BOTH INTERNATIONAL & INLAND You are on a vessel heading due north and see the lights shown in illustration D051RR below, one point on your port bow. What direction could this vessel be heading in?

Correct Answer: Option A — SE

Rule 21 sidelight arcs dictate that a green starboard sidelight is visible from right ahead to 22.5 degrees abaft the beam (112.5 degrees total). Observing this green light on your port bow while heading North indicates the target vessel's starboard side is open to you, which aligns with a Southeasterly heading.