Refraction of light by ice-crystals in clouds produces many beautiful color effects, such as halos of various types around sun or moon, vertical light pillars, circumzenithal arcs, and parhelia—"sun-dogs"—or paraselenæ—"moon-dogs"—which are luminous spots at equal altitudes with sun or moon—one to the left the other to the right, at an angular distance of 22°.
The most usual form of halo is that of 22° radius. This is a luminous ring of light surrounding sun or moon, with the inner edge red and sharply defined and the spectral colors proceeding outward in order; red is frequently the only color visible, the remainder of the ring appearing whitish. Since the halo is produced by refraction of light by ice-crystals, which exist in clouds of a certain type gathering at high altitudes, it is always a very good indicator of an approaching storm.
Coronas are luminous rings showing the spectral colors in the reverse order, that is, with the inner edge blue instead of red. They are usually of very small radius, scarcely two degrees, closely surrounding sun or moon and are produced—not by refraction—but by diffraction or a bending aside of the rays as they pass between—without entering—very small drops of water in clouds. As in the case of refraction, the red rays are turned from their course the least and the violet rays the most.
Many of these phenomena—halos, luminous spots, vertical pillars and arcs of light may, at times, be seen simultaneously, when clouds of ice-crystals are forming around the sun or moon. They then present a very complex and beautiful outline of luminous circles, arches and pillars that have a mysterious and almost startling appearance when the cause is not clearly understood.
We have found then that sunlight is made up of rays of many different wave-lengths and colors and that the atmosphere acts upon these rays in various ways. It reflects them or turns them back on their course; it refracts them as they pass through the gases of which the atmosphere consists, or through the water-vapor and ice-crystals suspended in it, thus sifting out and dispersing the rays of different colors and wave-lengths and producing beautiful color effects; it diffracts them or bends them aside as they pass between the fine dust particles and small drops of water in the air, again sifting out the rays of different colors and producing color effects similar to those produced by refraction; it also scatters and disperses, through the action of the molecules of oxygen and nitrogen in the upper strata, the blue and violet rays of shorter wave-length and thus produces the blue color and brightness of the sky; it produces beautifully colored auroral streamers and curtains and rays of light through the electrical discharges resulting when the rarefied gases in the upper air are bombarded by electrified particles shot forth from the sun.
It is our atmosphere, then, that we have to thank for all these beautiful displays of color that delight our eyes and give pleasure to our existence, as well as for the very fact of our existence upon a planet that without its presence would be an uninhabitable waste, covered only with barren rocks.
[XXVI]
KEEPING TRACK OF THE MOON
Of all celestial objects the nearest and most familiar is our satellite, the moon. Yet the mistakes and blunders that otherwise intelligent persons frequently make when they refer to the various aspects of the moon are quite unbelievable.