Sunset, also called sundown in some American English dialects, is the instant when the trailing edge of the sun's disk disappears below the horizon in the west. It should not be confused with dusk, which is the point at which darkness falls, some time after the beginning of twilight when the sun itself sets.

The sunset is often more brightly colored than the sunrise,Fact: date=March 2008 with the shades of red and orange being more vibrant. The atmosphere responds in a number of ways to exposure to the sun during daylight hours. 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.

Occurrence

The timing of sunset varies with the time of year and the latitude of the location from which it is viewed. The timing also varies in local time within a given time zone, determined by each location's precise longitude. 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 your latitude. The same phenomenon exists in the Southern Hemisphere except with the respective dates being some time before June 21 in winter and some time after 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 equator, sunrise and sunset shift several minutes back and forth through the year, along with solar noon. This effect is plotted by an analemma.

Due to Earth's axial tilt, the direction of sunset in the northern hemisphere 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 March and September equinoxes, and the duration of day and night are approximately 12 hours on the equinoxes for all viewers on Earth (precisely 12 hours if measured from the centre of the sun).