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==Colors==
==Colors==
[[File:Knysnasunset.jpg|250px|thumb|Sunset|Sunset in Knysna, South Africa, displaying the separation of orange colors in the direction from the sun to the observer and the blue components scattered from the surrounding sky.]]
[[File:Knysnasunset.jpg|250px|thumb|Sunset|Sunset in Knysna, South Africa, displaying the separation of orange colors in the direction from the sun to the observer and the blue components scattered from the surrounding sky.]]
In the atmosphere the incident solar white light is attenuated by scattering and absorption by the air molecules and [[Aerosol|airborne particles]] by a combination of [[Rayleigh scattering]] and Mie scattering.<ref name=saha>{{cite book
The incident solar light is attenuated in the atmosphere by scattering and absorption by air molecules and [[particulate matter]] suspended in the air.<ref>M.Z.Jacobson (2005),[http://books.google.com/books?id=WPwEf-1f73wC&pg=PA290#v=onepage&q&f=false Fundamentals of atmospheric modeling], 2nd ed., University Press, Cambridge, UK, p.290</ref>
Light scattering in the form of [[Rayleigh scattering]] preferentially scatters light at the short-[[wavelength]] end of the [[electromagnetic spectrum]] (violet-blue) with respect to the orange to red color of longer wavelengths. Therefore, the violet and blue components of white sun light are scattered out of the direct light beam from the Sun to an observer into other directions. During the day, when the Sun is high in the sky, this results in a perception of a yellow Sun and the blue sky.
|author=K. Saha
|title=The Earth's Atmosphere - Its Physics and Dynamics
|year=2008
|publisher=Springer
|isbn=978-3-540-78426-5
|page=107
}}</ref> The path that sunlight travels through the atmosphere is longer when the sun is on the horizon, and thus there is more attenuation by air molecules and light scattering by dust and aerosols at sunrise and sunset. Because shorter wavelengths (blues and greens) scatter more strongly, blues and greens are removed almost completely from the incident beam, leaving the longer wavelength orange and red [[hue]]s at [[sunrise]] and sunset to be further scattered by Mie scattering across the horizon to produce intense reds and oranges when there are soot, dust, or solid or liquid aerosols in the atmosphere.<ref name=guenther>{{cite book
|author=B. Guenther (ed.)
|title=Encyclopedia of Modern Optics
|publisher=Elsevier
|year=2005
|volume=Vol. 1
|page=186
}}</ref> The removal of the shorter wavelength of light is due to [[Rayleigh scattering]] by air molecules and small particles of sizes an order of magnitude smaller that the wavelength of visible light.<ref>[http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/blusky.html Hyperphysics, Georgia State University]</ref><ref>Craig Bohren (ed.), ''Selected Papers on Scattering in the Atmosphere'', SPIE Optical Engineering Press, Bellingham, WA, 1989</ref> The sun is actually white when observed without any air between the viewer and the sun, so, sunlight in outer space initially contains a mixture of violets, blues, greens, yellows, oranges and reds. Due to [[Rayleigh scattering]], the sun appears reddish or yellowish when we look at it, since the longer wavelengths of reds and yellow light are scattered the least, passing through the air to the viewer, and shorter wavelengths like blue and green light are effectively removed by air's [[Rayleigh scattering]].


