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SunsetSunset is the 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, occurring before and after it disappears below the horizon, are also commonly referred to as "sunset".

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.  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 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.

Sunset on a cloudy sky in Baltimore, MarylandThe timing of sunsets vary throughout the year, as determined by the viewer's longitude and latitude, elevation, the time of year, and time zone of the viewer's location.  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 does not fall on the summer solstice around June 21, but occurs later in June or in early July.  The precise date of the latest sunset depends on the viewer's latitude (connected with the slower Earth's movement around the aphelion around July 4).  Similarly, the earliest sunset does not occur on the winter solstice, but rather about two weeks earlier, again depending the viewer's latitude.  In the Northern Hemisphere the earliest sunset occurs in early December (influence from the Earth's faster movement near the perihelion which occurs around January 3).  Likewise, the same phenomena exists in the Southern Hemisphere except 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.

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 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.

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