The fact that the sky isn't absolutely dark at night can easily be observed. Were the sky (in the absence of moon and citylights) absolutely dark, one would not be able to see the silhouette of an object against the sky.
The intensity of the sky varies greatly over the day and the primary cause differs as well. During daytime when the sun is above the horizon direct scattering of sunlight (rayleigh scattering) is the overwhelmingly dominant source of light. In twilight, the period of time between sunset and sunrise, the situation is more complicated and a further differentiation is required. Twilight is divided in three segments according to how far the sun is below the horizon in segments of 6°.
After sunset the civil twilight sets in, and ends when the sun drops more than 6° below the horizon. This is followed by the nautical twilight, when the sun reaches heights of -6° and -12°, after which comes the astronomical twilight defined as the period from -12° to -18°. When the sun drops more than 18° below the horizon the sky generally attains its minimum brightness.
Several sources can be identified as the source of the intrinsic brightness of the sky, namely airglow, indirect scattering of sunlight, scattering of starlight, and artificial light pollution.
