Sunset

Source: Wikipedia, the free encyclopedia.
For other uses, see Sunset (disambiguation).
Not to be confused with Dusk.
Actual sunset: Two minutes before the Sun disappears below the horizon.

Sunset (or sundown) is the disappearance of the Sun below the horizon of the Earth (or any other astronomical object in the Solar System) due to its rotation. As viewed from everywhere on Earth, it is a phenomenon that happens approximately once every 24 hours, except in areas close to the poles. The equinox Sun sets due west at the moment of both the spring and autumn equinoxes. As viewed from the Northern Hemisphere, the Sun sets to the northwest (or not at all) in the spring and summer, and to the southwest in the autumn and winter; these seasons are reversed for the Southern Hemisphere.

The time of actual sunset is defined in astronomy as two minutes before the upper limb of the Sun disappears below the horizon.[1] Near the horizon, atmospheric refraction causes sunlight rays to be distorted to such an extent that geometrically the solar disk is already about one diameter below the horizon when a sunset is observed.

Sunset is distinct from twilight, which is divided into three stages. The first one is civil twilight, which begins once the Sun has disappeared below the horizon, and continues until it descends to 6 degrees below the horizon. The early to intermediate stages of twilight coincide with predusk. The second phase is nautical twilight, between 6 and 12 degrees below the horizon. The third phase is astronomical twilight, which is the period when the Sun is between 12 and 18 degrees below the horizon.[2] Dusk is at the very end of astronomical twilight, and is the darkest moment of twilight just before night.[3] Finally, night occurs when the Sun reaches 18 degrees below the horizon and no longer illuminates the sky.[4]

Locations further north than the

polar day or the polar night
persists continuously for 24 hours. At latitudes greater than within half a degree of either pole, the sun cannot rise or set on the same date on any day of the year, since the sun's angular elevation between solar noon and midnight is less than one degree.

Occurrence

See also: Analemma
Stages of the twilight period

The time of sunset varies throughout the year and is determined by the viewer's position on Earth, specified by

perihelion, which occurs around January 3).[citation needed
]

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 a few weeks surrounding both solstices, both sunrise and sunset get slightly later 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.[5][6]

Neglecting atmospheric refraction and the Sun's non-zero size, 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 almost exactly due west on the equinoxes for all viewers on Earth. Exact calculations of the azimuths of sunset on other dates are complex, but they can be estimated with reasonable accuracy by using the analemma.[citation needed]

As sunrise and sunset are calculated from the leading and trailing edges of the Sun, respectively, and not the center, the duration of a daytime is slightly longer than nighttime (by about 10 minutes, as seen from temperate latitudes). Further, because the light from the Sun is refracted as it passes through the Earth's atmosphere, the Sun is still visible after it is geometrically below the horizon. Refraction also affects the apparent shape of the Sun when it is very close to the horizon. It makes things appear higher in the sky than they really are. Light from the bottom edge of the Sun's disk is refracted more than light from the top, since refraction increases as the angle of elevation decreases. This raises the apparent position of the bottom edge more than the top, reducing the apparent height of the solar disk. Its width is unaltered, so the disk appears wider than it is high. (In reality, the Sun is almost exactly spherical.) The Sun also appears larger on the horizon, an optical illusion, similar to the moon illusion.[citation needed]

Locations north of the

polar day or the polar night persist continuously for 24 hours.[citation needed
]

Location on the horizon

Further information: Solar azimuth angle
A video time lapse of a sunset in Tokyo

Approximate locations of sunset on the horizon (azimuth) as described above can be found in Refs.[7][8] The figure on the right is calculated using the solar geometry routine as follows:[9]

  1. For a given latitude and a given date, calculate the declination of the Sun using longitude and
    solar noon
    time as inputs to the routine;
  2. Calculate the sunset hour angle using the
    sunset equation
    ;
  3. Calculate the sunset time, which is the solar noon time plus the sunset hour angle in degree divided by 15;
  4. Use the sunset time as input to the solar geometry routine to get the solar azimuth angle at sunset.

An interesting feature in the figure on the right is apparent hemispheric symmetry in regions where daily sunrise and sunset actually occur. This symmetry becomes clear if the hemispheric relation in sunrise equation is applied to the x- and y-components of the solar vector presented in Ref.[9]

Colors

See also: Sunrise § Colors
Further information: Atmospheric optics
Evening twilight in Joshua Tree, California, displaying the separation of yellow colors in the direction from the Sun below the horizon 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
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).[14][15][16]

Sunset colors are typically more brilliant than sunrise colors, because the evening air contains more particles than morning air.[10][11][13][16] Sometimes just before sunrise or after sunset a green flash can be seen.[17]

Ash from volcanic eruptions, trapped within the

stratus clouds containing sulfuric acid
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.

