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Sunset, also known as sundown, is the daily disappearance of the Sun below the
horizon The horizon is the apparent line that separates the surface of a celestial body from its sky when viewed from the perspective of an observer on or near the surface of the relevant body. This line divides all viewing directions based on whether i ...
due to
Earth's rotation Earth's rotation or Earth's spin is the rotation of planet Earth around its own Rotation around a fixed axis, axis, as well as changes in the orientation (geometry), orientation of the rotation axis in space. Earth rotates eastward, in retrograd ...
. As viewed from everywhere on Earth (except the North and South poles), the equinox Sun sets due west at the moment of both the spring and autumn equinoxes. As viewed from the
Northern Hemisphere The Northern Hemisphere is the half of Earth that is north of the Equator. For other planets in the Solar System, north is defined as being in the same celestial hemisphere relative to the invariable plane of the solar system as Earth's Nort ...
, 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 sunset is defined in astronomy as the moment when the upper limb of the Sun disappears below the horizon. Near the horizon, atmospheric refraction causes
sunlight Sunlight is a portion of the electromagnetic radiation given off by the Sun, in particular infrared, visible, and ultraviolet light. On Earth, sunlight is scattered and filtered through Earth's atmosphere, and is obvious as daylight when t ...
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 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. '' Dusk'' is at the very end of astronomical twilight, and is the darkest moment of twilight just before night. Finally, night occurs when the Sun reaches 18 degrees below the horizon and no longer illuminates the sky. Locations further north than the
Arctic Circle The Arctic Circle is one of the two polar circles, and the most northerly of the five major circles of latitude as shown on maps of Earth. Its southern equivalent is the Antarctic Circle. The Arctic Circle marks the southernmost latitude at w ...
and further south than the Antarctic Circle experience no full sunset or sunrise on at least one day of the year, when the
polar day The midnight sun is a natural phenomenon that occurs in the summer months in places north of the Arctic Circle or south of the Antarctic Circle, when the Sun remains visible at the local midnight. When the midnight sun is seen in the Arctic, th ...
or the polar night persists continuously for 24 hours.


Occurrence

The time of sunset varies throughout the year, and is determined by the viewer's position on Earth, specified by latitude and longitude, altitude, and time zone. Small daily changes and noticeable semi-annual changes in the timing of sunsets 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. During winter and spring, the days get longer and sunsets occur later every day until the day of the latest sunset, which occurs after the summer solstice. In the
Northern Hemisphere The Northern Hemisphere is the half of Earth that is north of the Equator. For other planets in the Solar System, north is defined as being in the same celestial hemisphere relative to the invariable plane of the solar system as Earth's Nort ...
, the latest sunset occurs late in June or in early July, but not on the Summer solstice of June 21. This date depends on the viewer's latitude (connected with the Earth's slower movement around the aphelion around July 4). Likewise, the earliest sunset does not occur on the winter solstice, but rather about two weeks earlier, again depending on the viewer's latitude. In the Northern Hemisphere, it occurs in early December or late November (influenced by the Earth's faster movement near its perihelion, which occurs around January 3). 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. 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. 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. Locations north of the
Arctic Circle The Arctic Circle is one of the two polar circles, and the most northerly of the five major circles of latitude as shown on maps of Earth. Its southern equivalent is the Antarctic Circle. The Arctic Circle marks the southernmost latitude at w ...
and south of the Antarctic Circle experience no sunset or sunrise at least one day of the year, when the
polar day The midnight sun is a natural phenomenon that occurs in the summer months in places north of the Arctic Circle or south of the Antarctic Circle, when the Sun remains visible at the local midnight. When the midnight sun is seen in the Arctic, th ...
or the polar night persist continuously for 24 hours.


Location on the horizon

Approximate locations of sunset on the horizon ( azimuth) as described above can be found in Refs. The figure on the right is calculated using the solar geometry routine as follows: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 # 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; # Calculate the sunset hour angle using the
sunset equation The sunrise equation or sunset equation can be used to derive the time of sunrise or sunset for any solar declination and latitude in terms of local solar time when sunrise and sunset actually occur. Formulation It is formulated as: :\cos ...
; # Calculate the sunset time, which is the solar noon time plus the sunset hour angle in degree divided by 15; # 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 The sunrise equation or sunset equation can be used to derive the time of sunrise or sunset for any solar declination and latitude in terms of local solar time when sunrise and sunset actually occur. Formulation It is formulated as: :\cos \ ...
is applied to the x- and y-components of the solar vector presented in Ref.


