Module 12 - Visual Aids to Navigation
Light Sectors
What you will learn
Use charted sector bearings and purpose to detect cross-track movement, account for transition uncertainty and execute a pre-briefed safe response.
A sector light shows different colours or rhythms over designated arcs. The observed sector supplies directional information from the light, but only the chart and local brief define what that information means for this vessel.
IALA notes that a sector or sector limit may mark a fairway, turn, channel junction, hazard, deepest water, anchorage or another feature. A white fairway sector is commonly flanked by green to starboard and red to port when approaching from seaward in Region A, with the colours reversed in Region B, but the white sector is not necessarily safe throughout its full radial length.
Read the charted bearings and purpose rather than using a universal 'turn towards white' rule. Draught, tidal height, traffic, cross-track trend and hazards inside the sector still govern. A red or green observation can mean different things at different sites.
The practical boundary may contain an angle of uncertainty or a mixed/oscillating transition. A colour change is useful directional evidence, not an infinitely precise line and not a complete two-dimensional fix. Brief which side is unsafe and what correction, hold or abort action remains available before entering close water.
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.
| Observation | Defensible meaning | Required check |
|---|---|---|
| Stable planned colour | Bearing lies within that sector | Depth, range along sector, traffic and trend |
| Mixed or alternating colours | Vessel may be in the transition/uncertainty region | Hold the safe margin and verify bearing/position |
| Unexpected flanking colour | The charted sector boundary has been approached or crossed | Execute the local brief, not a generic turn |
Brief the red boundary before it is seen
A Region A harbour approach uses a charted white sector. Shoal water lies beyond the red boundary, and the yacht has a safe holding area astern of the entry line.
1. Name the local geometry
Record the boundary bearing, danger side and the portion of the white sector that meets the yacht's depth requirement.
2. Preselect the response
Brief a correction while sea room remains, plus the point at which the approach is abandoned for the holding area.
3. Verify the result
After action, check colour trend, depth, traffic and independent position rather than steering until white merely reappears.
Sense check: If the crew cannot state which charted boundary is being controlled and where the safe exit lies, the sector is not yet a usable control limit.
Common mistake or limitation
- White does not remove hazards, depth limits or the need to know the sector's radial extent and local purpose.
- A first colour change is not an exact position line; transition uncertainty, light optics, visibility and observation angle affect what is perceived.
Recap
- Translate the observed colour through charted bearings and purpose.
- Allow for uncertainty at the transition and preserve a margin.
- Execute the pre-briefed correction, hold or abort and verify with independent evidence.
Optional quick check
Section 4 of 8
The planned white sector becomes mixed red and white near its charted boundary. What is the safest interpretation?
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 20 August 2026 - Compass Revision editorial review.
Practice pilotage with visual aids
Sector lights make more sense when combined with leading lines, clearing bearings, buoyage, and safe-water planning.