Astronomy:Sunset

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Sunset (or sundown) is the disappearance of the Sun at the end of the Sun path, 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 sunset is defined in astronomy as the moment 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 over the Delaware Bay at Sunset Beach, New Jersey, U.S., seen through cirrus clouds

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 farther north than the Arctic Circle and farther south than the Antarctic Circle experience no full sunset or sunrise on at least one day of the year, when 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: Astronomy:Analemma
Stages of the twilight period

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 the 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]


Locations within the Arctic and Antarctic Circles experience periods where the Sun does not rise or set for 24 hours or more, known as polar day and polar night. These phenomena occur due to Earth’s axial tilt, causing continuous sunlight or darkness at certain times of the year.[7]

Location on the horizon

File:Sunset tokyoarea-timelapse-2019-03-17.webm

Approximate locations of sunset on the horizon (azimuth) as described above can be found in Refs.[8][9] The figure on the right is calculated using the solar geometry routine as follows:[10]

  1. For a given latitude and a given date, calculate the declination of the Sun using 0∘ 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.[10] Solar geometry routines that model solar azimuth angles at sunset permit the calculation using latitude, date, and time parameters to be done precisely.[11]

Colors

Intense red clouds caused by Rayleigh scattering during a sunset over Meknes, Morocco
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 wavelength components, such as blue and green, scatter more strongly, these colors are preferentially removed from the beam.[12] At sunrise and sunset, when the path through the atmosphere is longer, the blue and green components are removed almost completely, leaving the longer wavelength orange and red hues we see at those times. The remaining reddened sunlight can then be scattered by cloud droplets and other relatively large particles to light up the horizon red and orange.[13] The removal of the shorter wavelengths of light is due to Rayleigh scattering by air molecules and particles much smaller than the wavelength of visible light (less than 50 nm in diameter).[14][15] The scattering by cloud droplets and other particles with diameters comparable to or larger than the sunlight's wavelengths (> 600 nm) is due to 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).[16][17][18]

Sunset over Tappan Zee (Mario Cuomo) Bridge on the Hudson River.

Sunset colors are typically more brilliant than sunrise colors, because the evening air contains more particles than morning air.[12][13][15][18] Sometimes just before sunrise or after sunset a green flash can be seen.[19]

Ash from volcanic eruptions, trapped within the troposphere, tends to mute sunset and sunrise colors, while volcanic ejecta that is instead lofted into the stratosphere (as thin clouds of tiny sulfuric acid droplets), can yield beautiful post-sunset colors called afterglows and pre-sunrise glows. A number of eruptions, including those of Mount Pinatubo in 1991 and Krakatoa in 1883, have produced sufficiently high 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.


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: Astronomy: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.[20]

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.

Sunset on the moon captured by Blue Ghost.

The Moon

On the Moon, due to the lack of an atmosphere, a sunset differs from earth due to the lack of atmosphere, making the change from light to dark less noticeable. Due to the tidal locking of the Moon to the Earth, it lasts longer, though not fully dark sometimes due to Earthshine.

Mars

See also: Astronomy:Astronomy on Mars
Sunset on Mars captured by the Mars rover Spirit.

On Mars, the setting Sun appears about two-thirds the size it does from Earth,[21] 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.[22] The colors of the Martian sunset differ from those on Earth. Mars has a thin atmosphere, lacking oxygen and nitrogen, so the light scattering is not dominated by a Rayleigh Scattering process. Instead, the air is full of red dust, blown into the atmosphere by high winds,[22] so its sky color is mainly determined by a 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.[22]

Cultural significance

Sunsets are an example of a natural phenomenon often considered beautiful by humans, and they are a common subject of visual artworks and in photography.[23][24][25]

In the Hebrew calendar,[26] the Islamic calendar,[27] and the Bahá'í calendar,[28], and in Orthodox Christianity[29] the day starts at sunset.

