The New Student's Reference Work/Zodiacal Light
Zodi′acal Light. If an observer in the tropical regions of the earth on a clear, moonless night watches the western sky from sunset till the last trace of twilight has disappeared, he will notice that the twilight seems to linger longer near the point where the sun sank below the horizon and that gradually a nebulous band of light, broad toward the horizon and narrowing, first rapidly and then more slowly upward, begins to stand put clearly from the vanishing twilight, which spreads along a much wider arc of the horizon. This is the zodiacal light. When seen on a perfectly clear night, it will be noticed that it fades imperceptibly on both sides and toward the vertex and that its light is distinctly brighter toward the base than at higher points. Its width at its base may be said to vary from 10° to 30°, and the height of its vertex from 40° to 90°. It is most clearly seen in our latitude during the spring months after sunset and in the autumnal months before sunrise. This is due not to any change in the light itself, but to the fact that at other seasons of the year the ecliptic makes so small an angle with the horizon that light lying in or near it does not rise sufficiently high above the mists of the horizon to be seen after the twilight has vanished.