Sunset
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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,[citation needed] 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.
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. 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 dates swapped. 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.
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 precisely 12 hours 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 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.
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[edit] Colors
The red hues of the sky at sunset and sunrise are caused by Rayleigh scattering, the same mechanism that causes the sky to be blue. [1] [2] [3]
As light travels through the atmosphere, some of it is scattered at large angles by small particles. This type of scattering, where the particles are much smaller than the wavelength, is called Rayleigh scattering. Because Rayleigh scattering is much stronger for shorter wavelengths, like blue, the light from a clear sky is blue. Likewise, as the blue component is scattered out of a ray of sunlight, the remaining light becomes yellower. At sunrise or sunset the light has traveled a long distance through the atmosphere leaving only the longest wavelengths like orange and red. The reddened sunlight illuminates clouds and other particles. The combination of orange and red clouds and the blue sky can produce a variety of colors.
Volcanic eruptions release particles into the atmosphere that affect scattering. 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.
[edit] Sunsets 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.[4] Although Mars lacks oxygen and nitrogen, 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 found that for up to two hours after twilight, sunlight continued to reflect off Martian dust high in the atmosphere, casting a diffuse glow.[5]
[edit] See also
[edit] References
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This article needs additional citations for verification. Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (May 2008) |
- ^ Red Sunset, Green Flash
- ^ Selected Papers on Scattering in the Atmosphere, edited by Craig Bohren ~SPIE Optical Engineering Press, Bellingham, WA, 1989
- ^ Science Made Simple
- ^ http://photojournal.jpl.nasa.gov/catalog/PIA07997 This article incorporates text from this source, which is in the public domain.
- ^ a b c http://apod.nasa.gov/apod/ap050620.html This article incorporates text from this source, which is in the public domain.
[edit] External links
- Sunrise and sunset calculator
- 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
- sun.exnatura.org Online sunrise/sunset calendar with interactive location finder
