Which of the following best describes the timeframe of Civil Twilight?

Civil Twilight is characterized as the time just before sunrise when the horizon is still visible. During this period, the sun is below the horizon, but its light is still scattered in the atmosphere, allowing for the illumination of the earth. This is the time when navigators can still see objects on the horizon and is a crucial period for those engaged in navigation, as it offers sufficient natural light for visual reference and assessment.

The other choices do not accurately encapsulate the essence of Civil Twilight. The beginning of Civil Twilight is tied to the moment when the sun is positioned at a specific angle below the horizon, not necessarily when the sun reaches the celestial horizon itself. The moment when the sun is exactly at the horizon indicates sunrise or sunset, rather than the twilight phase. Furthermore, the period being too dark for visual navigation refers to a time beyond Civil Twilight, where natural light is insufficient for navigation purposes. Understanding these distinctions facilitates a clearer grasp of twilight phases and their relevance in navigation contexts.