🔍 Key Concepts

• Rule 21 – Definitions of lights in the Navigation Rules (COLREGS/Inland) • Standard arcs of visibility: sidelights 112.5°, masthead light 225°, sternlight 135° • Orientation of the diagram: think of the top as the bow (ahead) and the bottom as the stern (astern)

💭 Think About

• Looking at sector D, from which general directions (ahead, abeam, astern) could another vessel see this light? • Does sector D cover only the bow area, only the stern area, or does it straddle across the bow or stern? How does that compare with the arcs defined for sidelights, masthead lights, and sternlights in Rule 21? • Compare the size of the black sector in D to a full circle: is it closer to a quarter, a third, or more than half? Which type of light has an arc closest to that?

✅ Before You Answer

• Re‑read Rule 21(a)-(c) and note the exact degree arcs and where they are measured from (dead ahead, abaft the beam, astern). • Confirm whether a green sidelight should be visible from dead ahead, abaft the beam, or astern—and see if D’s sector matches that pattern. • Check if a sternlight’s arc is centered on the stern or on the bow in the diagram, and whether D appears to be bow‑centered, stern‑centered, or something else.