Definition of Sunset

Sunset, also called sundown in some American English dialects, is the time at which the Sun disappears below the horizon in the west. It should not be confused with dusk, which is the point at which darkness falls, some time after the beginning of twilight when the Sun itself sets. The red hues of the sky at sunset and sunrise are caused by Mie Scattering, not Rayleigh Scattering. The colours of the sky throughout the day and at sunrise and sunset, are explained by the phenomena of both Rayleigh Scattering and Mie Scattering. The colour of the sky described by Rayleigh Scattering applies to the hues of blue, violet and green, not to the reds, oranges, peaches and purples of sunrise and sunset. Rayleigh Scattering is scattering of shorter wavelength light (e.g. blue & violet) by air atoms and molecules (not statistical variations in density of the Earth's atmosphere). The magnitude or strength of Rayleigh Scattering varies by the reciprocal of the wavelength raised to the fourth power, and hence does not explain the beautiful variations of reds, purples, oranges and peachy colours. The latter colours arise from Mie Scattering, low angle scattering of light off dust, soot, smoke and (ash) particles. Mie Scattering (producing the colours of sunset and sunrise) is beautifully recognizable down-wind of and after dust storms, forest fires and volcanic eruptions that inject large quantities of fine particulate matter into the atmosphere. A number of eruptions in recent times, such as those of Mount Pinatubo in 1991 and Krakatoa in 1883, have been sufficiently large to produce remarkable sunsets and sunrises all over the world. Sometimes just before sunrise or after sunset a green flash can be seen.