What is a key characteristic of Nautical Twilight?

A key characteristic of Nautical Twilight is that it is the phase where celestial bodies become visible. During this time, the sky is dark enough for sailors and navigators to take reliable readings of the stars and other celestial bodies, which is crucial for navigation. Nautical Twilight occurs when the sun is between 6 degrees and 12 degrees below the horizon, allowing for visibility of brighter stars and planets, which aids in navigation when out at sea.

The other options provide varying degrees of context about twilight phases, but they do not accurately describe the specific characteristics associated with Nautical Twilight. While it does occur after Civil Twilight, it is not meant to last until full darkness; instead, its purpose is related to navigation using celestial bodies. The visibility of the upper limb of the sun pertains more to Civil Twilight. The definition relating to the sun being 18 degrees below the horizon actually describes Astronomical Twilight, not Nautical Twilight. Thus, identifying Nautical Twilight as the phase where celestial bodies become visible encapsulates its significance in navigation.