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.'