Sky brightness
I want to introduct something about LED Underground Lamp,Underground Light,LED Light. Model: Underground Lamp ZHHW-DML03 Underground Lamp,Inground Lamp,LED Underground Light,Solar Underground Light,Inground Light,Underground Light,Solar Underground Lamp,Underground Lighting,LED Light,Underground Lamps,Solar LED Underground Light,LED Underground Lamp,LED Underground Lighting,Underground Lights,LED Underground Lights,LED Lighting,Solar LED Underground Light,Solar LED Underground Lamp,Solar Underground Lamps 12-240V 120PCS
Airglow made visible from aboard the ISS (NASA)
The fact that the sky is not completely dark at night can be easily 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.
Airglow
Spectrum of the night sky
When physicist Anders ?ngstr?m 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 557.7nm 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 557.7nm, a yellow doublet from sodium at 589.0 and 589.6nm, and a red lines from oxygen at 630.0 and 636.4nm.
The sodium emissions come from a thin sodium layer approximately 10km thick at an altitude of 90 – 100km, above the mesopause and in the D-layer of the ionosphere. The red oxygen lines originate at altitudes of about 300km, in the F-layer. The green oxygen emissions are more spatially distributed. How sodium gets to mesospheric heights is not yet well understood, but it is believed to be a combination of upward transport of sea salt and meteoritic dust.
In daytime sodium and red oxygen emissions are dominant, and are roughly 1000 times more luminous than nighttime emissions because in daytime the upper atmosphere is fully exposed to solar UV radiation. The effect is however not noticeable to the human eye since it totally fades in the glare of directly scattered sunlight.
Indirect scattering of Sunlight
Amount of air still illuminated after sunset, at the horizon. Normalized so that zenith is 1 airmass
Indirectly scattered sunlight comes from two directions. From the atmosphere itself, and from outer space. In the first case, the sun has just set but still illuminates the upper atmosphere directly. Because the amount of scattered sunlight is proportional to the number of scatterers (i.e. air molecules) in the line of sight, the intensity of this light decreases rapidly as the sun drops further below the horizon and illuminates less and less of the atmosphere.
When the sun’s altitude is < -6 99% of the atmosphere in zenith is in the earth’s shadow and second order scattering takes over. At the horizon, however, 35% of the atmosphere along the line of sight is still directly illuminated, and continues to be until the sun reaches -12. From -12 to -18 only the uppermost parts of the atmosphere along the horizon, directly above the spot where the sun is, is still illuminated. After that, all direct illumination ceases and astronomical darkness sets in.
A second source sunlight is the zodiacal light, which is caused by reflection and scattering of sunlight on interplanetary dust. Zodiacal light varies quite a lot in intensity depending on the position of the earth, location of the observer, time of year, and composition and distribution of the reflecting dust.
Scattered light from extraterrestrial sources
Not only sunlight is scattered by the molecules in the air. Starlight and the diffuse light of the milky way are also scattered by the…(and so on) To get More information , you can visit some products about iron pendant light, floating pool light, . The LED Underground Lamp,Underground Light,LED Light products should be show more here!
himfr can provide you most popular hot products from china!
Recent Comments