During sunrise and sunset, however, when the Sun is close to, or even below the horizon, the path of sunlight through the atmosphere is much longer than during the day, resulting in much stronger attenuation and lower overall light intensity. The complete removal of light in the blue end of the spectrum from the incident beam reduces the yellow day time image of the Sun to orange or orange-red [[hue]]s at sunrise and sunset.<ref name=guenther>{{cite book
[[Rayleigh scattering]] is the [[elastic scattering]] of electromagnetic radiation due to the polarizability of the electron cloud in molecules, or by particles much smaller than the wavelength of visible light. Rayleigh scattering intensity is fairly omnidirectional and has a strong reciprocal 4th-power wavelength dependency and, thus, the shorter wavelengths of violet and blue light are effected much more than the longer wavelengths of yellow to red light. During the day, this scattering results in the increasingly intense blue color of the sky away from the direct line of sight to the Sun, while during sunrise and sunset, the much longer path length through the atmosphere results in the complete removal of blue and green light from the incident rays, leaving weak intensities of orange to red light.<ref name=hecht>{{cite book
|author=E. Hecht
|title=Optics
|edition=4th
|year=2002
|publisher=Addison Wesley
|isbn=0321188780
|page=88
}}</ref>
[[File:SDIM0241b.jpg|thumb|250px|Typical less intense sunset colors from just Rayleigh scattering, with very little Mie scattering, due to the lack of atmospheric aerosols. This picture was taken about one hour after sunset at 500m altitude, looking at the horizon where the sun had set.]]
The viewing of red and orange colors of sunsets and sunrises is then enhanced by the presence of particulate matter, in form of dust, soot, water droplets, or other aerosols in the atmosphere, (notably sulfuric acid droplets from volcanic eruptions). Particles much smaller than the wavelength of the incident light efficiently enhance the blue colors for off-axis short path lengths through air (Rayleigh scattering intensity increases as the sixth power of the particle diameter). Larger particles, instead, with sizes comparable to and longer than the wavelength of light, scatter by mechanisms treated, for spherical shapes, by the [[Mie theory]]. Mie scattering is largely wavelength insensitive. Its spacial distribution is highly preferential in the forward direction of the incident light being scattered, thus having its largest effect when an observer views the light of the rising or setting Sun directly, rather than looking in other directions. It generally causes a diffuse white halo around the Sun decreasing the perception of blue color in the direction toward the Sun. Mie scattering off of large particles and droplets off axis from the sun, transmits all wavelengths of light fairly similarly, which is why milk and clouds appear white. At sunset and sunrise Mie scattering off of particles and aerosols across the horizon then transmits the red and orange light that remains after [[Rayleigh scattering]] has depleted the blue light. This explains why sunsets without soot, dust, or aerosols are dull and fairly faint red, while sunsets and sunrises are brilliantly intense when there are lots of soot, dust, or other aerosols in the air.
<ref>{{cite web|url=http://www.spc.noaa.gov/publications/corfidi/sunset/}}</ref>
<ref>{{cite web|url=http://www.nasa.gov/centers/langley/news/factsheets/Aerosols.html}}</ref>

Sunset colors are typically more brilliant than sunrise colors, because the evening air contains generally more particles and aerosols and clouds than morning air. Cloud droplets are much larger than the wavelength of light; so they scatter all colors equally by Mie scattering, which makes them appear white when illuminated by white sunlight during the daytime. The clouds glow orange and red due to Mie scattering during a sunset because they are illuminated with the orange and red light that remains after multiple prior Rayleigh scattering events of the light from the setting sun.
<ref name=guenther>{{cite book
|author=B. Guenther (ed.)
|author=B. Guenther (ed.)
|title=Encyclopedia of Modern Optics
|title=Encyclopedia of Modern Optics

Revision as of 17:06, 10 December 2010

For other uses, see Sunset (disambiguation).
The Sun, about a minute before astronomical sunset.

Sunset or sundown is the daily disappearance of the Sun below the horizon as a result of Earth's rotation.

The time of sunset is defined in astronomy as the moment the trailing edge of the Sun's disk disappears below the horizon in the west. The ray path of light from the setting Sun is highly distorted near the horizon because of atmospheric refraction, making the apparent astronomical sunset occur when the Sun’s disk is already about one diameter below the horizon. Sunset is distinct from dusk, which is the moment at which darkness falls, which occurs when the Sun is approximately eighteen degrees below the horizon. The period between the astronomical sunset and dusk is called twilight.

Locations north of the Arctic Circle and south of the Antarctic Circle experience no sunset or sunrise at least one day of the year, when the polar day or the polar night persist 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.

Occurrence

The time of sunset varies throughout the year, and is determined by the viewer's position on Earth, specified by longitude and latitude, and elevation. Small daily changes and noticeable semi-annual changes in the timing of sunsets are driven by the axial tilt of Earth, daily rotation of the Earth, the planet's movement in its annual elliptical orbit around the Sun, and the Earth and Moon's paired revolutions around each other. In the summertime, the days get longer and sunsets occur later every day until the day of the latest sunset, which occurs after the summer solstice. In the Northern Hemisphere, the latest sunset occurs late in June or in early July, but not on the summer solstice of June 21. This date depends on the viewer's latitude (connected with the slower Earth's movement around the aphelion around July 4). Similarly, the earliest sunset does not occur on the winter solstice, but rather about two weeks earlier, again depending on the viewer's latitude. In the Northern Hemisphere it occurs in early December (influence from the Earth's faster movement near the perihelion which occurs around January 3).

