What does "in sight of one another" signify in navigational rules?

In navigational rules, "in sight of one another" signifies that vessels are able to be observed visually, meaning they can see each other with the naked eye. This concept is crucial because many regulations and navigational practices depend on the visual ability to recognize and assess the proximity and actions of other vessels. The conditions under which vessels must operate, such as who has the right of way or what actions need to be taken to avoid collision, are significantly influenced by whether they can see each other clearly.

The other choices address different forms of visibility. For instance, radar detection (as mentioned in one of the options) does not fulfill the requirement of "in sight"; situations may arise where vessels are detectable by radar but not visually. The stipulation that only the captain can see the other vessel refers to a narrow framework of observation, which does not encapsulate the interaction required by navigational rules, as it is typically understood to mean any crew members aboard can see the other vessel. Mentioning a specific distance, such as a half-mile, is also misleading as "in sight of one another" does not define a specific range but rather focuses on the visual capability regardless of distance when conditions allow for it.