Definitions for "NAUTICAL TWILIGHT"
When the centre of the Sun is between 6º and 12º below the horizon; the marine horizon becomes invisible.
The interval of incomplete darkness between sunrise or sunset and the time at which the center of the sun’s disc is 12° below the astronomical horizon.
The period when the sun is from 0 degrees to 12 degrees below the horizon; the period when most observations of stars and planets are made by the navigator.