Brightness
The fact that the sky is not completely dark at night can be easily observed. When the sky is(in the absence of moon and city lights) 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.
Read more about this topic: Night Sky
Famous quotes containing the word brightness:
“on thy brow
Shall sit a nobler grace than now.
Deep in the brightness of the skies
The thronging years in glory rise.
And, as they fleet,
Drop strength and riches at thy feet.”
—William Cullen Bryant (1794–1878)
“For pain is perhaps but a violent pleasure? Who could determine the point where pleasure becomes pain, where pain is still a pleasure? Is not the utmost brightness of the ideal world soothing to us, while the lightest shadows of the physical world annoy?”
—Honoré De Balzac (1799–1850)
“Writing is to descend like a miner to the depths of the mine with a lamp on your forehead, a light whose dubious brightness falsifies everything, whose wick is in permanent danger of explosion, whose blinking illumination in the coal dust exhausts and corrodes your eyes.”
—Blaise Cendrars (1887–1961)