Encyclopædia Britannica, Ninth Edition/Twilight

TWILIGHT.The light of what is called the "sky" depends upon the scattering or reflexion of direct sunlight in the earth's atmosphere, mainly if not entirely due to those fine dust particles which (as we have recently learned) form the necessary nuclei for condensation of aqueous vapour. Were it not for these particles the sky would appear by day as it does in a clear winter night, and the stars would be always visible. Alpine climbers and aeronauts, when they have left the grosser strata of the atmosphere below them, find this state of things approximated to; and even at the sea-level the blue of the sky is darker when the air contains but few motes. After the sun has set, its rays continue for a time to. pass through parts of the atmosphere above the spectator's horizon, and the scattered light from these is called twilight. It is, of course, most brilliant in the quarter where the sun has set. Before sunrise we have essentially the same phenomenon, but it goes by the name of "dawn." The brilliancy of either depends upon several conditions, of which the chief is, of course, the degree by which the sun has sunk below the horizon. But the amount of dust in the air affects the phenomenon in two antagonistic ways: it diminishes the amount of sunlight which reaches the upper air after passing close to the earth and it increases the fraction of this light which is scattered to form twilight. Hence no general law can be laid down as to the duration of twilight; but it is usual to state (roughly) that it lasts until the sun is about 18 under the horizon. If we make this assumption, it is a simple matter of calculation to solve questions as to the duration of twilight at a given place at a given time of year, the maximum duration of twilight at a given place, fee. In the older works on astronomy such questions were common enough, but they have now little beyond antiquarian interest. The more complex phenomena of twilight, such as the "after-glow," &c., probably depend upon the precipitation of moisture on the dust particles as the air becomes gradually colder. This will of course alter the amount of scattering; but it may also lead (by reflexion from strata of such particles) to an increase in the amount of light to be scattered.