319. _The General Appearance of the Zodiacal Light._--The phenomenon known as the _zodiacal light_ consists of a very faint luminosity, which may be seen rising from the western horizon after twilight on any clear winter or spring evening, also from the eastern horizon just before daybreak in the summer or autumn. It extends out on each side of the sun, and lies nearly in the plane of the ecliptic. It grows fainter the farther it is from the sun, and can generally be traced to about ninety degrees from that luminary, when it gradually fades away. In a very clear, tropical atmosphere, it has been traced all the way across the heavens from east to west, thus forming a complete ring. The general appearance of this column of light, as seen in the morning, in the latitude of Europe, is shown in Fig. 360. [Illustration: Fig. 360.] Taking all these appearances together, they indicate that it is due to a lens-shaped appendage surrounding the sun, and extending a little beyond the earth's orbit. It lies nearly in the plane of the ecliptic; but its exact position is not easily determined. Fig. 361 shows the general form and position of this solar appendage, as seen in the west. [Illustration: Fig. 361.] 320. _The Visibility of the Zodiacal Light._--The reason why the zodiacal light is more favorably seen in the evening during the winter and spring than in the summer and fall is evident from Fig. 362, which shows the position of the ecliptic and the zodiacal light with reference to the western horizon at the time of sunset in March and in September. It will be seen that in September the axis of the light forms a small angle with the horizon, so that the phenomenon is visible only a short time after sunset and low down where it is difficult to distinguish it from the glimmer of the twilight; while in March, its axis being nearly perpendicular to the horizon, the light may be observed for some hours after sunset and well up in the sky. Fig. 363 gives the position of the ecliptic and of the zodiacal light with reference to the eastern horizon at the time of sunrise, and shows why the zodiacal light is seen to better advantage in the morning during the summer and fall than during the winter and spring. It will be observed that here the angle made by the axis of the light with the horizon is small in March, while it is large in September; the conditions represented in the preceding figure being thus reversed. [Illustration: Fig. 362.] [Illustration: Fig. 363.] 321. _Nature of the Zodiacal Light._--Various attempts have been made to explain the phenomena of the zodiacal light; but the most probable theory is, that it is due to an immense number of meteors which are revolving around the sun, and which lie mostly within the earth's orbit. Each of these meteors reflects a sensible portion of sunlight, but is far too small to be separately visible. All of these meteors together would, by their combined reflection, produce a kind of pale, diffused light.