Tuesday, 17 June 2014
Sunset or sundown is the daily disappearance of the Sun below the western half of the horizon, i.e. at an azimuth greater than 180 degrees, as a result of Earth's rotation, and not, as was previously believed, of any movement of the sun. 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. Source
