ZODIACAL LIGHT. °The zodiacal light is a faint column of light rising from the west ern horizon of ter twilight in winter or spring evenings; and before daybreak in summer or autumn. It extends out on each side of the sun, and lies nearly in the plane of the ecliptic. Near the equator it can be seen all the year, and has been traced all the across the heavens from cast to west, forming a complete ring.' In our latitudes it can seldom be traced more than 90° from the sun. Parts of the column near the sun arc somewhat bright, but distant portions are extremely and can only be seen on the very clearest nights; though the cause of the zodiacal light cannot be said to been definitely proved, it is very probable that it is sunlight reflected by myriads of small meteoric bodies revolving around the sun, nearly in the plane of the ecliptic. This theory re quires a thin flat ring of these meteors to ex tending heyond the orbit of the earth. The name zodiacal light was given to this phenome non by Cassini in 1653, who described it as a flat luminous ring encircling the sun nearly in the plane of the ecliptic. Kepler supposed it to be the atmosphere of the sun; but Laplace showed that the atmosphere of the sun could not extend to anything lace the distance from the sun which is reached by the zodiacal light. In 1853 an extended series of observations was made by Jones from different parts of the Pacific Ocean. From these observations he deduced the theory that the zodiacal light was caused by a ring of matter surrounding the earth, and not the sun.
Professor Wright, of Yak University. has determined that die spectrum of the light is continuous, and therefore is essentially reflected sunlight. He has also determined that die light is partially polarized in a plane passing through the sun, and that the amount of the polarization is between IS and 20 per cent. The origin of the minute particles which reflect the light has been accounted for in many ways. By some they arc believed to have been thrown out from the corona of the sun; by others to be com posed of dust thrown out from the equatorial regions of the sun; but by most they are be lieved to be an immense cloud of meteoroids filling the space between the earth and the sun. The meteoric theory of the sun's heat presup poses a multitude of these meteoric bodies con stantly falling into the sun to supply the loss by radiation, as well as multitudes of others which never reach the surface. It is not prop able, however, that these meteors if they exist play any part in the phenomena of the zodiacal light.
A most valuable and important contribution to our knowledge of the zodiacal light is due to a recent research of Seeliger. It has lmg been known that the slow, secular changes of some of the elements of the orbits of the plan ets of the solar system, when these are derived from observation, are not in exact agreement when these same changes are determined from a computation based upon the law of universal gravitation. In particular, it is found that the
computed motions of the perihelia of the orbits of Mercury and Mars and of the node of Venus differ from the motions derived from observa tion by amounts far in excess of the uncertain ties inherent in the observations themselves. Many attempts to account for the first discrep ancy have been made. Thus, Leverrier assumed that it arose from the disturbing pull of a small planet near the sun, which for many years found its place in our astronomical books under the name of Vulcan. Other astronomers have supposed that the disturbance might be due to the pull of a ring of particles about the sun, to an unequal distribution of the material within the body of the sun itself, or even to an inex actness in the accepted statement of the law of gravity. Analysis shows, however, that none of these explanations are admissible, for all of them lead to disturbances in other directions, either of the planets or of the moon, which are in discordance with observation. But Seeliger has shown that if there be ascribed to the zo diacal light an even almost inconceivable tenuity, its gravitational pull will exactly account for the discrepancies observed, without at the same time introducing any new discrepancies. The densi ties to be assumed for the inner and outer por tions of the zodiacal light in order that its at traction may be sufficient to balance the former discrepancies are as follows: Average density of the inner portion, extend ing from the sun to 0.24X, the distance of the earth 2 52 X X, the sun's density; Average density of the outer portion, extend ing from the sun to 1.20X, the distance of the earth .•-• 2.52 X X, the sun's density. Aver age density of the outer portion, extending from the sun to 1.20 X, the distance of the earth = 0.0036 X ler" X, the sun's density.
ZOR, eh% (Gr. Z4-4), empress of the East: b. about 978; d. 1050. She was the daughter of Constantine VIII. and became the wife of Ro mantis III in 10213. In 1034 Roir.anns was put to death by Zoe and Michael the Paphlagonian, whom she married and raised to the throne as Michael IV. The latter dying, was succeeded by his nephew, Michael V, who was deposed by the people in 1042. Zoe and her sister Theodora were then proclaimed joint sovereigns. She dis played great ability and firmness in the govern man, and in 1042 married Constantine IX, Monomachus, who supplanted Theodora- She reigned till her death.