Sky brightness

Sky brightness refers to the visual perception of the sky and how it reflects and radiates light. The fact that the sky is not completely dark at night can be easily observed. If light sources were removed from the sky (such as the Moon and light pollution from cities) it would appear absolutely dark. Silhouettes of an object against the sky would not be possible.

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, civil twilight sets in, and ends when the sun drops more than 6° below the horizon. This is followed by nautical twilight, when the sun reaches heights of –6° and –12°, after which comes 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.

When physicist Anders Ångström examined the spectrum of the aurora borealis, he discovered that even on nights when the aurora was absent, its characteristic green line was still present. It was not until the 1920s that scientists were beginning to identify and understand the emission lines in aurorae and of the sky itself, and what was causing them. The green line Angstrom observed is in fact an emission line with a wavelength of 557.7 nm, caused by the recombination of oxygen in the upper atmosphere.

Airglow is the collective name of the various processes that occur in the upper atmosphere that result in the emission of photons, with the driving force being primarily UV-radiation from the sun. Several emission lines are dominant, a green line from oxygen at 557.7 nm, a yellow doublet from sodium at 589.0 and 589.6 nm, and red lines from oxygen at 630.0 and 636.4 nm.

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Wikipedia