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'''Sunset''', also called '''sundown''' in some ] ]s, is the ] at which the ] disappears below the ] in the ]. It should not be confused with ], which is the point at which ] falls, some time after the beginning of ] when the sun itself sets. '''Sunset''', also called '''sundown''' in some ] ]s, is the ] at which the ] disappears below the ] in the ]. It should not be confused with ], which is the point at which ] falls, some time after the beginning of ] when the sun itself sets.
] dividing night from day, running across Europe and Africa. Observers on the surface of the earth along this terminator will see a sunset.]] ] dividing night from day, running across Europe and Africa. Observers on the surface of the earth along this terminator will see a sunset.]]
]<font color="ffffff">Photo: Marc Averette </font>]] ] sunset in ]]]
The sunset is often more brightly coloured than the ], 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 ] and increasing wind speed and ], which serves to lift dust into the air. However, differences between sunrise and sunset may in some cases depend more on the ] particulars of the location from which they are viewed. For example, on a west-facing ], sunset occurs over water while sunrise occurs over land. The sunset is often more brightly coloured than the ], 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 ] and increasing wind speed and ], which serves to lift dust into the air. However, differences between sunrise and sunset may in some cases depend more on the ] particulars of the location from which they are viewed. For example, on a west-facing ], sunset occurs over water while sunrise occurs over land.


Revision as of 00:29, 30 January 2008

For other uses, see Sunset (disambiguation). "Sundown" redirects here. For other uses, see Sundown (disambiguation).
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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.

A composite image showing the terminator dividing night from day, running across Europe and Africa. Observers on the surface of the earth along this terminator will see a sunset.
Mock mirage sunset in San Francisco

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 also varies in local time within a given time zone, determined by each location's precise longitude. Changes in timing of sunset are driven by the axial tilt of Earth, the spherical shape of the Earth, and the planet's movement in its annual orbit around the sun. Some apparent anomolies exist however. In the Northern Hemisphere, the earliest sunset does fall on the winter solstice around December 21, but instead it occurs earlier in December. Likewise, the latest sunset does not fall on the summer solstice around June 21, but instead it happens later in June or in early July, depending on your latitude. 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 and annual orbit around the sun, sunsets in the Northern Hemisphere appear in the northwest quadrant of the sky from the March equinox to the September equinox and in the southwest quadrant from the September equinox to the March equinox. This effect is reversed for viewers in the Southern Hemisphere, where sunsets appear in the southwest quadrant of the sky from the March equinox to the September equinox and in the northwest quadrant from the September equinox to the March equinox. Sunsets occur precisely due west on the March and September equinoxes, and the duration of day and night are precisely 12 hours on the Equinoxes for all viewers on Earth.

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. The sun also appears larger on the horizon, which is another optical illusion, similar to the moon illusion.

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.

Technical detail

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.

Types of sunset

See also

External links

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