Chart No. 1 · P 40.1
Sector light
Lights
Dashed lines and lettered arcs show a light that changes color by direction — colors mark danger and safe sectors.
What it means
Dashed radial lines from a light show its sector limits; arcs between them are lettered with the color shown in each direction (R, W, G). From your boat you see one color at a time.
Why it matters
Red sectors usually cover dangers: if a sector light turns from white to red, you have crossed into hazardous water — alter course until it shows white again. The sector limits are bearings toward the light, as seen from seaward.