Sunset or sundown is the daily disappearance of the Sun below the horizon as a result of Earth's rotation. The Sun will set exactly due west at the equator on the spring and fall equinoxes, each of which occurs only once a year.
The time of sunset is defined in astronomy as the moment when the trailing edge of the Sun's disk disappears below the horizon. Near to the horizon, atmospheric refraction causes the ray path of light from the Sun to be distorted to such an extent that geometrically the Sun's disk is already about one diameter below the horizon when a sunset is observed.