Morning nautical twilight is defined to begin when the geometric center of the sun is 12º below the horizon According to some definitions, it ends when the geometric center of the sun is 0º50' below the horizon.
Morning nautical twilight is defined to begin when the center of the sun is geometrically 12 degrees below the horizon and to end at sunrise. Evening nautical twilight is defined to begin at sunset and end when the center of the sun is 12 degrees below the horizon.