Sunset

Sunset or sundown is the daily disappearance of the Sun below the horizon, as a result of Earth's rotation.
The time of sunset is defined in astronomy as the moment when the trailing edge of the Sun's disk disappears below the horizon. The ray path of light from the setting Sun is highly distorted near the horizon because of atmospheric refraction, making the 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 time at which the sky becomes completely dark, 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 persists continuously for 24 hours. 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 colour of the beam the viewer sees. Because the shorter wavelength components, such as blue and green, scatter more strongly, these colours 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.Sunset colours are typically more brilliant than sunrise colours, because the evening air contains more particles than morning air.

[Wikipedia]