the time of morning or evening when the sun is 12° below the horizon. (Solar zenith angle is 102°, solar elevation angle is -12°.) See also civil twilight and astronomical twilight.
The time after civil twilight, when the brighter stars used for celestial navigation have appeared and the horizon may still be seen. It ends when the center of the sun is 12 degrees below the horizon, and it is too difficult to perceive the horizon, preventing accurate sighting of stars. Related term: twilight
That period when the upper limb of the sun is below the visible horizon and the center of the sun is not more than 12 degrees below the celestial horizon.
The time periods beginning before sunrise and ending after sunset when the sun is 12 degrees below the horizon; characterized by illumination sufficient to distinguish the general outlines of ground objects but not sufficient to distinguish the horizon or carry out detailed outdoor operations
