Sunset Photography At 3 Dec 2016 at Tezpur
Sunset or sundown, is the daily disappearance of the Sun below the horizon as a result of Earth’s rotation. The Sun will set exactly due west at the spring and fall equinoxes, which each occur only once a year.
The time of sunset is defined in astronomy as the moment when the trailing edge of the Sun’s disk disappears below the horizon. Near to the horizon, atmospheric refractioncauses the ray path of light from the Sun to be distorted to such an extent that geometrically the Sun’s disk is already about one diameter below the horizon when sunset is observed.
Sunset is distinct from twilight, which has three phases, the first being civil twilight, which begins once the Sun has disappeared below the horizon, and continues until it descends to 6 degrees below the horizon; the second phase is nautical twilight, between 6 and 12 degrees below the horizon; and the third is astronomical twilight, which is the period when the Sun is between 12 and 18 degrees below the horizon. Dusk is at the very end of astronomical twilight, and is the darkest moment of twilight just before night. Night occurs when the Sun reaches 18 degrees below the horizon and no longer illuminates the sky,
Locations north of the Arctic Circle and south of the Antarctic Circle experience no sunset or sunrise on at least one day of the year, when the polar day or the polar night persists continuously for 24 hours.
Sunset creates unique atmospheric conditions such as the often intense orange and red colors of the Sun and the surrounding sky.
The 16th-century astronomer Nicolaus Copernicus was the first to present to the world a detailed and eventually widely accepted mathematical model supporting the premise that the Earth is moving and the Sun actually stays still, despite the impression from our point of view of a moving Sun.
As a ray of white sunlight travels through the atmosphere to an observer, some of the colors are scattered out of the beam by air molecules and airborne particles, changing the final color of the beam the viewer sees. Because the shorter wavelength components, such as blue and green, scatter more strongly, these colors are preferentially removed from the beam. At sunrise and sunset, when the path through the atmosphere is longer, the blue and green components are removed almost completely, leaving the longer wavelength orange and red hues we see at those times. The remaining reddened sunlight can then be scattered by cloud droplets and other relatively large particles to light up the horizon red and orange. The removal of the shorter wavelengths of light is due toRayleigh scattering by air molecules and particles much smaller than the wavelength of visible light (less than 50 nm in diameter).The scattering by cloud droplets and other particles with diameters comparable to or larger than the sunlight’s wavelengths (> 600 nm) is due to Mie scattering and is not strongly wavelength-dependent. Mie scattering is responsible for the light scattered by clouds, and also for the daytime halo of white light around the Sun (forward scattering of white light).  Sunset colors are typically more brilliant than sunrise colors, because the evening air contains more particles than morning air.
Ash from volcanic eruptions, trapped within the troposphere, tends to mute sunset and sunrise colors, while volcanic ejecta that is instead lofted into the stratosphere (as thin clouds of tiny sulfuric acid droplets), can yield beautiful post-sunset colors calledafterglows and pre-sunrise glows. A number of eruptions, including those of Mount Pinatubo in 1991 and Krakatoa in 1883, have produced sufficiently high stratospheric sulfuric acid clouds to yield remarkable sunset afterglows (and pre-sunrise glows) around the world. The high altitude clouds serve to reflect strongly reddened sunlight still striking the stratosphere after sunset, down to the surface. Sometimes just before sunrise or after sunset a green flash can be seen.