ZODIACAL LIGHT, a luminous appearance seen at certain times after sunrise and before sunset, from which it is inferred that there is a slight degree of nebulosity about the sun, if indeed it do not arise from the denser parts of that medium which [Comm.] is mere than conjectured to occupy the spaces in which the bodies of our system move.
The following description of the zodiacal light is from the pen of that careful observer, Sir John Herschel :—" The zodiacal light, as its name imports, invariably appears in the zodiac, to speak mole precisely, in the plane of the sun's equator, which is 7 degrees inclined to the zodiac, and which plane, seen from the sun, intersects the ecliptic in longitude 78° and 25S°, or so much in advance of the equinoctial points. In consequence it is seen to the best advantage at, or a little after, the equinoxes, after sunset at the spring, and before sunrise at the autumnal equinox, not only because the direction of its apparent axis lies at those times more nearly perpendicular to the horizon, but also because at those epochs we are approaching the situation when it is seen most completely in section.
"At the vernal equinox the appearance of the zodiacal light is that of a pretty broad pyramidal, or rather lenticular, body of light, which begins to be visible as soon as the twilight decays. It is very bright_ at its lower or broader part near the horizon, and (if there be broken clouds about) often appears like the glow of a distant conflagration, or of the rising moon, only less red. At higher altitudes its light fades gradually, and is seldom traceable much beyond the Pleiades, which it usually, however, attains and involves ; and its axis at the venial equinox is always inclined (to the northward of the equator) at an angle of between 60° and 70° to the horizon • and it is most luminous at its base, resting on the horizon, where it is broadest, occupying, in fact, an angular breadth of somewhere about 10° or 12° in ordinary clear weather." ZONE (the Greek Coma, "a belt"), a portion of a sphere inter cepted between two parallel planes. When, on the globe of the
earth, one plane is the equator, and four others are drawn parallel to the equator, two of which contain the circles in which the sun is vertical at the summer and winter solstices, and the two others, the circles of which are as far distant (on the earth) from tho poles as the former are from the equator [Ancric Clucks], the earth is divided into six zones (the polar segments being called by that name as well as the others). Of these the portions which contain the two poles are called the north and south frigid zones : throughout these zones the sun never rises during a part of the winter, and never sets during a part of the summer. The parts intercepted between the arctic circle and the summer solstice parallel, and between the antarctic circle and the winter solstice parallel, are called the northern and southern temperate zones : in every part of these there is always rising and setting of the sun for every day in the year ; but in no part of them is the sun ever vertical The parts between the summer solstice parallel and the equator, and between the winter solstice parallel and the equator, are called the northern and southern torrid zones : in these there is always night and day, and at every point the noon-day sun is vertical twice in the year.
The torrid and frigid zones deserve their names ; but the temperate zones partake of both excessive heat and cold in those parts which are near the boundaries of the torrid and frigid zones. Every zone, in fact, partakes of all the qualities of the adjacent zones in those parts which are near the boundary. Thus near the arctic circle there are places where the shortest day is only ten minutes, and the shortest night no longer ; near the solstice parallels there are places at which a part of the sun's body may be vertical, though the centre of the sun cau never be so ; all being ,within the temperate zone.