Collier's New Encyclopedia (1921)/Twilight

TWILIGHT, in astronomy, the faint diffused light which appears 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. Popularly, the term is only applied to the evening twilight, the morning twilight being called dawn. Twilight is produced by the diffused reflection of light from and among the atmosphere after the direct rays of the sun have ceased to reach the earth. When the sun descends below the horizon, its rays pass through the atmosphere strata, and some of them are reflected toward the earth and illuminate its surface. At first the light, falling on the lowest and densest strata, is reflected in great abundance, but as the sun descends to a greater distance below the horizon, the rays fall on higher, and therefore rarer, atmospheric strata. Consequently fewer rays undergo reflection, and as the number of reflected rays diminishes as the sun descends, the strength of the twilight diminishes in the same proportion, till at last the solar rays fall on strata so rare as to be incapable of reflecting light, and the twilight accordingly disappears. In the morning the change from darkness to light takes place in a similar manner, but in inverted order.