Visual perception of the sky and how it scatters and diffuses light.
This article is primarily about the brightness of the night sky. For daytime situations, see diffuse sky radiation and Rayleigh scattering.
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Airglow made visible from aboard the ISS
Sky brightness refers to the visual perception of the sky and how it scatters and diffuses light. The fact that the sky is not completely dark at night is easily visible. If light sources (e.g. the Moon and light pollution) were removed from the night sky, only direct starlight would be visible.
The sky's brightness varies greatly over the day, and the primary cause differs as well. During daytime, when the Sun is above the horizon, the direct scattering of sunlight is the overwhelmingly dominant source of light. During twilight (the duration after sunset or before sunrise until or since, respectively, the full darkness of night), the situation is more complicated, and a further differentiation is required.
Twilight (both dusk and dawn) is divided into three 6° segments that mark the Sun's position below the horizon. At civil twilight, the center of the Sun's disk appears to be between 1/4° and 6° below the horizon. At nautical twilight, the Sun's altitude is between –6° and –12°. At astronomical twilight, the Sun is between –12° and –18°. When the Sun's depth is more than 18°, the sky generally attains its maximum darkness.
Sources of the night sky's intrinsic brightness include airglow, indirect scattering of sunlight, scattering of starlight, and light pollution.[1]
^F. Patat. "The Brightness of the Night Sky". ESO. Retrieved 2015-11-27.
Skybrightness refers to the visual perception of the sky and how it scatters and diffuses light. The fact that the sky is not completely dark at night...
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...
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assigning the brightness of Vega as the definition of zero magnitude at any specified wavelength. Apart from small corrections, the brightness of Vega still...
use polarization to distinguish skybrightness from the image of the corona: both coronal light and skybrightness are scattered sunlight and have similar...
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...
(Na), and lithium (Li). The skybrightness is typically measured in units of apparent magnitude per square arcsecond of sky. In order to calculate the...
features of bright objects, especially Moon and planets. These filters have no effect on observing faint objects. Red: Reduces skybrightness, particularly...
constellations. The area of sky that the Milky Way obscures is called the Zone of Avoidance. The Milky Way has a relatively low surface brightness. Its visibility...
arranged by their apparent magnitude – their brightness as observed from Earth. It includes all stars brighter than magnitude +2.50 in visible light, measured...
of a two-week span of dates near the new moon removes any effect on skybrightness from scattered moonlight, and observing well after sunset prevents any...
methane, moving across Titan, using the James Webb Space Telescope. Skybrightness and viewing conditions are expected to be quite different from Earth...
sky, soon hundreds of satellites may be clearly visible to the human eye at dark sites. It is estimated that the overall levels of diffuse brightness...
A low-surface-brightness galaxy, or LSB galaxy, is a diffuse galaxy with a surface brightness that, when viewed from Earth, is at least one magnitude...
Dark-sky movement Dark-sky preserve Light pollution List of astronomical societies Skybrightness "We have a new name and logo". www.DarkSky.org. 1 June 2023...
"Large peculiar motion of the solar system from the dipole anisotropy in skybrightness due to distant radio sources". The Astrophysical Journal. 742 (2): L23–L27...
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Horizons saw a sky ten times darker than the sky seen by the Hubble Space Telescope because of the absence of diffuse background skybrightness from the zodiacal...
Bingen; Erkki Kyrölä; Iuri Mateshvili; Giuli Mateshvili (2005). "Twilight skybrightness measurements as a useful tool for stratospheric aerosol investigations"...
and Lake Tekapo (town). The night skybrightness in the reserve ranks at a level 2 on the nine-level Bortle Dark-Sky Scale, representing a truly dark site...
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