Page:The American Cyclopædia (1879) Volume XVI.djvu/861

ZODIACAL LIGHT 831 ill ; Sagittarius, the Archer, ; Capricornus, the Sea Goat, -V3 ; Aquarius, the Water-Bearer, $? and Pisces, the Fishes, ^. These names were given from a fanciful resemblance to the objects designated which was supposed to be presented by the configuration of the stars. This division is still employed. The width of the zodiac was originally determined by the widest range of planetary motion N. and S. of the ecliptic, for Venus is at times nearly 9 N. of the ecliptic and at times nearly 9 S. of that circle. If the motions of the planets between Mars and Jupiter were considered, the zodiac would have to be nearly 70 wide instead of 18, and would cover much more than half of the celestial sphere. 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 trabes, though Humboldt dissents from this supposition. Kep- ler described it, and supposed it to be the at- mosphere of the sun. Dominique Cassini be- gan 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 incli- nation 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. Cas- sini 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 sphe- roid. 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 25 days ; that is to say, only as far as 849 VOL. xvi. 53 ^ of Mercury's distance from the sun. The theory of this philosopher, which astronomers have generally adopted, is thus presented in his Systeme du monde, in connection with his fa- mous doctrine of the genesis of the solar sys- tem (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 with- out 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 equa- tor. The remarkable meteor shower of 1833 gave an impulse to speculations respecting the zodiacal light. It was suspected that this me- teoric 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. 0. Houzeau to question the justice of Cassini's conclusion, and in 1844 he announced in the Astronomische Nachrichten that " the supposition of the ex- istence 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 " Transac- tions of the Eoyal Society of Edinburgh," vol. xx., part iii., an account of valuable observa- tions made at the Cape of Good Hope in 1845. In April, 1853, the Rev. George Jones, chap- lain of the United States Japan expedition, be- gan in the Pacific ocean a series of observa- tions, which were conducted almost daily du- ring two years, with results of considerable import ; 341 successful observations were made, all of which were charted down. They are especially valuable from being, in the ob- server's language, "independent of hypothe- ses, and independent of each other." These charts, together with accompanying explana- tions, were published as a supplementary vol- ume in the report of that expedition. Hum- boldt and others had noticed intermittent va- riations 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,