The period in which the sun is between 6° - 12° below the horizon in the morning and evening
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
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
occurs while the Sun is between 6° and 12° below the horizon At the end of nautical twilight it is not possible to make out the horizon
is defined to begin in the morning, and to end in the evening, when the center of the sun is geometrically 12 degrees below the horizon At the beginning or end of nautical twilight, under good atmospheric conditions and in the absence of other illumination, general outlines of ground objects may be distinguishable, but detailed outdoor operations are not possible, and the horizon is indistinct
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
The time when the center of the Sun passes a reference altitude of 12 degrees below a mathematical horizon