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. [Illustration: _Figure 7._ Displacement of light image by temperature inversion.] Normally the air is warmest at the surface of the earth and steadily gets colder at greater and greater heights. Sometimes this condition is reversed, particularly in the broad deserts and prairies of the Southwest, where the changes between the day’s heat and the night’s cold may be sudden and extreme. The ground cools off rapidly during the night and imparts this coldness to the layer of air immediately above. Thus the air may be warmer some distance above the ground. When such a “temperature inversion” occurs, light going through the air bends in a peculiar way (see Figure 7), so that the image is displaced far more than normally. The inversion may produce fuzzy or greatly distorted images, and when there are several layers of alternating hot and cold air, the effects may be spectacular. At the boundaries between the layers the distortion and displacement increase greatly. A star or a planet seen through such an atmosphere may display apparently violent motions, peculiar shapes, and fantastic color changes; light clouds drifting over the bright stars may increase this illusion of motion[IV-3]. The rising or setting sun, although actually below the horizon, may project upward several images of itself, one on the top of another, to form a kind of Chinese pagoda, or a “bell pepper.” And the twinkling top rung of the pagoda may simulate a whirling propeller. _The “Whipping Girl” of Saucerdom_