🔍 Key Concepts

• Masthead light definition and which vessels must carry it (Navigation Rules, Rule 21(a)) • Required arc of visibility for different lights: masthead, sidelights, sternlight (Navigation Rules, Rule 21) • How the full horizon (360°) is divided among masthead light, sidelights, and sternlight on a power-driven vessel

💭 Think About

• First, picture a power-driven vessel at night and imagine where you see the masthead light from ahead, on the beam, and astern. From which sectors should you NOT see it? • Compare the arcs: sidelights are visible over a certain angle from right ahead to a bit abaft the beam; sternlight covers the stern sector. What remains for the masthead light? • Ask yourself: if you could see the masthead light all the way around, would that help you know whether a vessel is coming toward you or going away from you?

✅ Before You Answer

• Verify in Rule 21 the exact arc of visibility (in degrees) specified for a masthead light. • Confirm how many degrees each sideline covers, and how many degrees the sternlight covers, then see what arc is left for the masthead light. • Make sure your choice matches a light that is visible from ahead and to a little past the beam, but not all the way around the vessel.