everything, there yet remains something to be found out; something to excite speculation and recompense research. Such a subject is the zodiacal light, which, for nearly two centuries past, has at different times occupied the attention of astronomers and other observers of celestial phenomena, though it is only of late years that the theories concerning it have acquired anything like a precise character. Many ingenious hypotheses have been thrown out, which may perhaps be accepted as steps towards a true explanation; and while waiting the result of further inquiry, we shall endeavour to make our readers acquainted with the interesting phenomenon.
The zodiacal light is a peculiar brightness, pyramidal or wedge-like in form, seen at certain periods of the year in the eastern or western sky, before sunrise and after sunset. Its direction is in the line of the zodiac, whence its name—not perpendicular to the horizon, but at a varying angle, being in the spring from 60 to 70 degrees. The base of the wedge, which has a breadth generally of from 10 to 12 degrees, is below, and the sides rise in a line, curving outwards, to the apex, but so vague and diffuse as to be frequently indefinable. In our latitudes, it is best seen at or just after the equinoxes; before sunrise in autumn, and after sunset in spring; and becomes invisible as twilight increases, or if the moon shines; the light even of Venus and Jupiter is sufficient to render its discovery difficult. It is brightest at the base, and grows fainter the further it stretches from the horizon, vanishing entirely at the point. Unpractised observers would be apt to overlook it altogether, and those accustomed to watch the heavens are at times obliged to fix one eye on a dark space of sky, while they search for the light with the other, and discover it only by the contrast. A stratum of black cloud resting on the horizon often affords a means of detection, as the light can then be seen shooting from it with comparative distinctness. The soft, clear atmosphere which usually precedes or follows rain, is very favourable to a view of the light.
The luminous wedge varies in length with the progress of the seasons: sometimes but little more than its point is visible; at others, it is seen extending over a space of 120 degrees. Astronomically speaking, the axis of the zodiacal light is said to lie in the plane of the solar equator, with an angle of more than 7 degrees to the ecliptic, which it consequently intersects, the points of intersection becoming its nodes, and these nodes are the parts through which the earth passes in March and September. The light travels forward along the zodiacal signs from Gemini to Cancer and Leo from August to November, keeping pace with the sun. It grows dim towards the end of November, and fades more and more until January; but while this decrease has been going on in the east, and in the morning, the light has presented itself with increasing brightness in the west, and in the evening, and pursues its course until the end of February at about the same rate of motion. In March, it is slow, and travels through not more than one sign, and fades in April, and is lost in May, to reappear again at the end of summer, and perform the same route.
Lengthened twilight is not favourable to the appearance of the zodiacal light; it can, therefore, be observed successfully in the temperate latitudes only by patient and long-continued watching. But in tropical regions, the deep azure of the sky, and the brief twilight, give it a distinctness and luminosity never witnessed elsewhere. In Egypt, we are told it is clearly 'visible every night, except when the light of the moon is too great, from January to June;' and in India its appearance is described as that of 'a pyramid of faint aurora-borealis like light' usually preceding the dawn. Humboldt tells us, that he has seen it shine with greater brightness than the Milky Way, from different parts of the coast of South America, and from places on the Andes more than 13,000 feet above the sea-level.
'Those who have dwelt long,' he writes, 'in the zone of palms, must retain a pleasing remembrance of the mild radiance of this phenomenon, which, rising pyramidally, illumines a portion of the unvarying length of the tropical nights.' And once, during a voyage from Lima to Mexico, he saw it in greater magnificence than ever before. 'Long narrow clouds, scattered over the lovely azure of the sky, appeared low down in the horizon, as if in front of a golden curtain, while bright varied tints played from time to time on the higher clouds: it seemed a second sunset. Towards that side of the heavens, the light diffused appeared almost to equal that of the moon in her first quarter.'
The zodiacal light can hardly fail of having been observed by astronomers in the past ages of the world; but the earliest known mention of it occurs in the Britannia Baconica, published by Childrey in 1661. The writer says: 'There is another thing which I recommend to the observation of mathematical men—which is, that in February, and for a little before and a little after that month—as I have observed for several years together—about six in the evening, when the twilight hath almost deserted the horizon, you shall see a plainly discernible way of the twilight, striking up towards the Pleiads, and seeming almost to touch them. It is so observed any clear night, but it is best illæ nocte. There is no such way to be observed at any other time of the year. But what the cause of it in nature should be, I cannot yet imagine, but leave it to further inquiry.' The further inquiry followed soon afterwards, for Cassini, the eminent French astronomer, having carefully observed the phenomenon from 1683 to 1688, communicated the results to the Académie des Sciences. Some of his views and determinations were well founded; and from them we gather that the zodiacal light was nearly or quite the same in his day as at present. Others also devoted considerable attention to it, and noticed the variations in brightness in different years, which subsequent observations have verified. Since then, it has been made more or less a subject of investigation by modern astronomers, and has been observed in many parts of the world; the first observations in the southern hemisphere being those made by Professor Smyth at the Cape of Good Hope, from 1843 to 1845. In that latitude, the zodiacal light is best seen in spring evenings, at an angle of 30 degrees, visible long after sunset; its opposite peak is discernible at daybreak, but has scarcely come into view before the rising sun overpowers it. In autumn, the reverse takes place; the best appearance is in the morning.
