A pretty phenomenon observed chiefly in the late afternoon and early morning consists of beams of light radiating from the sun, known technically as crepuscular rays. The beams are made visible by the presence of abundant dust or water droplets in the atmosphere, and the intervening dark spaces are the shadows of clouds. When the sun is above the horizon and the beams are directed downward, the phenomenon is popularly described as “the sun drawing water” and is regarded as a sign of rain. Sailors call these beams the “backstays of the sun,” and they have several other names based upon the legendry associated with them in different parts of the world. After sunset or before sunrise a fanlike sheaf of the beams often extends upward from the western or eastern horizon, respectively. The Homeric expression “rosy-fingered dawn” probably refers to this phenomenon. In all cases the apparent divergence of these beams is an effect of perspective, as they are really parallel. A rarer phenomenon is that of anticrepuscular rays, which appear to converge to a point opposite the sun. In this case the beams and shadows are projected entirely across the sky, but their paths can very seldom be traced in the upper part of the heavens because in this direction the observer’s line of sight passes through a comparatively shallow extent of dusty atmosphere.
An analogous phenomenon is seen in the shadows which near-by isolated mountain peaks frequently cast upon the sky opposite the sun at sunrise and sunset. Travelers have described such shadows cast by Adam’s Peak in Ceylon, Pike’s Peak in the Rocky Mountains, and Fujiyama in Japan. The phenomenon is said to be especially striking in the polar regions, where the air is often heavily charged with particles of ice.
One more optical phenomenon of sunrise and sunset that requires mention here seems to be comparatively little known to the nonscientific public, notwithstanding the fact that it has supplied the subject and title of a diverting novel by Jules Verne. The conditions required for its appearance are a clear and steady atmosphere and a sharply defined horizon, such as that of the ocean. At the instant the sun is appearing or disappearing, and when only a very small segment of its disk is visible above the horizon, this portion appears to be colored a bright emerald green, sometimes blending into blue. This transient phenomenon is known as the green flash. It is best explained as due to the different degrees of refraction undergone by rays of different wave lengths coming to us from the sun. The effect of refraction in elevating the solar image as a whole when near the horizon has already been mentioned. This effect is a little greater for the green and blue rays than for the orange and red. It is still more pronounced for the violet and indigo rays, but these are mostly sifted out of the solar beams in their long passage through the atmosphere when the sun is low. Hence at the upper edge of the solar image there is a narrow green or blue fringe, which is not, however, perceptible except when a screen is interposed between the eye and the bright image of the sun. A sharp horizon furnishes such a screen. Through a telescope it is possible, in suitable weather, to see the green flash—and also a corresponding “red flash” at the lower edge of the sun—by placing an opaque diaphragm in the focal plane of the object glass. Another explanation of the green flash—which could, however, account only for its appearance at sunset and not at sunrise—is that it is a physiological effect; the eye, fatigued by the reds and yellows that predominate in the light of the setting sun, sees an “after image” of complementary hue the instant after the real image has disappeared.
We now turn to a group of phenomena, also due to atmospheric refraction, which includes some of the most bizarre of optical illusions. The simplest of these phenomena consists of a slight apparent elevation of all objects in the surrounding landscape through terrestrial refraction, which is identical in principle with astronomical refraction and depends upon the difference in density, and hence of refractive power, of the air at different levels above the earth.
Normally the air decreases in density at a nearly regular rate with increasing altitude. Sometimes, however, this change in density is greatly modified by local effects of temperature. Over a cold surface of land or water the adjacent air may be abnormally dense, resulting in an unusually rapid decrease of density with altitude. Over a hot surface, as in the case of a desert under strong sunshine, the adjacent air may become so much rarefied that, for a certain distance upward, there is actually an increase of density with ascent, instead of the reverse. The rays of light coming to us from distant objects are bent in different directions and to various degrees by virtue of these abnormalities in the density of the atmosphere. The apparent positions of such objects depend upon the angle at which the light rays, coming from them, strike the eye of the observer. Sometimes the objects appear to be lifted far above their true positions (a phenomenon known as looming) and sometimes depressed far below them; and occasionally local irregularities in air density produce curiously distorted images of these objects.
Most of these strange effects are known collectively as mirage. There are many varieties. There is the “desert mirage,” first made famous through the experience of Napoleon’s soldiers in Egypt. There are mirages that suspend the images of remote objects in the sky; sometimes inverted, sometimes right side up. There is the lateral mirage, occasionally seen when one looks along the face of a heated wall or cliff. Lastly, there are the complex displacements and distortions of objects known as the Fata Morgana—a name originally applied to a phenomenon of this kind visible, on rare occasions, at the Straits of Messina, but now used generically for similar appearances in other parts of the world. Some of the finest examples of Fata Morgana are witnessed in the polar regions.
In the desert mirage an image of the lower part of the sky is brought down to earth and simulates the appearance of water, while the images of terrestrial objects, also depressed and inverted by the mirage, look like the reflections of the same objects upon the liquid surface. Humphreys says: “This type of mirage is very common on the west coast of Great Salt Lake. Indeed, on approaching this lake from the west one can often see the railway over which he has just passed apparently disappearing beneath a shimmering surface. It is also common over smooth-paved streets, provided one’s eyes are just above the street level.” The confusing and obscuring effects of the desert mirage were illustrated during the fighting between the British and Turks in Mesopotamia, in April, 1917, when, according to the report of General Maude, a battle had to be suspended on account of one of these optical disturbances.
FATA MORGANA ON THE COAST OF GREENLAND
(From drawing by Scoresby)