Sunset

Sunset, also called sundown in some American English dialects, is the instant when the trailing edge of the sun's disk 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.

The sunset is often more brightly colored than the sunrise,Fact|date=March 2008 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 particular geographical features of the location from which they are viewed. For example, on a west-facing coastline, sunset occurs over water while sunrise occurs over land.

Occurrence

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 anomalies exist however, the main one caused by the Earth's axial tilt and the Earth's elliptical orbit. In the Northern Hemisphere, the earliest sunset does not 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 respective dates being some time "before" June 21 in winter and some time "after" December 21 in summer, possibly in January of the following year. 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. [ [http://www.starrynight.com/sntimes/2007/01/ Starry Night® Times - January 2007] (explains why Sun appears to cross slow before early January)] [ [http://www.analemma.com/Pages/framesPage.html 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 2, the sun drifts back toward the east']

Due to Earth's axial tilt, the direction of sunset in the northern hemisphere is always to the northwest from the March equinox to the September equinox, and to the southwest 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 approximately 12 hours on the equinoxes for all viewers on Earth (precisely 12 hours if measured from the centre of the sun).

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.

Colors

The intense red and orange hues of the sky at sunset and sunrise are mainly caused by scattering of sunlight off dust particles, soot particles, other solid aerosols, and liquid aerosols floating in the earth's atmosphere. These enhanced red and orange colors at sunset and sunrise are mathematically explained by Mie theory or the discrete dipole approximation. When there are no particulates in the troposphere, such as after a big rain storm, then the remaining less intense reds are explained by Rayleigh Scattering of sunlight off of air molecules. The brilliance and intensity of the colors of sunsets are typically greater than sunrises, since there are generally more particles in the evening air than in the morning air. Nighttime air is usually cooler and less windy which allows the dust and soot particles to settle out of the atmosphere, reducing the amount of Mie Scattering and the amount of red and orange scattered light at sunrise. 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.

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

unsets on other planets

Sunsets on other planets appear different because of the differences in the distance from the planet to the sun and in different atmospheric compositions.

Because Mars is farther from the Sun than the Earth is, the Sun appears only about two-thirds the size that it appears in a sunset seen from the Earth. [http://photojournal.jpl.nasa.gov/catalog/PIA07997 This article incorporates text from this source, which is in the public domain.] Although Mars lacks oxygen and nitrogen, it is covered in red dust frequently hoisted into the atmosphere by fast but thin winds.http://apod.nasa.gov/apod/ap050620.html This article incorporates text from this source, which is in the public domain.] At least some Martian days are capped by a sunset significantly longer and redder than typical on Earth. One study found that for up to two hours after twilight, sunlight continued to reflect off Martian dust high in the atmosphere, casting a diffuse glow.

ee also

*Afterglow
*Astronomy on Mars
*Dawn
*Day length
*Diffuse sky radiation
*Noon
*Sunrise
*Sunrise equation
*Twilight

References

External links

* [http://commons.wikimedia.org/wiki/Sunset#Sunsets_on_other_planets Sunsets on other planets] (Wikimedia Commons gallery)
* [http://www.sunrisesunset.com/ Sunrise and sunset calculator]
* [http://math.ucr.edu/home/baez/physics/General/BlueSky/blue_sky.html Full physical explanation in simple terms]
* [http://www.ecy.wa.gov/programs/eap/models/twilight.zip Excel workbook] with VBA functions for sunrise, sunset, solar noon, twilight (dawn and dusk), and solar position (azimuth and elevation); by [http://www.ecy.wa.gov/programs/eap/models.html Greg Pelletier] , translated from NOAA's online calculators for [http://www.srrb.noaa.gov/highlights/sunrise/azel.html solar position] and [http://www.srrb.noaa.gov/highlights/sunrise/sunrise.html sunrise/sunset]
* [http://sun.exnatura.org sun.exnatura.org] Online sunrise/sunset calendar with interactive location finder
* [http://www.icelandphotoblog.com/index.php?x=browse&tag=sunset Sunset photos from Iceland at ICELANDPHOTOBLOG.COM]