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'''Sunset''' or '''sundown''' is the daily disappearance of the [[Sun]] below the [[horizon]] in the [[west]] as a result of [[Earth's rotation]].
'''Sunset''' or '''sundown''' is the daily disappearance of the [[Sun]] below the [[horizon]] in the [[west]] 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 sunset appear to 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 sunset and dusk is called [[twilight]].
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 sunset appear to 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 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.
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.

Revision as of 13:07, 26 September 2011

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

Sunset or sundown is the daily disappearance of the Sun below the horizon in the west 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 sunset appear to 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 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 Earth's slower 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 (by about 10 minutes). 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.

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.[3] 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.[4] The removal of the shorter wavelengths of light is due to Rayleigh scattering by air molecules and particles much smaller than the wavelength of visible light (less than 50 nm in diameter).[5][6] 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). Without Mie scattering at sunset and sunrise, the sky along the horizon has only a dull reddish appearance, while the rest of the sky remains mostly blue and sometimes green.[7][8][9]

Sunset colors are typically more brilliant than sunrise colors, because the evening air contains more particles than morning air.[3][4][9][6]

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 called afterglows 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.[10]

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 that it appears in a sunset seen from the Earth,[11] because Mars is further 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.[12] At least some Martian days are capped by a sunset significantly longer and redder than typical on Earth.[12] 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.[12]

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. ^ a b K. Saha (2008). The Earth's Atmosphere - Its Physics and Dynamics. Springer. p. 107. ISBN 978-3-540-78426-5.
  4. ^ a b B. Guenther (ed.) (2005). Encyclopedia of Modern Optics. Vol. Vol. 1. Elsevier. p. 186. {{cite book}}: |author= has generic name (help); |volume= has extra text (help)
  5. ^ Hyperphysics, Georgia State University
  6. ^ a b Craig Bohren (ed.), Selected Papers on Scattering in the Atmosphere, SPIE Optical Engineering Press, Bellingham, WA, 1989
  7. ^ Corfidi, Stephen F. (February 2009). "The Colors of Twilight and Sunset". Norman, OK: NOAA/NWS Storm Prediction Center.
  8. ^ "Atmospheric Aerosols: What Are They, and Why Are They So Important?". nasa.gov. August 1996.
  9. ^ a b E. Hecht (2002). Optics (4th ed.). Addison Wesley. p. 88. ISBN 0321188780.
  10. ^ "Red Sunset, Green Flash".
  11. ^ "A Moment Frozen in Time". Jet Propulsion Laboratory. June 10, 2005. Retrieved September 7, 2011. This article incorporates text from this source, which is in the public domain.
  12. ^ a b c "Astronomy Picture of the Day:Sunset Over Gusev Crater". NASA. June 20, 2005. Retrieved September 6, 2011. 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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