Sunset or sundown, is the daily disappearance of the Sun below the western 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. 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 sunset is observed.
The spinning Earth lit by the Sun as seen from far above the North Pole. All along the terminator, the rays from the sun hit Earth horizontally, neglecting any atmospheric effects and Earth's orbital motion.
Sunset is distinct from dusk, which is the time when the sky becomes completely dark (apart from artificial light). This occurs when the Sun is about 18 degrees below the horizon. The period between sunset and dusk is called twilight.
Uploaded: Apr 27, 2016