Earth’s Distorting Atmosphere
In everyday life we often look at familiar objects through a distorting medium. Houses and persons seen through a pane of poor window glass look peculiar and wrongly shaped, and images of trees and clouds reflected in a pool or a stream of rippling water may continually shift and break, but these distortions do not deceive us because we are used to them. The child who stands before the crazy mirrors in an amusement park may laugh at himself for looking so fat or so thin, so tall or so short. Knowing that the image is only a ludicrous approximation to his real appearance, he is able to recognize himself without difficulty. But a stranger, placed so that he could see only the distorted image and not the person who made it could not make the necessary corrections and probably would not recognize the child if they met in the street.
Like window glass, water, or mirrors, a mere layer of air can distort an image. For the astronomer, the earth’s atmosphere is a lifelong frustration. Acting as an imperfect lens, it continually falsifies the true position, color, size, and shape of the heavenly bodies he tries to study. Under certain conditions it can change the image of a star or a planet into an unrecognizable stranger. When light enters the atmosphere, the rays are bent or “refracted” so that the image is moved upward, somewhere above the true position of the star (see [Figure 6]). When we are admiring a sunset and think we are watching the very top rim of the sinking sun as it drops below the horizon, we are actually seeing only its projected image. The sun itself has already set, but its light is bent upward by the air that clings to the earth’s surface. The greater the density of the air, the greater the displacement of the image. If there were no air at the earth’s surface, the sun would vanish and darkness would come instantaneously, with no intervening period of twilight.
Figure 6. Bending of light by the atmosphere. A star below the horizon is visible because refraction raises the image.
A star’s light does not bend uniformly, however. Light rays of different wave lengths bend at different angles, so that when white light is scattered or “dispersed” into its component colors, the blues and greens are bent more than the reds. The density and the temperature of the air also affect the beam, so that as a star’s light travels from the thin upper atmosphere to the denser air near the earth, the colors shift constantly and the star seems to twinkle, flicker, and change in color and brightness.
Such changes are most noticeable when a star is low on the horizon at dawn or at dusk, so that its light reaches us only after traveling through miles of dense atmosphere. The sun displays these effects dramatically. At sunrise and sunset its scattered light may illuminate the entire horizon. Clouds turn red and gold, hills and the tops of buildings take on a ruddy glow, and the entire sky may flame. The red wave lengths remain, while most of the blues and greens have been scattered out of the beam or may appear briefly at the top of the sun’s disk, as a “green flash,” at the instant it sinks below the horizon.
Similarly, a star or planet observed low on the horizon at sunrise or sunset may appear extraordinarily large and brilliant. It may seem to have structure, showing an intense red glow at the bottom and bright blue at the top. Watching it, the startled observer may see the object apparently in motion, hovering, pulsating, and flashing red and green lights. If he is so inclined, he can easily interpret the image as a strange machine, the red as the glow from an exhaust, and the blue as the illumination system of an interplanetary craft.
Figure 7. Displacement of light image by temperature inversion.