Some of the most varied colors at sunset can be found in the opposite or eastern sky after the Sun has set during twilight. Depending on weather conditions and the types of clouds present, these colors have a wide spectrum, and can produce unusual results.[citation needed]

Names of compass points

In some languages, points of the compass bear names etymologically derived from words for sunrise and sunset. The English words "orient" and "occident", meaning "east" and "west", respectively, are descended from Latin words meaning "sunrise" and "sunset". The word "levant", related e.g. to French "(se) lever" meaning "lift" or "rise" (and also to English "elevate"), is also used to describe the east. In Polish, the word for east wschód (vskhud), is derived from the morpheme "ws" – meaning "up", and "chód" – signifying "move" (from the verb chodzić – meaning "walk, move"), due to the act of the Sun coming up from behind the horizon. The Polish word for west, zachód (zakhud), is similar but with the word "za" at the start, meaning "behind", from the act of the Sun going behind the horizon. In Russian, the word for west, запад (zapad), is derived from the words за – meaning "behind", and пад – signifying "fall" (from the verb падать – padat'), due to the act of the Sun falling behind the horizon. In Hebrew, the word for east is 'מזרח', which derives from the word for rising, and the word for west is 'מערב', which derives from the word for setting.

Historical view

See also: History of astronomy

The 16th-century astronomer Nicolaus Copernicus was the first to present to the world a detailed and eventually widely accepted mathematical model supporting the premise that the Earth is moving and the Sun actually stays still, despite the impression from our point of view of a moving Sun.[18]

Planets

Sunsets on other planets appear different because of differences in the distance of the planet from the Sun and non-existent or differing atmospheric compositions.

Mars

See also: Astronomy on Mars
Sunset on Mars

On Mars, the setting Sun appears about two-thirds the size it does from Earth,[19] due to the greater distance between Mars and the Sun. The colors are typically hues of blue, but some Martian sunsets last significantly longer and appear far redder than is typical on Earth.[20] The colors of the Martian sunset differ from those on Earth. Mars has a thin

Mie Scattering process, resulting in more blue hues than an Earth sunset. One study also reported that Martian dust high in the atmosphere can reflect sunlight up to two hours after the Sun has set, casting a diffuse glow across the surface of Mars.[20]

See also

References

  1. ISBN 978-0-19-960905-5
    , retrieved 2021-10-05
  2. ^ "Definitions from the US Astronomical Applications Dept (USNO)". Archived from the original on 2015-08-14. Retrieved 2016-06-17.
  3. ^ "Full definition of Dusk".
  4. ^ "Sunset vs Dusk [What Is The Difference Between The Two?]". Astronomy Scope. 2020-12-03. Retrieved 2021-10-03.
  5. ^ Starry Night Times – January 2007 (explains why Sun appears to cross slow before early January)
  6. ^ The analemma Archived 2006-10-18 at the Wayback Machine, 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'
  7. ^ Karen Masters (October 2004). "Curious About Astronomy: How does the position of Moonrise and Moonset change?". Curious About Astronomy? Ask an Astronomer. Cornell University Astronomy Department. Retrieved 2016-08-11.
  8. ^ "Where Do the Sun and Stars Rise?". Stanford Solar Center. Retrieved 2012-03-20.
  9. ^ a b Zhang, T., Stackhouse, P.W., Macpherson, B., and Mikovitz, J.C., 2021. A solar azimuth formula that renders circumstantial treatment unnecessary without compromising mathematical rigor: Mathematical setup, application and extension of a formula based on the subsolar point and atan2 function. Renewable Energy, 172, 1333-1340. DOI: https://doi.org/10.1016/j.renene.2021.03.047
  10. ^
    ISBN 978-3-540-78426-5
    .
  11. ^ a b B. Guenther, ed. (2005). Encyclopedia of Modern Optics. Vol. 1. Elsevier. p. 186.
  12. ^ "Hyperphysics, Georgia State University". Hyperphysics.phy-astr.gsu.edu. Retrieved 2012-04-07.
  13. ^ a b Craig Bohren (ed.), Selected Papers on Scattering in the Atmosphere, SPIE Optical Engineering Press, Bellingham, WA, 1989
  14. ^ Corfidi, Stephen F. (February 2009). "The Colors of Twilight and Sunset". Norman, OK: NOAA/NWS Storm Prediction Center.
  15. ^ "Atmospheric Aerosols: What Are They, and Why Are They So Important?". nasa.gov. August 1996.
  16. ^
    ISBN 0-321-18878-0
    .
  17. ^ "Red Sunset, Green Flash".
  18. ^ "The Earth Is the Center of the Universe: Top 10 Science Mistakes". Science.discovery.com. 2012-01-23. Archived from the original on 2012-11-18. Retrieved 2012-04-07.
  19. ^ "A Moment Frozen in Time". Jet Propulsion Laboratory. June 10, 2005. Retrieved September 7, 2011.
  20. ^ a b c Nemiroff, R.; Bonnell, J., eds. (June 20, 2005). "Sunset Over Gusev Crater". Astronomy Picture of the Day. NASA. Retrieved September 6, 2011.

External links

Wikiquote has quotations related to Sunset.
Look up sunset in Wiktionary, the free dictionary.
Wikimedia Commons has media related to Sunset.
Wikimedia Commons has media related to Sunsets.
Retrieved from "https://en.wikipedia.org/w/index.php?title=Sunset&oldid=1217824881"
This page is based on the copyrighted Wikipedia article: Sunset. Articles is available under the CC BY-SA 3.0 license; additional terms may apply.Privacy Policy