Module 12 - Visual Aids to Navigation
Sector Lights in Navigation: Red, White and Green Explained
What you will learn
Use the charted purpose and bearings of a sector light to detect cross-track movement and execute a pre-briefed safe response.
A sector light shows a colour or character over a defined arc. The observed signal tells you which charted sector contains the vessel's bearing from the light; the local chart defines what that sector means.
Many navigation lights show different colours or characters in different directions. White often marks a planned or preferred sector and red or green may mark flanking or danger sectors, but this is not a universal red-left/green-right steering rule. Read the charted purpose, bearings and any sailing directions.
Sector limits are charted from the light. The transition may include an angle of uncertainty in which colour or rhythm is mixed or difficult to distinguish, so do not treat the first colour change as an infinitely precise line.
In pilotage, sector lights are useful because they provide continuous feedback. If the planned approach keeps you in the white sector, seeing red or green tells you that tide, leeway, or steering error has pushed you off track.
A colour change is a prompt to execute the brief, not improvise. Confirm the vessel's side of the charted limit, check depth and traffic, then make the planned correction, hold or abort as conditions require.
Worked example · plan view
The same light can show three colours
Each boat sees the colour shining into its part of the water.
Red
Left of the plan
White
On the planned track
Green
Right of the plan
How to use the colour
- 1Find your boat relative to the light and its charted sector boundaries.
- 2Match the colour you see to the sector containing your boat.
- 3If it changes, check position, depths and your planned correction before steering.
Example only: sector bearings and the safe route come from the chart, not from a universal red-left/green-right rule.
Example only: the chart defines sector bearings, purpose and the safe response.
Brief the boundary before the approach
The route uses a charted white sector, with shoal water beyond its red boundary and a holding area astern of the entry line.
1. Name the limit
Record the boundary bearing and which observed colour lies on the danger side.
2. Choose the response
Brief a safe correction while sea room remains, plus the holding or abort point.
3. Verify after action
Check trend, depth and position rather than steering only until white reappears.
Sense check: If the colour meaning or safe side is not explicit on the chart and brief, the light is not yet a usable control limit.
Common mistake or limitation
- White is not universally safe in every direction or for every draught; charted depths and purpose still govern.
- A sector transition has practical uncertainty and is not a complete two-dimensional position fix.
Recap
- Read sector colour or character against charted bearings and purpose.
- Pre-brief correction, hold and abort choices before the boundary is reached.
- Confirm the result with trend, depth and independent position evidence.
Optional quick check
Section 4 of 6
A planned white-sector approach changes to red. What should happen first?
Common questions
What is a sector light in navigation?
A sector light is a navigation light that shows different colours in different directions. The colour you see helps identify safe water, channel limits, or nearby danger.
What does it mean when a sector light changes from white to red?
It means you have crossed a sector boundary. Red often warns that you are leaving the safe sector or approaching a danger.
Keep revising this topic
Sources and factual review
Reviewed 5 July 2026 - Day Skipper Revision.
Practice pilotage with visual aids
Sector lights make more sense when combined with leading lines, clearing bearings, buoyage, and safe-water planning.