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zodiacal light or zodiacal band, a faint band of light sometimes seen in the western sky just after sunset in the spring, extending up from the horizon at the point where the sun has just set, or in the eastern sky just before sunrise in the autumn. The light is so faint as to be obscured by moonlight. It is caused by the reflection and scattering of sunlight by a sparse band of tiny dust particles that appears to be an extension of the solar corona, stretching out beyond the orbit of Jupiter. Concentrated in the plane of the ecliptic, the faint light is best seen in the region of the sky called the zodiac. Near the equator the zodiacal light sometimes seems to stretch completely across the sky. It was first investigated and explained by the astronomer Gian Domenico Cassini about 1690.
See also gegenschein.
a faint glow extending along the ecliptic (that is, in the region of the zodiac) and observable in the stellar sky in the west soon after the onset of darkness or in the east before sunrise, when the ecliptic is sufficiently steeply inclined to the horizon. The glow is accounted for by the scattering of sunlight by dust particles moving around the sun.
Zodiacal light has the form of cones that widen toward the horizon. With increasing distance from the horizon, the brightness of the cones diminishes, and they gradually develop into the zodiacal band—a weakly glowing band about 10° in width, barely discernible against the background of the night sky. In the region opposite the sun, the gegenschein, or counterglow, is superposed on the zodiacal light. In the middle latitudes, the cone of zodiacal light is clearly visible from September through November before morning twilight (autumnal, or morning zodiacal light), and the western cone can be seen from January through March after evening twilight (vernal, or evening zodiacal light). During summer nights the zodiacal light is observed as a diffuse glow along the northern horizon (northern, or summer zodiacal light). The cones of the zodiacal light can be seen most clearly in the equatorial countries when the ecliptic is perpendicular to the horizon. The diffuse glow of the zodiacal light spreads over the entire sky; however, its intensity decreases rapidly with increasing distance from the sun and the ecliptic. On the average, the glow of the zodiacal light is about 15 percent of the total radiation of the night sky in the visible region of the spectrum, although the cones of zodiacal light are two to three times brighter than the night sky background. The emission of zodiacal light is partially polarized, and the degree of polarization depends on the angular distance from the sun. The distribution of the energy in the spectrum of the zodiacal light is close to that of the sun; as in the solar spectrum, the spectrum of zodiacal light includes absorption lines.
The study of zodiacal light is of high scientific interest, since it can yield information on the distribution of dust in interplanetary space. It has been established that dust particles in interplanetary space are concentrated near the sun in the form of an ellipsoidlike cloud (zodiacal cloud), whose dust content decreases with increasing distance from the sun and the plane of the ecliptic. Zodiacal light is investigated by photometric and spectral measurements. Observations made from artificial earth satellites are most valuable for its study, since here the effect of the earth’s atmosphere on the results of investigations is excluded.
N. B. DIVARI