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the aggregate of colorful light phenomena in the atmosphere at sunset or just before sunrise; it is a regular change in the color of the sky caused by the location of the sun with respect to the horizon.
When the sun is moving down toward the horizon and there are no clouds, the sky above the horizon is colored yellow or reddish orange, sometimes with a brownish hue. This is due to the fact that when the rays of the sun take a long oblique path in the atmosphere, the blue and violet rays are attenuated as a result of the scattering of light, whereas the remaining rays, and with them the atmosphere that they illuminate, acquire a yellowish or reddish color. After sunset the earth’s shadow begins to rise above the horizon in the east; its upper edge is colored grayish blue because the light rays passing through at altitudes of 15–25 km enter the atmospheric ozone layer, which absorbs yellow rays. When the sun is below the horizon, a bright red spot—the afterglow—appears above it, and red and orange bands, whose color gradually takes on greenish and bluish shades, appear in the east above the bluish shadow of the earth. When the sun drops 2°-3° below the horizon, a rosy spot of purplish color, which becomes brighter and brighter (until the sun has dropped to 5°) and gradually takes on the shape of a section located above the set sun and illuminating the landscape (especially a snowy landscape) with a pinkish light, appears in the glow. This light apparently occurs where the reddish rays of the sun enter the dusty layer of the atmosphere, which was recently discovered at an altitude of about 20 km. Later the purple light fades, but the orange or red bands above the horizon become brighter. At this time the whitish or bluish spot of the glow slowly moves downward and grows dull. Darkness sets in quickly as the purple light fades. When the sun has dropped to 7° it is no longer possible to read without an artificial light source (this is the end of civil twilight). The earth’s shadow in the east and its colored border become blurred and almost invisible against the darkening sky. When the sun has dropped to 12° the colored bands in the west become greenish, and at 18° they disappear altogether. The weakest stars can be seen, and night sets in.
The morning glow develops in a similar manner but in the reverse order.
REFERENCESMinnaert, M. Svet i tsvetvprirode. Moscow, 1969. (Translated [from Dutch].)
A. KH. KHRGIAN
Available from Andrew Arnblaster, Bollostraat 6, B-3140 Keerbergen, Belgium, for Mac or MS-DOS.
[Byte's UK edition, May 1992, p.84UK-8].