To understand what is meant by the 'opposite peak,' we are to regard the zodiacal light, of which we see only one end in our latitudes, as a body extending all round the sun in the same form, presenting at a distance the appearance of one of those flat elongated oval nebulæ seen in the heavens. Its direction is at right angles to that of the sun's rotation, a straight line drawn from either pole of the great luminary divides it in the centre. From its outline resembling that of a lens in section, it is frequently described as a 'cosmical body of lenticular form.'
From this account of what the zodiacal light appears to be, we proceed to consider what it is. Some inquirers—arguing from the 'nebular theory,' which assumes the formation of the several planets, one after another, from nebulous matter—have supposed the zodiacal light to be a remnant of that matter yet unconcentrated. In this view, it may be a nebula, brightest in the centre, as is the case with most, and fainter towards the margin. According to Humboldt, 'we may with great probability attribute the zodiacal light to the existence of an extremely oblate ring of nebulous matter, revolving freely in space between the orbits of Venus and Mars.' On several occasions he witnessed its fluctuations, night after night, from the plains of South America, shewing itself at times greatly collapsed or condensed, with intermittences of vividness and faintness, in the course of a few minutes, as is observed of the aurora. The light of the stars, of even the fifth or sixth magnitudes, can be seen through it: the same has been remarked of comets; and it is known also that the tails of comets undergo frequent flashings or pulsations, so that the two phenomena may be analogous in character. It is necessary, however, to distinguish the fluctuations from such effects as may be produced by movements in the lower strata of the atmosphere.
Mairan, who wrote in 1731, was of opinion that the zodiacal light consisted of particles thrown off from the sun by its rapid rotation, or a species of atmosphere peculiar to the central orb. Others have supposed the luminosity to be composed of 'revolving planetary particles,' shining by a direct or reflected light. But, according to Professor Olmsted, of Yale College, Massachusetts, it is something which has a motion of its own around the sun, notwithstanding that the general steadiness of its movements had warranted the notion that it was in some way attached to the body of the sun itself. Olmsted's conclusions are drawn from a diligent observation of the light during a period of six years, and are on this account, as well as from his scientific reputation, entitled to respect. He states the light to be, in constitution, colour, and density, similar to that of the tail of a comet, the portion nearest the sun being brightest, and both admitting of stars being seen through them. We may, therefore, infer it to be a nebulous ring surrounding the sun, in the same way that the magnificent rings of Saturn surround that planet. Of such nebulæ as this there are from 2000 to 3000 visible in the regions of space, compared with which the dimension of ours is insignificant: at the same distance, and sought for with the same instruments, it would be invisible.
In one point, Professor Olmsted's views are particularly interesting, as, out of one mysterious phenomenon, he endeavours to explain another, and inquires: 'Whether or not the zodiacal light is the origin of the meteoric showers of November and August, and especially those of November?' Many readers know that for some years past great numbers of falling-stars, or showers of meteors, have been observed periodically in November: the fall seen in the United States in 1834—when, as is estimated, more than 240,000 stars fell as thick as snow-flakes, in the space of nine hours—being the most remarkable hitherto known. The explanation is, that the zodiacal light is a nebulous body revolving round the sun, and arrives at its aphelion on the 13th November in that part of the earth's orbit which the earth then reaches, and, coming into contact with our atmosphere, portions of the nebulous matter are detached, and, taking fire as they pass through, appear to us as shooting-stars. This explanation of the phenomenon in question is one not hastily conceived; the reasoning on which it is founded is altogether satisfactory, as well with regard to the movement of the nebulous matter, as to that of the earth.
Professor Olmsted, in a communication addressed to the 'American Association for the Advancement of Science,' sums up his views as follows:
'1. The zodiacal light, as we have found, in our inquiry into its nature and constitution, is a nebulous body.
'2. It has a revolution round the sun.
'3. It reaches beyond and lies over the earth's orbit at the time of the November meteors, and makes but a small angle with the ecliptic.
'4. Like the nebulous body, its periodic time is commensurable with that of the earth, so as to perform a certain whole number of revolutions while the earth performs one, and thus to complete the cycle in one year, at the end of which the zodiacal light and the earth return to the same relative position in space. This necessarily follows from the fact, that at the same season of the year it occupies the same position one year with another, and the same now as when Cassini made his observations nearly 170 years ago.
'5. In the meteoric showers of November, the meteors are actually seen to come from the extreme portions of the zodiacal light, or rather a little beyond the visible portions.'
There is much that is suggestive in this summary, and, as we said at the commencement, the subject is one of a nature to stimulate inquiry and research, and to lead to further explanations of cosmical phenomena. M. Mathieson's observations, published in the Comptes Rendus of the Académie des Sciences for 1843, shew, that when tested with the thermo-multiplier, the zodiacal light was found to radiate heat as well as light—a fact which, if further verified, will support the evidence in favour of an independent luminous ring.