Twilight, the faint light which appears in the sky a little before sunrise, and again for some time after sunset, the amount and duration of the light varying materially in different latitudes and at different seasons. The light is caused by the reflection of the sun's rays, when below the horizon, from the vapors and minute solid particles floating in it, and perhaps from the material atoms of the air itself. To this property of reflection possessed by the atmosphere its illumination is due beyond the direct reach of the rays proceeding from the sun, as under the shadow of clouds and behind opaque objects upon the surface, where, unless the light were directed upon some principle of general diffusion, intense darkness would prevail. So also a sudden illumination would attend the rising of the sun and instantaneous darkness accompany his setting. As the sun sets to any point upon the surface of the earth, the atmosphere above this point all around the horizon is illuminated by direct rays, and the reflection from so broad an illuminated surface brings to the earth a large amount of light; but as the dark shadow of the earth, in consequence of the continued sinking of the sun, rises higher and higher into the atmosphere at this locality, the reflected light steadily diminishes, and finally disappears when no direct rays from the sun reach the higher regions of the atmosphere above the horizontal line extended toward the sunset.
By observing the time after the setting of the sun to the disappearance of the reflected rays, data are afforded upon which an approximate estimate may be made of the height of the atmosphere, or at least of that portion of the atmosphere which is capable of being illuminated in the manner above described; and it is on this method chiefly that this calculation is based. On the equator, when the sun is in the equinoctial, and apparently descending vertically, and occupying as much time below as above the horizon, the duration of the twilight is an hour and 12 minutes, or one tenth of the semicircumference, equal to 18°; whence it is concluded that such must be its depression below the horizon at any place before the twilight can end; and it is reckoned from this that the height of the atmosphere is a little over 52 m. But this cannot be otherwise than a rude approximation, the calculation proceeding on the supposition of there being but one direct reflection, whereas the rays may be reflected again and again, and no account being made of the refraction the rays must experience in their direct passage through the dense stratum of air near the surface, and entering it again when turned back from the upper strata.
By a different calculation, founded on observations of the progress of the edge of the dark shadow (known till it reaches the zenith as the anticrepuscular, and afterward as the crepuscular curve), made in the pure and transparent air at the summit of high mountains, the height of the atmosphere has been found by French astronomers (whose observations are recorded in the Annuaire météorologique de France, 1850) to be 71.46 m. This curve they found set when the sun was 17° below the horizon. The variable length of the twilight at the same place in different seasons results from the varying declination of the sun and the consequent difference of time required to sink 17° or 18° below the horizon, as his course is vertical or more or less approaching it. Near the poles, where the sun attains at noon no great height above the horizon, it also keeps near it after disappearing each night; and if its depression does not exceed 18°, the twilight is continuous into the dawn. This is the case in some portion of the summer in all places for which the least polar distance of the sun is only 18° greater than the latitude, that is, all places in higher N. or S. latitude than 48½ (the complement of 18° + 23½°, or 41½°). At a place in 48½° N. or S. latitude, the midnight depression of the sun at midsummer amounts to 41½°, the complement of the latitude, diminished by 23½°, the obliquity of the ecliptic, that is, to 18°; and there is therefore no twilight at midnight at this the most favorable season of the year.
At lower latitudes there is of course no twilight at midnight. In higher and higher latitudes, as the sun is less and less depressed below the horizon at midnight in summer, the twilight increases in brilliancy, and is finally lost during the period that the circuit of the sun is above the horizon. - A beautiful feature attending the twilight is the rich coloring of the clouds upon which the direct rays of the sun strike, and from which they are reflected in gorgeous tints, which slowly change their hues as the angle of reflection varies. This phenomenon is seen in greatest perfection in mountainous regions and over wide districts, where the air remains in a uniform condition unaffected by local causes. The presence of much moisture is also favorable for the display of the finest colors. Thus at sea, especially in the warm atmosphere of the Gulf stream, these exhibitions are of the finest character, as also over the waters of inland seas. Even when no clouds are formed, brilliantly colored bands are produced along the horizon, which change in a somewhat regular order with the continued rising or declining of the sun. These, tints are due to the different powers of penetration possessed by the different rays of which white light is composed.
In the same manner as the solar rays are decomposed and present different colors in passing through a piece of glass covered with smoke in layers of different thickness, these rays are also decomposed in penetrating the dense and humid lower strata of the atmosphere. A slight obstruction of this kind extinguishes the blue rays, and causes those which pass through to appear yellowish red; next to this the yellow is arrested, and the light is orange; till with further obstruction the yellow entirely disappears, and the red rays alone reach the surface.