When must navigation lights be displayed on vessels?

The requirement to display navigation lights on vessels is critical for ensuring safe navigation and collision avoidance on the water. Navigation lights must be shown from sunset to sunrise, as this is when darkness typically obscures visibility. In addition, these lights are required when visibility is restricted, such as during fog, rain, or snow. The purpose of these lights is to make vessels visible to others, allowing for proper identification of their size, type, and direction of travel, which is vital for maintaining safety.

During daylight hours, vessels usually rely on visual observation and have other signaling methods to avoid collisions, which is why the other options that suggest navigation lights should be used only during the day or simply at sunset are not comprehensive enough. The darkness of night and restricted visibility conditions necessitate navigation lights to ensure that all vessels can see and be seen, thereby promoting navigation safety in various environmental conditions.