What marks the beginning or end of Nautical Twilight?

The beginning or end of Nautical Twilight is defined by the position of the sun relative to the celestial horizon, specifically when the center of the sun is 12 degrees below the horizon. During Nautical Twilight, there is enough natural light for sailors to navigate using visible stars while still allowing for minor visibility of the horizon. This phase occurs before the darkness of night and after the light of sunset during the evening, or conversely, before dawn.

This specific measurement of 12 degrees is significant because it indicates a transitional period where the horizon is still discernible, enabling navigation without the aid of artificial lights, but dim enough for stars to become visible. This makes it a critical time for mariners who rely on celestial navigation.

In contrast, the other options refer to different phases of twilight or daylight. The moment when the sun is 6 degrees below the horizon marks Civil Twilight, which occurs before Nautical Twilight. When the center of the sun is fully above the horizon, it marks the transition to daytime. The visibility of stars alone does not define twilight but rather indicates darkness, which corresponds with Astronomical Twilight when the sun is 18 degrees below the celestial horizon.