Colors

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. 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. 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).Craig Bohren (ed.), ''Selected Papers on Scattering in the Atmosphere'', SPIE Optical Engineering Press, Bellingham, WA, 1989 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). Sunset colors are typically more brilliant than sunrise colors, because the evening air contains more particles than morning air. Sometimes just before sunrise or after sunset a green flash can be seen. 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 The stratosphere () is the second layer of the atmosphere of the Earth, located above the troposphere and below the mesosphere. The stratosphere is an atmospheric layer composed of stratified temperature layers, with the warm layers of air ...
(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 Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid ( Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen and hydrogen, with the molecular formu ...
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.


Names of compass points

In some languages,
points of the compass The points of the compass are a set of horizontal, radially arrayed compass directions (or azimuths) used in navigation and cartography. A compass rose is primarily composed of four cardinal directions—north, east, south, and west—each sep ...
bear names etymologically derived from words for sunrise and sunset. The English words "
orient The Orient is a term for the East in relation to Europe, traditionally comprising anything belonging to the Eastern world. It is the antonym of ''Occident'', the Western World. In English, it is largely a metonym for, and coterminous with, the c ...
" 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

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.


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

On Mars, the setting Sun appears about two-thirds the size it does from Earth, 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. The colors of the Martian sunset differ from those on Earth. Mars has a thin
atmosphere An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...
, 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, 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.


Gallery

File:SunsetHikkaduwa-April2012.JPG, Clear half-disk view of a sunset in Hikkaduwa,
Sri Lanka Sri Lanka (, ; si, ශ්‍රී ලංකා, Śrī Laṅkā, translit-std=ISO (); ta, இலங்கை, Ilaṅkai, translit-std=ISO ()), formerly known as Ceylon and officially the Democratic Socialist Republic of Sri Lanka, is an ...
File:Закат над Онего - panoramio.jpg, Sunset in the
Republic of Karelia The Republic of Karelia (russian: Респу́блика Каре́лия, Respublika Kareliya; ; krl, Karjalan tašavalta; ; fi, Karjalan tasavalta; vep, Karjalan Tazovaldkund, Ludic: ''Kard’alan tazavald''), also known as just Karelia (rus ...
, Russia File:Хорватия, Закат в Брела, 2007-06.jpg, Sunset in Brela, Croatia File:Sunset with coconut palm tree, Fiji.jpg,
Fiji Fiji ( , ,; fj, Viti, ; Fiji Hindi: फ़िजी, ''Fijī''), officially the Republic of Fiji, is an island country in Melanesia, part of Oceania in the South Pacific Ocean. It lies about north-northeast of New Zealand. Fiji consists ...
File:A view of some houses at sun down.jpg, Takoradi, Ghana File:Cristo Redentor night.jpg, Silhouette of '' Christ the Redeemer'' statue at sunset, Rio de Janeiro, Brazil


See also

* Afterglow * Analemma * Astronomy on Mars * Dawn *
Daytime Daytime as observed on Earth is the period of the day during which a given location experiences natural illumination from direct sunlight. Daytime occurs when the Sun appears above the local horizon, that is, anywhere on the globe's hemis ...
* Diffuse sky radiation * Dusk *
Earth's shadow Earth's shadow (or Earth shadow) is the shadow that Earth itself casts through its atmosphere and into outer space, toward the antisolar point. During the twilight period (both early dusk and late dawn), the shadow's visible fringe – sometimes ...
, visible at sunset * Golden hour * Sundown town * Sunrise *
Sunrise equation The sunrise equation or sunset equation can be used to derive the time of sunrise or sunset for any solar declination and latitude in terms of local solar time when sunrise and sunset actually occur. Formulation It is formulated as: :\cos \ ...
* Twilight


References


External links


Full physical explanation in simple terms

The colors of twilight and sunset


* ttp://sunsetsunrisetime.com/ Geolocation service to calculate the time of sunrise and sunset {{Authority control Earth phenomena Parts of a day Solar phenomena Daily events Evening