The illumination of the Earth's atmosphere after sunset, visible until the Sun is about 18° below the horizon. Its duration depends upon the time required for the Sun to traverse this distance. At the Equator this requires about an hour at any time of year, but during Summer lasts for a much longer period. As one passes beyond 40° N. latitude, the interval is lengthened in the Summer and shortened in the Winter.
ASTROLABE ENCYCLOPEDIA
