Page:Popular Science Monthly Volume 60.djvu/280

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the negative charge in the upper atmosphere is high enough, discharges are brought about by the powerful ultra-violet radiation from the sun, and particles are driven off radially from the earth on the side turned towards the sun, only to be drifted back with the other streams into the tail. The effect will be as if a sheaf of light projected from her towards the sun.

Compare with this the description of the Zodiacal Light (Newcomb, 'Popular Astronomy,' p. 416):

This object consists of a very soft faint column of light, which may be seen rising from the western horizon after twilight on any clear winter or spring evening; it may also be seen rising from the eastern horizon just before daybreak in the summer or autumn. It really extends out on each side of the sun, and lies nearly in the plane of the ecliptic. . . Near the equator, where the ecliptic always rises high above the horizon, the light can be seen about equally well all the year round. . . It is due to a lens-shaped appendage of some sort surrounding the sun, and extending out a little beyond the earth's orbit. The nature of the substance from which this light emanates is entirely unknown. . . Professor Wright of Yale College finds its spectrum to be continuous. Accepting this, we should be led to the conclusion that the phenomenon in question is due to reflected sunlight, probably from an immense cloud of meteorites, filling up the space between the earth and the sun.

The difficulty in this view is that the orbits of such swarms of meteorites as are known to us are distributed irregularly with regard to the ecliptic. On the other hand, Arrhenius' streams of particles, when near enough to be visible, necessarily lie in or near the ecliptic, as required by observation. More than this, the particles emitted by the earth herself should be most abundant over those regions which have been exposed for many hours to the sun. Now it has been observed that the zodiacal light is stronger on what Arrhenius calls the 'evening side' of the earth (i. e., that side which is in the act of turning away from the sun, and has the sun in the west) than on the 'morning side.'

Even at night, when the sun is below the horizon, faint reflections should reach us from the streamers behind the earth, and by an effect of perspective, these should have a maximum in the point opposite to the sun, where they will appear most dense. Let Professor Newcomb describe the Gegenschein:

Another mysterious phenomenon associated with the zodiacal light is known by its German appellation, the Gegenschein. It is said that in that point of the heavens directly opposite the sun there is an elliptical patch of light, a few degrees in extent, of such extreme faintness that it can be seen only by the most sensitive eyes, under the best conditions, and through the clearest atmosphere. This phenomenon seems so difficult to account for that its existence is sometimes doubted; yet the testimony in its favor is difficult to set aside.

How is it that the moon does not exhibit such tails? The moon has