bridge under sunset

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.

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 oflight 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.