The durations of nautical twilight and astronomical twilight are, respectively, the intervals between sunrise or sunset and the times at which the centre of the Sun is 12 and 18 degrees below the horizon. It is difficult to give precise statements on the degree of illumination at varying angles of depression of the Sun, and in any case, such illumination is dependent upon other causes such as moonlight and weather conditions. It will be found, in general that civil twilight marks the time when ordinary outdoor operations are difficult without artificial light, although there will be still ample light to make possible large scale operations, requiring outline only. The limits of astronomical twilight are times at which complete darkness save moonlight and starlight, begins in the evening and ends in the morning. Nautical twilight represents an intermediate state of illumination when the general outline will still be visible, although the horizon probably cannot be distinguished.
All detailed operations are impossible and all brighter stars can be seen.