We will bring this chapter to a close by alluding briefly to two strange appearances which are sometimes seen in our night skies. These are known respectively as the Zodiacal Light and the Gegenschein.
The Zodiacal Light is a faint cone-shaped illumination which is seen to extend upwards from the western horizon after evening twilight has ended, and from the eastern horizon before morning twilight has begun. It appears to rise into the sky from about the position where the sun would be at that time. The proper season of the year for observing it during the evening is in the spring, while in autumn it is best seen in the early morning. In our latitudes its light is not strong enough to render it visible when the moon is full, but in the tropics it is reported to be very bright, and easily seen in full moonlight. One theory regards it as the reflection of light from swarms of meteors revolving round the sun; another supposes it to be a very rarefied extension of the corona.
The Gegenschein (German for "counter-glow") is a faint oval patch of light, seen in the sky exactly opposite to the place of the sun. It is usually treated of in connection with the zodiacal light, and one theory regards it similarly as of meteoric origin. Another theory, however—that of Mr. Evershed—considers it a sort of tail to the earth (like a comet's tail) composed of hydrogen and helium—the two lightest gases we know—driven off from our planet in the direction contrary to the sun.
[13] Every one knows the simple experiment in which a coin lying at the bottom of an empty basin, and hidden from the eye by its side, becomes visible when a certain quantity of water has been poured in. This is an example of refraction. The rays of light coming from the coin ought not to reach the eye, on account of the basin's side being in the way; yet by the action of the water they are refracted, or bent over its edge, in such a manner that they do.
CHAPTER XVI
THE MOON
What we call the moon's "phases" are merely the various ways in which we see the sun shining upon her surface during the course of her monthly revolutions around the earth ([see Fig. 14], p. 184). When she passes in the neighbourhood of the sun all his light falls upon that side which is turned away from us, and so the side which is turned towards us is unillumined, and therefore invisible. When in this position the moon is spoken of as new.
As she continues her motion around the earth, she draws gradually to the east of the sun's place in the sky. The sunlight then comes somewhat from the side; and so we see a small portion of the right side of the lunar disc illuminated. This is the phase known as the crescent moon.