Zodiacal Light, a triangular track of light, seen within the tropics, after sunset and before sunrise, stretching up from the horizon 50° or more according to the season, its axis nearly or quite corresponding with the ecliptic. It is of a warm, yellowish tint, its light stronger at the central parts, and diffused toward the boundaries. In higher latitudes it is visible under favorable circumstances during spring and autumn. It is most conspicuous when the ecliptic makes the greatest angle with the spectator's horizon, at which time in moderate latitudes it reaches nearly to the zenith, having near the horizon a striking brilliancy, and thence fading upward. Near the equator it often has at the horizon a brilliancy equal to the sky in the east as the sun is about to rise. - The few ancient records of this phenomenon are unsatisfactory. Pliny has been thought to allude to it under the name of trales, though Humboldt dissents from this supposition. Kepler described it, and supposed it to be the atmosphere of the sun.
Dominique Cassini began to notice it in 1683, and during 11 years accumulated a greater mass of observations than all others together up to those of Piazzi Smyth at Cape Town in 1845, and later at Teneriffe, and of Jones in 1853-'5. Cassini, finding, as he supposed, that the northern edge of the light bent away more and more from the ecliptic during March and April, when the sun's equator was similarly increasing its inclination to the ecliptic, concluded the cause to be a solar emanation; and this opinion has biassed and misled astronomers ever since. He assigned to this emanation a lenticular shape, having in June a diameter equal to that of the sun, and in March twice as great. Cassini gave to the phenomenon the name it now bears. It was noticed in 1731 by Mairan, who considered it to be a reflection from the sun's atmosphere stretched out into a flattened spheroid. But Laplace has demonstrated that this is impossible from the extent of the heavens covered by the light, taken in connection with the fact that the sun's atmosphere can extend no further 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 25½ days; that is to say, only as far as 9/20 of Mercury's distance from the sun.
The theory of this philosopher, which astronomers have generally adopted, is thus presented in his Systeme du mondey in connection with his famous doctrine of the genesis of the solar system (see Nebulae Hypothesis): "If in the zones abandoned by the atmosphere of the sun there are any molecules too volatile to be united to each other or to the planets, they ought, in continuing to circulate around this star, to offer all the appearances of the zodiacal light without opposing any sensible resistance to the different bodies of the planetary system, either on account of their extreme rarity, or because their motion is nearly the same as that of the planets with which they come in contact." This rotating ring Laplace supposed to be somewhere between the orbits of Venus and Mercury. All these theories are based on Cassini's erroneous conclusion that the axis of this light has a fixed relation to the sun's equator. The remarkable meteor shower of 1833 gave an impulse to speculations respecting the zodiacal light. It was suspected that this meteoric display was owing to the passage of the earth through the substance of the light. This theory found an advocate in Biot, who argued that the earth then passed near the node of this substance.
This led J. C. Houzeau to question the justice of Cassini's conclusion, and in 1844 he announced in the Astronomische NacJiricMen that "the supposition of the existence of this light in the plane of the sun's equator does not satisfy the observations made," and that the cause of the appearance "may be more local than has been hitherto supposed." Prof. C. Piazzi Smyth gives, in the "Transactions of the Royal Society of Edinburgh," vol. xx., paft iii., an account of valuable observations made at the Cape of Good Hope in 1845.. - In April, 1853, the Rev. George Jones, chaplain of the United States Japan expedition, began in the Pacific ocean a series of observations, which were conducted almost daily during two years, with results of considerable import; 341 successful observations Mere made, all of which were charted down. They are especially valuable from being, in the observer's language, "independent of hypotheses, and independent of each other." These charts, together with accompanying explanations, were published as a supplementary volume in the report of that expedition. Humboldt and others had noticed intermittent variations in the lustre of the light, not in the nature of pulsations so much as of a rapid fading away, and a gradual brightening again.
