Sunset is the perceived daily disappearance of the sun below the horizon as a result of the Earth's rotation. The atmospheric conditions created by the setting of the sun are also commonly referred to as "a sunset". These conditions often appear before and after the precise moment the sun apparently disappears below the horizon, and as a result the term "sunset" in common usage has a substantially looser meaning than in scientific and technical applications.

    In astronomy the time of sunset is defined as the moment the trailing edge of the sun's disk disappears below the horizon in the west. Due to refraction of light in the atmosphere, the ray path of the setting sun is highly distorted near the horizon making the apparent astronomical sunset to occur when the sun’s disk is already about one diameter below the horizon. Sunset should not be confused with dusk, which is the moment at which darkness falls, when the sun is about eighteen degrees below the horizon. The period between the astronomical sunset and dusk is called twilight.

     The sunset is often more brightly colored than the sunrise,^{[citation needed]} with the shades of red and orange being more vibrant. The atmosphere responds in a number of ways to exposure to the sun. In particular, there tends to be more dust in the lower atmosphere at the end of the day than at the beginning. During the day, the sun heats the surface of the Earth, lowering the relative humidity and increasing wind speed and turbulence, which serves to lift dust into the air. However, differences between sunrise and sunset may in some cases depend more on the particular geographical features of the location from which they are viewed. For example, on a west-facing coastline, sunset occurs over water while sunrise occurs over land.

     The timing of sunset varies with the time of year and the latitude of the location from which it is viewed. The precise local time of sunset also depends upon each location's precise longitude within a given time zone. Small daily changes and noticable semi-annual changes in timing of sunset are driven by the axial tilt of Earth, the spherical shape of the Earth, and the planet's movement in its annual orbit around the sun. Some apparent anomalies exist however, the main one caused by the Earth's axial tilt and the Earth's elliptical orbit. In the Northern Hemisphere, the earliest sunset does not fall on the winter solstice around December 21, but instead it occurs earlier in December. Likewise, the latest sunset does not fall on the summer solstice around June 21, but instead it happens later in June or in early July, depending on one's latitude. The same phenomenon exists in the Southern Hemispherebefore June 21 in winter and some time afterequator, sunrise and sunset shift several minutes back and forth through the year, along with solar noon. This effect is plotted by an analemma. except with the respective dates being some time December 21 in summer, possibly in January of the following year. For one or two weeks surrounding both solstices, both sunrise and sunset get slightly later or earlier each day. Even on the

     Due to Earth's axial tilt, whenever and wherever sunset occurs, sunset is always to the northwest from the March equinox to the September equinox, and to the southwest from the September equinox to the March equinox. Sunsets occur precisely due west on the equinoxes, and the duration of day and night are approximately equal on the equinoxes for all viewers on Earth (precisely 12 hours if measured from the geometric (unrefracted) centre of the sun).

     As sunrise and sunset are calculated from the leading and trailing edges of the sun, and not the centre, the duration of "day" is slightly longer than "night". Further, because the light from the sun is bent by the atmospheric refraction, the sun is still visible after it is geometrically below the horizon. The sun also appears larger on the horizon, which is another optical illusion, similar to the moon illusion.

     The intense red and orange hues of the sky at sunset and sunrise are mainly caused by scattering of sunlight by dust particles, soot particles, other solid aerosols, and liquid aerosols floating in the earth's atmosphere. These enhanced red and orange colors at sunset and sunrise are mathematically explained by Mie theory or the discrete dipole approximation. When there are no particulates or aerosols in the troposphere, such as after a big rain storm, then the remaining less intense reds are explained by Rayleigh Scattering of sunlight by air molecules. Sunrise colors are typically less brilliant and less intense than sunset colors, since there are generally fewer particles and aerosols in the morning air than in the evening air. Nighttime air is usually cooler and less windy, which allows dust and soot particles to settle out of the atmosphere, reducing the amount of Mie Scattering. The reduced Mie Scattering correspondingly reduces the amount of red and orange scattered light at sunrise. Sunrise color intensities can however exceed sunset's intensities when there are nighttime fires, volcanic eruptions or emissions, or dust storms to the east of the viewer. A number of eruptions in recent times, such as those of Mount Pinatubo in 1991 and Krakatoa in 1883, have been sufficiently large to produce remarkable sunsets and sunrises all over the world.