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 the Sun’s disk is already about one diameter below the horizon when sunset is observed. 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.
Uploaded: Oct 13, 2015