Likewise, the same phenomenon exists in the Southern Hemisphere, but with the respective dates reversed, with the earliest sunsets occurring some time before June 21 in winter, and latest sunsets occurring some time after December 21 in summer, again depending on one's southern latitude. 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. These effects are plotted by an analemma.[1][2]

Due to Earth's axial tilt, whenever and wherever sunset occurs, it is always in the northwest quadrant from the March equinox to the September equinox, and in the southwest quadrant from the September equinox to the March equinox. Sunsets occur precisely due west 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 center, the duration of a day time is slightly longer than night time. Further, because the light from the Sun is refracted, the Sun is still visible after it is geometrically below the horizon. The Sun also appears larger on the horizon, an optical illusion, similar to the moon illusion.

Locations north of the Arctic Circle and south of the Antarctic Circle experience no sunset or sunrise at least one day of the year, when the polar day or the polar night persist continuously for 24 hours.

Colors

Sunset in Knysna, South Africa, displaying the separation of orange colors in the direction from the sun to the observer and the blue components scattered from the surrounding sky.

The incident solar light is attenuated in the atmosphere by scattering and absorption by air molecules and particulate matter suspended in the air.[3] Light scattering in the form of Rayleigh scattering preferentially scatters light at the short-wavelength end of the electromagnetic spectrum (violet-blue) with respect to the orange to red color of longer wavelengths. Therefore, the violet and blue components of white sun light are scattered out of the direct light beam from the Sun to an observer into other directions. During the day, when the Sun is high in the sky, this results in a perception of a yellow Sun and the blue sky.

During sunrise and sunset, however, when the Sun is close to, or even below the horizon, the path of sunlight through the atmosphere is much longer than during the day, resulting in much stronger attenuation and lower overall light intensity. The complete removal of light in the blue end of the spectrum from the incident beam reduces the yellow day time image of the Sun to orange or orange-red hues at sunrise and sunset.[4][5][6]

The color of the sunset or sunrise sky is often affected or enhanced by the presence of dust, moisture, or other contamination in the atmosphere. Other light scattering mechanisms (Mie scattering) are largely color-neutral but sensitive to the size of airborne particles. Its spacial distribution is highly preferential in the forward direction of the incident light beam, thus having its largest effect when an observer views the light of the rising or setting Sun directly, rather than looking in other directions. It generally causes only a diffuse colorless haze around the Sun. Brilliant colors may result in the sky from reflections from cloud layers or particulate concentrations at various altitudes.

While ash from volcanic eruptions tends to mute sunset colors[7] when trapped within the troposphere, when lofted into the stratosphere, thin clouds of tiny sulfuric acid droplets from volcanoes can yield beautiful post-sunset colors called afterglows. 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.[8]

Planets

Sunset on Mars.

Sunsets on other planets appear different because of the differences in the distance of the planet from the Sun, as well as different atmospheric compositions.

On Mars, the Sun appears only about two-thirds of the size than it appears in a sunset seen from the Earth,[9] because Mars is farther from the Sun than the Earth. Although Mars lacks oxygen and nitrogen gas in the atmosphere, it is covered in red dust frequently hoisted into the atmosphere by fast but thin winds.[10] At least some Martian days are capped by a sunset significantly longer and redder than typical on Earth.[10] One study reported that for up to two hours after twilight, sunlight continued to reflect off Martian dust high in the atmosphere, casting a diffuse glow.[10]

See also

References

  1. ^ Starry Night Times - January 2007 (explains why Sun appears to cross slow before early January)
  2. ^ The analemma, elliptical orbit effect. 'July 3rd to October 2nd the sun continues to drift to the west until it reaches its maximum "offset" in the west. Then from October 2 until January 21, the sun drifts back toward the east'
  3. ^ M.Z.Jacobson (2005),Fundamentals of atmospheric modeling, 2nd ed., University Press, Cambridge, UK, p.290
  4. ^ B. Guenther (ed.) (2005). Encyclopedia of Modern Optics. Vol. Vol. 1. p. 186. {{cite book}}: |author= has generic name (help); |volume= has extra text (help); Unknown parameter |Publisher= ignored (|publisher= suggested) (help)
  5. ^ Selected Papers on Scattering in the Atmosphere, edited by Craig Bohren ~SPIE Optical Engineering Press, Bellingham, WA, 1989
  6. ^ E. Hecht (2002). Optics (4th ed.). Addison Wesley. p. 88. ISBN 0321188780.
  7. ^ K. Saha (2008). The Earth's Atmosphere - Its Physics and Dynamics. Springer. p. 107. ISBN 978-3-540-78426-5.
  8. ^ "Red Sunset, Green Flash".
  9. ^ NASA This article incorporates text from this source, which is in the public domain.
  10. ^ a b c NASA, This article incorporates text from this source, which is in the public domain.
Wikimedia Commons has media related to Sunset.
Wikimedia Commons has media related to Sunset on Mars.
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