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.

The sunset is often more brightly coloured than the sunrise, with the shades of red and orange being more vibrant. The atmosphere responds in a number of ways to exposure to the Sun during daylight hours. In particular, there tends to be more dust in the lower atmosphere at the end of the day than at the beginning. During the day, the Sun heats the surface of the Earth, lowering the relative humidity and increasing wind speed and turbulence, which serves to lift dust into the air. However, differences between sunrise and sunset may in some cases depend more on the geographical particulars of the location from which they are viewed. For example, on a west-facing coastline, sunset occurs over water while sunrise occurs over land.

The timing of sunset varies with the time of year and the latitude of the location from which it is viewed. The timing can also vary in local time, with the location's precise longitude. Changes in timing of sunset are generally driven by the axial tilt of Earth and the planet's movement around its orbit, but some differences exist. For example, in the Northern Hemisphere, the earliest sunset is not at the winter solstice around December 21, but instead it occurs earlier in December. Likewise, the latest sunset is not at the summer solstice around June 21, but later in June or in early July. The same phenomenon exists in the Southern Hemisphere except with the dates swapped. For one or two weeks surrounding both solstices, both sunrise and sunset get slightly later or earlier each day. Even on the equator, sunrise and sunset shift several minutes back and forth through the year, along with solar noon. This effect is plotted by an analemma.

Due to Earth's axial tilt, the direction of sunset is always to the northwest from the March equinox to the September equinox, and to the southwest from the September equinox to the March equinox.

As sunrise and sunset are calculated from the leading and trailing edges of the Sun, and not the centre, this slightly increases the duration of "day" relative to "night". Further, because the light from the Sun is bent by the atmospheric refraction, the Sun is still seen after it is below the horizon. This effect is a daily illusion along with sunrise.

As a visual motif, sunset is often associated with summer, and (particularly when paired with a coconut palm) beach living and surfing culture. This may be due in the first instance to people spending more time outdoors in the evening during summer than during winter, and also because pictures of sunsets over the sea are often more spectacular than daytime beach scenes (see images below). Sunset is also a symbol of west, old age, ending, and closure.Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, with no Front-Cover Texts, and with no Back-Cover Texts.
Virtual Magic is a human knowledge database blog. Text Based On Information From Wikipedia, Under The GNU Free Documentation License. Copyright (c) 2007 Virtual Magic. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.1 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts and no Back-Cover Texts. A copy of the license is included in the section entitled "GNU Free Documentation License".

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