Sunset, also known as sundown, is the daily disappearance of

the Sun below the horizon due to Earth's rotation. As viewed from the Equator, the equinox Sun sets exactly due west in both spring and fall. As viewed from the middle latitudes, the local summer Sun sets to the northwest for the Northern Hemisphere, but to the southwest for the Southern Hemisphere.

The time of sunset is defined in astronomy as the moment when the upper limb of the Sun disappears below the horizon. Near the horizon, atmospheric refraction causes sunlight rays to be distorted to such an extent that geometrically the solar disk is already about one diameter below the horizon when a sunset is observed.

Source: Wikipedia