What does it mean when a vessel is "in sight" of another vessel?

When a vessel is described as "in sight" of another vessel, it specifically means that one vessel can be observed visually from the other vessel. This is important for navigation safety, as being able to see another vessel allows crew members to take the necessary measures to avoid collisions and maintain safe distances. Visibility is crucial in maritime navigation to ensure that the actions of other vessels can be anticipated and managed effectively.

The other options address related but distinct concepts. Being observed visually or by radar pertains to detection methods, but for the term "in sight," the emphasis is on direct visual observation. Knowing the heading of another vessel is important for navigation, but simply being "in sight" does not imply that one can ascertain the vessel's heading—only that it is visible. Hearing a fog signal pertains to audio cues rather than visual contact, which is not aligned with the definition of "in sight."