MIKE MOSS SAYS: Jack, Civil Twilight is defined to begin (in the morning) or end (evening) when the center of the sun's disk is six degrees of arc below the horizon. In practical terms, this means that enough natural illumination is available for most outdoor activities to continue without the aid of artificial lighting. During the somewhat darker nautical twilight, which begins or ends when the center of the sun's disk is 12 degrees below the horizon, general shapes and outlines remain visible, but most activities require additional light sources in order to proceed normally. Finally, astronomical twilight begins or ends when the center of the sun is 18 degrees below the horizon. Below this threshold there is no longer any light from the sun illuminating the sky.