Which statement best describes Nautical Twilight?

• A. Time of incomplete darkness beginning when the center of the sun is 12° below horizon
• B. The sun is above horizon at morning
• C. Time of complete darkness at night
• D. Time when the upper limb of the sun is 12° above horizon

Answer: (A)

Nautical twilight is defined by the Sun’s depression below the horizon while enough light remains to see the horizon at sea for navigation. The boundary used in this context is when the center of the Sun is about 12 degrees below the horizon, marking the transition from broader daylight to darker twilight, yet still allowing horizon visibility for sights. That’s why this statement best describes nautical twilight in many navigation references.

The other scenarios describe different light conditions: the Sun above the horizon means daylight, the sky is still fully dark at night, and the Sun’s upper limb being above the horizon indicates sunrise, not nautical twilight.