What does "in sight" mean in the Navigation Rules?

According to Navigation Rule 3(k, or j of Inland) “vessels shall be deemed to be in sight of one another only when one can be observed visually from the other.”  If you can see the other vessel only on radar or AIS, it is not in sight. Many of the Navigation Rules apply specifically to when vessels are in sight of each other. The text of Rule 3 is available at www.navcen.uscg.gov/?pageName=navRulesContent.