This appearance is confirmed by Mr. Jones, who speaks of a swelling out laterally and upward of the pyramid, with an increase of brightness in the light itself; then in a few minutes a shrinking back of the boundaries and a dimming of the light, almost at times as if quite dying away; and so back and forth for about three quarters of an hour. The light, though stronger at the central parts, does not shade off uniformly to the borders, but has two distinct degrees of lustre - a triangle within a triangle - two different kinds of light as it were, as if the matter was more condensed at its central parts and thinned out beyond. The inner is termed by Mr. Jones the stronger light, and the outer the diffuse light. These are not bounded by sharp lines, but melt away by degrees; still there is between the two a line of greater suddenness of transition, while the experienced eye has no difficulty in tracing the outer boundary of the diffuse light. The stronger he found to be approximately 60° in its greatest width, and the diffuse 90°. The data furnished by these observations were as follows: 1, when his position was N. of the ecliptic, the main body of the zodiacal light was on the N. side of that line; 2, when his position was 8. of the ecliptic, the main body of the zodiacal light was on the S. side of that line; 3, when his position was on or near the ecliptic, the light was equally divided by the ecliptic, or nearly so; 4, when by the earth's rotation on its axis he was during the night carried rapidly to or from the ecliptic, the change of the apex and of the direction of the boundary lines was equally great, and corresponded to his change of place; 5, as the ecliptic changed its position as respects the horizon, the entire shape of the light became changed, which would result from new portions of the nebulous matter coming into position for giving him visible reflection, while portions lately visible were no longer giving him such a reflection.
The first four of these results were not absolutely invariable, but the exceptions were few. Mr. Jones inferred from these observations that the zodiacal light is caused by a ring of matter surrounding the earth, not the sun; for the changes resulting from the observer's change of position on the earth, as well as from the change of position caused by the earth's rotation, seemed to him much greater than could be explained if the ring were not relatively near to the earth. These changes of appearance also seemed to correspond in character with the theory thus advanced. But it is certain that no ring surrounding the earth could possibly explain the phenomena of the zodiacal light when these are all considered together, however competent to explain the particular phenomena observed by Mr. Jones. It is to be noted in particular that the phenomena observed in high latitudes, though not so striking as those observed in low latitudes, are in reality even more instructive. It will be manifest that if there were a ring surrounding the earth at a distance so moderate that a traveller in tropical regions could recognize the zodiacal's change of position as he passed from the northern to the southern side of the equator, it would be invisible from places in high latitudes.
This is clearly shown in the writer's treatise on Saturn, where the configuration of the rings viewed from different Saturnian latitudes has been carefully calculated (not merely surmised from general considerations). Even in moderately high latitudes the zodiacal, if Mr. Jones's theory were sound, ought to be seen far toward the S. point of the horizon; whereas, so far is this from being the case, that in England the average position of the zodiacal's axis in the horizon is nearly identical with the ecliptic. - The most probable interpretation of the zodiacal light is that which regards it as caused by multitudes of minute bodies travelling around the sun. At the same time two points must bo carefully noted. In the first place, there are phenomena of the zodiacal which indicate some resemblance between its structure and that of comets' tails, so that not meteoric matter alone, but cometic matter also, is probably present in it. Secondly, it is highly improbable that the greater portion of the matter forming the zodiacal light travels on orbits of small eccentricity around the sun.
Knowing that the orbits of meteors extend far out into space, even beyond the orbits of Uranus and Neptune, we must suppose the meteoric and cometic matter of the zodiacal to travel on paths similarly eccentric, so that the matter composing the zodiacal light at one time will at another be far beyond the bounds of its visible extent. This, indeed, so far from introducing a difficulty, helps to remove one. For it is manifest that according to this theory the zodiacal light should vary markedly in appearance from time to time, which is precisely what had been observed and had remained unexplained until the eccentric nature of meteoric orbi.ts was recognized. The spectrum of the zodiacal appears not to be monochromatic as Angstrom supposed, but continuous, indicating that the zodiacal light is reflected sun light.