See also

References

  1. ↑ Ridpath, Ian (2012-01-01), "sunset" (in en), A Dictionary of Astronomy (Oxford University Press), doi:10.1093/acref/9780199609055.001.0001, ISBN 978-0-19-960905-5, https://www.oxfordreference.com/view/10.1093/acref/9780199609055.001.0001/acref-9780199609055-e-3625, retrieved 2021-10-05 
  2. ↑ "Definitions from the US Astronomical Applications Dept (USNO)". http://aa.usno.navy.mil/faq/docs/RST_defs.php. 
  3. ↑ "Full definition of Dusk". https://www.merriam-webster.com/dictionary/dusk. 
  4. ↑ "Sunset vs Dusk [What Is The Difference Between The Two?"] (in en-us). 2020-12-03. https://www.astronomyscope.com/sunset-vs-dusk/. 
  5. ↑ Starry Night Times – January 2007 (explains why Sun appears to cross slow before early January)
  6. ↑ 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'
  7. ↑ "Equinox" (in en). https://education.nationalgeographic.org/resource/equinox/. 
  8. ↑ 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. https://curious.astro.cornell.edu/our-solar-system/the-moon/46-our-solar-system/the-moon/observing-the-moon/128-how-does-the-position-of-moonrise-and-moonset-change-intermediate. 
  9. ↑ "Where Do the Sun and Stars Rise?". Stanford Solar Center. https://solar-center.stanford.edu/AO/sunrise.html. 
  10. ↑ 10.0 10.1 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
  11. ↑ Team, GML Web. "Solar Calculator - NOAA Global Monitoring Laboratory" (in en). https://gml.noaa.gov/grad/solcalc/index.html. 
  12. ↑ 12.0 12.1 K. Saha (2008). The Earth's Atmosphere – Its Physics and Dynamics. Springer. p. 107. ISBN 978-3-540-78426-5. https://archive.org/details/earthsatmosphere00saha_371. 
  13. ↑ 13.0 13.1 B. Guenther, ed (2005). Encyclopedia of Modern Optics. 1. Elsevier. p. 186. 
  14. ↑ "Hyperphysics, Georgia State University". Hyperphysics.phy-astr.gsu.edu. http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/blusky.html. 
  15. ↑ 15.0 15.1 Craig Bohren (ed.), Selected Papers on Scattering in the Atmosphere, SPIE Optical Engineering Press, Bellingham, WA, 1989
  16. ↑ Corfidi, Stephen F. (October 2024). "The Science of Sunsets". Norman, OK: NOAA/NWS Storm Prediction Center. https://www.spc.noaa.gov/publications/corfidi/sunset.html. 
  17. ↑ "Atmospheric Aerosols: What Are They, and Why Are They So Important?". nasa.gov. August 1996. http://www.nasa.gov/centers/langley/news/factsheets/Aerosols.html. 
  18. ↑ 18.0 18.1 E. Hecht (2002). Optics (4th ed.). Addison Wesley. p. 88. ISBN 0-321-18878-0. https://archive.org/details/optics00ehec. 
  19. ↑ "Red Sunset, Green Flash". http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/redsun.html. 
  20. ↑ "The Earth Is the Center of the Universe: Top 10 Science Mistakes". Science.discovery.com. 2012-01-23. http://science.discovery.com/top-ten/2009/science-mistakes/science-mistakes-02.html. 
  21. ↑ "A Moment Frozen in Time". Jet Propulsion Laboratory. June 10, 2005. http://photojournal.jpl.nasa.gov/catalog/PIA07997. 
  22. ↑ 22.0 22.1 22.2 Nemiroff, R.; Bonnell, J., eds (June 20, 2005). "Sunset Over Gusev Crater". Astronomy Picture of the Day. NASA. https://apod.nasa.gov/apod/ap050620.html. 
  23. ↑ "Sunsets in Fine Art: Capturing Ephemeral Beauty and Emotional Depth" (in en-GB). 2024-08-26. https://thebrokenspine.co.uk/2024/08/26/sunsets-in-fine-art-capturing-ephemeral-beauty-and-emotional-depth/. 
  24. ↑ (in en) Photography Reframed: New Visions in Contemporary Photographic Culture. Taylor & Francis. 2018. pp. 74. ISBN 9781000210927. 
  25. ↑ Baofu, Peter (2005) (in en). The Future of Beauty in Theatre, Literature and the Arts. Cambridge Scholars Publishing. pp. 47. ISBN 1904303595. 
  26. ↑ Kurzweil, Arthur (2011). The Torah For Dummies. John Wiley & Sons. ISBN 9781118051832. https://books.google.com/books?id=t8VZga76bw4C&q=%22jewish+day+begins%22+evening&pg=PA169. 
  27. ↑ "The Islamic Calendar of Turkey". https://webspace.science.uu.nl/~gent0113/islam/diyanetcalendar.htm. 
  28. ↑ "The Baháʼí Calendar". https://www.bahai.org/action/devotional-life/calendar. 
  29. ↑ "Orthodox Worship". https://www.goarch.org/-/orthodox-worship. 



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