Sunset or
sundown is the daily disappearance of the
Sun below the
horizon in the
west as a result of
Earth's rotation.
The time of sunset is defined in
astronomy as the moment the trailing edge of the Sun's disk disappears below the horizon in the west. The ray path of
light from the setting Sun is highly distorted near the horizon because of
atmospheric refraction, making sunset appear to occur when the Sun’s disk is already about one diameter below the horizon. Sunset is distinct from
dusk, which is the moment at which darkness falls, which occurs when the Sun is approximately eighteen degrees below the horizon. The period between sunset and dusk is called
twilight.
Locations north of the
Arctic Circle and south of the
Antarctic Circle experience no sunset or sunrise at least one day of the year, when the
polar day or the
polar night persist continuously for 24 hours.
Sunset creates unique atmospheric conditions such as the often intense orange and red colors of the Sun and the surrounding sky.
Occurrence
Sunset in San Francisco area
The time of sunset varies throughout the year, and is determined by the viewer's position on Earth, specified by longitude and latitude, and elevation. 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. In the summertime, 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 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 (influence from the Earth's faster movement near the 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 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. These effects are plotted by an
analemma.
[2][3]Due to Earth's axial tilt, 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 precisely due west 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 center, the duration of a day time is slightly longer than night time (by about 10 minutes). Further, because the light from the Sun is refracted, the Sun is still visible after it is geometrically below the horizon. The Sun also appears larger on the horizon, an optical illusion, similar to the
moon illusion.
Locations north of the
Arctic Circle and south of the
Antarctic Circle experience no sunset or sunrise at least one day of the year, when the
polar day or the
polar night persist continuously for 24 hours.
[edit] Colors
Sunset in Knysna, South Africa, displaying the separation of orange colors in the direction from the sun 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.
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.
[5] 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).
[6][7] 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). Without Mie scattering at sunset and
sunrise, the sky along the horizon has only a dull-reddish appearance, while the rest of the sky remains mostly blue and sometimes green.
Sunset colors are typically more brilliant than
sunrise colors, because the evening air contains more particles than morning air.
[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 stratospheric sulfuric acid clouds 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.
Sometimes just before sunrise or after sunset a
green flash can be seen.
Planets
Sunsets on other planets appear different because of the differences in the distance of the planet from the Sun, as well as different atmospheric compositions.
On Mars, the Sun appears only about two-thirds of the size that it appears in a sunset seen from Earth,
because Mars is farther from the Sun than the Earth is. Although Mars lacks oxygen and nitrogen in the atmosphere, it is covered in red dust frequently hoisted into the atmosphere by fast but thin winds.
At least some Martian days are capped by a sunset significantly longer and redder than typical on Earth.
One study reported that for up to two hours after twilight, sunlight continued to reflect off Martian dust high in the atmosphere, casting a diffuse glow.
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