Sunset: Difference between revisions
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==Colors== |
==Colors== |
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[[File:Sunset at Land's end in San Francisco.jpg|250px|thumb|Red sunset at [[Lands End, San Francisco]].]] |
[[File:Sunset at Land's end in San Francisco.jpg|250px|thumb|Red sunset at [[Lands End, San Francisco]].]] |
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The intense red and orange [[hue]]s of the sky at [[sunrise]] and sunset are the result of the |
The intense red and orange [[hue]]s of the sky at [[sunrise]] and sunset are the result of the scattering of red and orange components of sun light off of atmospheric dust and atmospheric aerosols, explained by [[Mie scattering]]. At sunset, sunlight has a much longer path length through the lower dust-laden atmosphere and aerosol laden parts of the middle sections of the atmosphere during morning and evening, than it has during mid-day, and sunset and sunrise sunlight light has a smaller [[angle of incidence]] with respect to the surface of Earth for a stationary observer. |
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| ⚫ | The color of sunsets or sunrises is mainly caused by the scattering of sunlight off of particulate matter (dust) and off of other solid and liquid aerosols in the atmosphere. Scattering from these particles (larger than the wavelength of light) is described by [[Mie theory]], which is a solution of [[Maxwell's equations]], and it can also be described by the [[discrete dipole approximation]]. Mie scattering is largely wavelength insensitive, with the important term for scattering intensity having the sixth power dependency on the diameter of the particles. Mie scattering's 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 setting sun directly, rather than looking in other directions. |
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Rayleigh scattering is the [[elastic scattering]] of electromagnetic radiation due to the polarizability of the electron cloud in atoms and molecules, particles much smaller than the wavelength of visible light. The Rayleigh scattering intensity is fairly omnidirectional and has a strong reciprocal 4th-power wavelength dependency and, thus, the shorter wavelength of blue light are effected much more than the longer wavelengths of yellow to red light. The enhanced scattering increases the path length for blue light preferentially, increasing its absorption in the atmosphere, resulting in drastically diminished blue intensity in sunlight reaching an observer.<ref>Selected Papers on Scattering in the Atmosphere, edited by Craig Bohren ~SPIE Optical Engineering Press, Bellingham, WA, 1989</ref> |
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<ref>{{cite web|url=http://ucsu.colorado.edu/~kuesterm/RTweb/startRT.html|title=Science Made Simple}}</ref> |
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| ⚫ | For this reason sunset colors are typically more brilliant than [[sunrise]] colors, because there are generally more particles in the evening air than in the morning air due to daytime winds. Since cloud droplets and dust particles in clouds are much larger than the wavelength of visible light, clouds appear white in the daytime, and they glow red during a sunset because they are illuminated with the red light from the setting sun. Sunrise colors can be more intense than sunset colors when there have been heavy rain storms that wash out particulate matter, or when there is high altitude ash and aerosols from volcanoes or fires in the eastern sky. |
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| ⚫ | The color of sunsets or sunrises is |
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| ⚫ | For this reason sunset colors are typically more brilliant than [[sunrise]] colors, because there are generally more particles in the evening air than in the morning air. |
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While ash from volcanic eruptions tends to mute sunset colors 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 [[afterglow]]s. 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. |
While ash from volcanic eruptions tends to mute sunset colors 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 [[afterglow]]s. 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. |
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Revision as of 03:05, 16 November 2010
Sunset or sundown is the daily disappearance of the sun below the horizon as a result of the Earth's rotation. The atmospheric conditions created by the setting of the sun, occurring before and after it disappears below the horizon, are also commonly referred to as "sunset".
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. Due to refraction of light in the atmosphere, the ray path of the setting sun is highly distorted near the horizon making the apparent astronomical sunset occur when the sun’s disk is already about one diameter below the horizon. Sunset should not be confused with dusk, which is the moment at which darkness falls, when the sun is about eighteen degrees below the horizon. The period between the astronomical sunset and dusk is called twilight.
Occurrence
The timing of sunsets vary throughout the year, as determined by the viewer's longitude and latitude, elevation, the time of year, and time zone of the viewer's location. 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 does not fall on the summer solstice around June 21, but occurs later in June or in early July. The precise date of the latest sunset 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 the viewer's latitude. In the Northern Hemisphere the earliest sunset occurs in early December (influence from the Earth's faster movement near the perihelion which occurs around January 3). Likewise, the same phenomena exists in the Southern Hemisphere except 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 "day" is slightly longer than "night". Further, because the light from the sun is bent by the atmospheric refraction, the sun is still visible after it is geometrically below the horizon. The sun also appears larger on the horizon, which is another optical illusion, similar to the moon illusion.
Colors
The intense red and orange hues of the sky at sunrise and sunset are the result of the scattering of red and orange components of sun light off of atmospheric dust and atmospheric aerosols, explained by Mie scattering. At sunset, sunlight has a much longer path length through the lower dust-laden atmosphere and aerosol laden parts of the middle sections of the atmosphere during morning and evening, than it has during mid-day, and sunset and sunrise sunlight light has a smaller angle of incidence with respect to the surface of Earth for a stationary observer.
The color of sunsets or sunrises is mainly caused by the scattering of sunlight off of particulate matter (dust) and off of other solid and liquid aerosols in the atmosphere. Scattering from these particles (larger than the wavelength of light) is described by Mie theory, which is a solution of Maxwell's equations, and it can also be described by the discrete dipole approximation. Mie scattering is largely wavelength insensitive, with the important term for scattering intensity having the sixth power dependency on the diameter of the particles. Mie scattering's 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 setting sun directly, rather than looking in other directions.
For this reason sunset colors are typically more brilliant than sunrise colors, because there are generally more particles in the evening air than in the morning air due to daytime winds. Since cloud droplets and dust particles in clouds are much larger than the wavelength of visible light, clouds appear white in the daytime, and they glow red during a sunset because they are illuminated with the red light from the setting sun. Sunrise colors can be more intense than sunset colors when there have been heavy rain storms that wash out particulate matter, or when there is high altitude ash and aerosols from volcanoes or fires in the eastern sky.
While ash from volcanic eruptions tends to mute sunset colors 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.[3]
Planets
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,[4] 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.[5] At least some Martian days are capped by a sunset significantly longer and redder than typical on Earth.[5] 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.[5]
See also
- Afterglow
- Astronomy on Mars
- Day length
- Diffuse sky radiation
- Earth's shadow, visible at sunset
- Sunrise equation
References
- ^ Starry Night Times - January 2007 (explains why Sun appears to cross slow before early January)
- ^ 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'
- ^ "Red Sunset, Green Flash".
- ^ NASA This article incorporates text from this source, which is in the public domain.
- ^ a b c NASA, This article incorporates text from this source, which is in the public domain.
External links
- Accurate Sun Position Calculator - Sunrise/Sunset times worldwide
- Sunrise and sunset calculator
- Sunset at your location by Wolfram Alpha
- Full physical explanation in simple terms
- Excel workbook with VBA functions for sunrise, sunset, solar noon, twilight (dawn and dusk), and solar position (azimuth and elevation); by Greg Pelletier, translated from NOAA's online calculators for solar position and sunrise/sunset
- The colors of twilight and sunset
- Calculate & view sunrise and sunset times for an entire year
