Definitions for "CIVIL TWILIGHT"
Keywords:  horizon, twilight, sunset, sun, zenith
When the centre of the Sun is less than 6º below the horizon; normal daylight activities are possible.
The interval of incomplete darkness between sunrise (or sunset) and the time when the center of the sun’s disc is 6o below the horizon. The limiting angle of 6o was chosen to correspond approximately to the minimum sky illumination needed to carry on normal work out of doors. As in the case of any of the several defined types of twilight, the actual duration of civil twilight varies considerably with latitude and time of year.
The period when the sun is from 0 degrees to 6 degrees below the horizon.