Actual sunset: Two minutes before the Sun disappears below the horizon.
Sunset (orsundown) is the disappearance of theSun at the end of theSun path, below thehorizon of theEarth (or any otherastronomical object in theSolar System) due to itsrotation. As viewed from everywhere on Earth, it is a phenomenon that happens approximately once every 24 hours, except in areas close to thepoles. Theequinox Sun sets due west at the moment of both the spring and autumn equinoxes. As viewed from theNorthern 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 theSouthern Hemisphere.
The sunset is defined inastronomy the moment the upper limb of the Sun disappears below the horizon.[1] Near the horizon,atmospheric refraction causessunlight 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 fromtwilight, which is divided into three stages. The first one iscivil 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 withpredusk. The second phase isnautical twilight, between 6 and 12 degrees below the horizon. The third phase isastronomical 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 beforenight.[3] Finally, night occurs when the Sun reaches 18 degrees below the horizon and no longer illuminates the sky.[4]
Locations further north than theArctic Circle and further south than theAntarctic Circle experience no full sunset orsunrise on at least one day of the year, when thepolar day or thepolar 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.
The time of sunset varies throughout the year and is determined by the viewer's position on Earth, specified bylatitude and longitude,altitude, andtime zone. Small daily changes and noticeable semi-annual changes in the timing of sunsets are driven by theaxial tilt of the 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 theNorthern Hemisphere, 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 theaphelion 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 itsperihelion, which occurs around January 3).[citation needed]
Likewise, the same phenomenon exists in theSouthern 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 ananalemma.[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 theMarch equinox to theSeptember 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 theazimuths 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 adaytime 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 themoon illusion.[citation needed]
Locations within theArctic andAntarctic Circles experience periods where the Sun does not rise or set for 24 hours or more, known aspolar day andpolar night. These phenomena occur due toEarth’s axial tilt, causing continuous sunlight or darkness at certain times of the year.[7]
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]
For a given latitude and a given date, calculate the declination of the Sun using longitude andsolar noon time as inputs to the routine;
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 insunrise 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]
Eveningtwilight inJoshua Tree, California, displaying the separation of yellow colors in the direction from the Sun below thehorizon 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 andairborne particles, changing the final color of the beam the viewer sees.Because the shorterwavelength 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 redhues 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 toRayleigh 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 toMie 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 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 agreen flash can be seen.[19]
Ash from volcanic eruptions, trapped within thetroposphere, tends to mute sunset and sunrise colors, while volcanic ejecta that is instead lofted into thestratosphere (as thin clouds of tiny sulfuric acid droplets), can yield beautiful post-sunset colors calledafterglows and pre-sunrise glows. A number of eruptions, including those ofMount Pinatubo in 1991 andKrakatoa in 1883, have produced sufficiently highstratus clouds containingsulfuric 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 theSun has set during twilight. Depending on weather conditions and the types ofclouds present, these colors have a wide spectrum, and can produce unusual results.[citation needed]
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. InPolish, the word foreastwschód (vskhud), is derived from themorpheme "ws" – meaning "up", and "chód" – signifying "move" (from the verbchodzić – meaning "walk, move"), due to the act of the Sun coming up from behind the horizon. The Polish word forwest,zachód (zakhud), is similar but with the word "za" at the start, meaning "behind", from the act of the Sun going behind the horizon. InRussian, the word for west,запад (zapad), is derived from the wordsза – meaning "behind", andпад – signifying "fall" (from the verbпадать –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.
The 16th-centuryastronomerNicolaus 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]
Sunsets on other planets appear different because of differences in the distance of the planet from theSun and non-existent or differing atmospheric compositions.
OnMars, the setting Sun appears about two-thirds the size it does fromEarth,[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 thinatmosphere, lackingoxygen andnitrogen, so the light scattering is not dominated by aRayleigh Scattering process. Instead, the air is full ofred dust, blown into the atmosphere by high winds,[22] so its sky color is mainly determined by aMie Scattering process, resulting in more blue hues than anEarth 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]
^The analemmaArchived 2006-10-18 at theWayback 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'
^abZhang, 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
