TWILIGHT, that light, whether in the morning before sun-rise, or in the evening after sun-set, supposed to begin and end when the least stars that can be seen
Twinkling by the naked eye cease or begin to appear. By means of the atmosphere, it happens that though none of the sun's direct rays can come to us after it is set, yet we still enjoy its reflected light for some time, and night approaches by degrees. For after the sun is hid from our eyes, the upper part of our atmosphere remains for some time exposed to its rays, and from thence the whole is illuminated by reflection. But as the sun grows lower and lower, that portion of the atmosphere which is above our horizon becomes enlightened till the sun has got 18° below it; after which it ceases to be illuminated thereby, till it has got within as many degrees of the eastern side of the horizon; at which time it begins to illuminate the atmosphere again, and in appearance to diffuse its light throughout the heavens, which continues to increase till the sun be up. Hence it is that during that part of the year in which the sun is never 18° below our horizon, there is a continued twilight from sun-setting to sun-rising.
As the twilight depends on the quantity of matter in the atmosphere fit to reflect the sun's rays, and also on the height of it; for the higher the atmosphere is, the longer will it be before the upper parts of it will cease to be illuminated, the duration of it will be various. For instance, in winter, when the air is condensed with cold, and the atmosphere upon that account lower, the twilight will be shorter; and in summer, when the limits of the atmosphere are extended by the rarefaction and dilatation of the air, of which it consists, the duration of the twilight will be greater. And for the like reason, the morning twilight, the air being at that time condensed and contracted by the cold of the preceding night, will be shorter than the evening one, when the air is more dilated and expanded.
TWINKLING of the STARS. See STARS.