Sunset 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”.

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 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 intense red and orange hues of the sky at sunrise and sunset are mainly caused by scattering of sunlight by dust particles, soot particles, other solid aerosols, and liquid aerosols in the Earth’s atmosphere. These enhanced red and orange colors at sunrise and sunset are mathematically explained by the Mie theory or the discrete dipole approximation. When there are no particulates 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. Sunset colors are typically more brilliant and more intense than sunrise colors, since there are generally more particles and aerosols in the evening air than in the morning 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 at sunrise. 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.