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==Colors==
==Colors==
[[Image:Jamaica sunrise.JPG|right|thumb|270px|A sunrise with the typical orange color in the sky (south beach of Jamaica).]]
[[Image:Jamaica sunrise.JPG|right|thumb|270px|A sunrise with the typical orange color in the sky (south beach of Jamaica).]]
The intense red and orange [[hue]]s of the sky at sunrise and sunset are the result of the removal of blue components of sun light due to [[Rayleigh scattering]] and absorption in the atmosphere. The path length of sunlight through the atmosphere during morning and evening hours is much longer than during the day when the light has a small [[angle of incidence]] with respect to the surface of Earth for a stationary observer.
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 sunrise, 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.
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>
<ref>{{cite web|url=http://ucsu.colorado.edu/~kuesterm/RTweb/startRT.html|title=Science Made Simple}}</ref>


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.
⚫
The color of sunsets or sunrises is often enhanced by the presence of particulate matter, in form of dust or moisture, in the atmosphere. Scattering from particles of sizes comparable to the wavelength of light results from the [[Mie theory]], which is a solution of [[Maxwell's equations]] in the [[discrete dipole approximation]]. Mie scattering is largely wavelength insensitive and the important term for scattering is the sixth power dependency of the intensity in the diameter of the particles. 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 setting sun directly, rather than looking in other directions.


Sunrise color intensities can however exceed sunset's intensities when there are nighttime fires, volcanic eruptions or emissions, or dust storms to the east of the viewer. 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.
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.


Sometimes just before sunrise or after sunset a [[green flash]] can be seen.
Sometimes just before sunrise or after sunset a [[green flash]] can be seen.<ref>{{cite web|url=http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/redsun.html|title=Red Sunset, Green Flash}}</ref>
<ref>
[http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/redsun.html Red Sunset, Green Flash]
</ref>
<ref>
Selected Papers on Scattering in the Atmosphere, edited by Craig
Bohren ~SPIE Optical Engineering Press, Bellingham, WA, 1989
</ref>
<ref>
[http://ucsu.colorado.edu/~kuesterm/RTweb/startRT.html Science Made Simple]
</ref>


== See also ==
== See also ==

Revision as of 03:13, 16 November 2010

For other uses, see Sunrise (disambiguation).
Sunrise over the Bristol Channel, England.
Sunrise over the Dead Sea seen from Masada, Israel.
Sunrise over the Black sea, Varna, Bulgaria.
Sunrise over Igоumenitsa.

Sunrise is the instant at which the upper edge of the Sun appears above the horizon in the east. Sunrise should not be confused with dawn, which is the (variously defined) point at which the sky begins to lighten, some time before the sun itself appears, ending twilight. Because atmospheric refraction causes the sun to be seen while it is still below the horizon, both sunrise and sunset are, from one point of view, optical illusions. The sun also exhibits an optical illusion at sunrise similar to the moon illusion.

The apparent westward revolution of Sun around the earth after rising out of the horizon is due to the Earth's eastward rotation, a counter-clockwise revolution when viewed from above the North Pole. This illusion is so convincing that most cultures had mythologies and religions built around the geocentric model. This same effect can be seen with near-polar satellites as well.

Sunrise and sunset are calculated from the leading and trailing edges of the Sun, and not the center; this slightly increases the duration of "day" relative to "night". The sunrise equation, however, is based on the center of the sun.

The timing of sunrises vary throughout the year, as determined by the viewer's longitude and latitude, altitude, the time of year, and time zone of the viewer's location. Small daily changes and noticeable semi-annual changes in the timing of sunrises 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 sunrises occur earlier every day until the day of the earliest sunrise, which occurs before the summer solstice. In the Northern Hemisphere, the earliest sunrise does not fall on the summer solstice around June 21, but occurs earlier in June. The precise date of the earliest sunrise depends on the viewer's latitude (connected with the slower Earth's movement around the aphelion around July 4). Similarly, the latest sunrise does not occur on the winter solstice, but rather about two weeks later, again depending the viewer's latitude In the Northern Hemisphere the latest sunrise occurs in early January (influence from the Earth's faster movement near the perihelion which occurs around January 3). Likewise, the same phenomena exist in the Southern Hemisphere except with the respective dates reversed, with the latest sunrises occurring some time after June 21 in winter and earliest sunrises occurring some time before 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 sunrise occurs, it is always in the northeast quadrant from the March equinox to the September equinox and in the southeast quadrant from the September equinox to the March equinox. Sunrises occur precisely due east on the March and September 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, 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

A sunrise with the typical orange color in the sky (south beach of Jamaica).

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 sunrise, 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]

See also

This is a False Sunrise, a very particular kind of Parhelion

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 2nd until January 21, the sun drifts back toward the east'
  3. ^ "Red Sunset, Green Flash".
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