Sector lights: colour by direction
1The lesson in plain words
A sector light shows a different colour or rhythm depending on your bearing from it. Its limits are true bearings from seaward, read clockwise.
What a sector is
A sector light shows different colours, or different rhythms, over set arcs of the horizon. The colour you see tells you which arc you are in. Sector lights mark safe approaches, turning points, channel junctions and dangers. An arc where the light cannot be seen is obscured; the arc where it shows at all is its arc of visibility.
How bearings are given
Sector limits in the List of Lights and on the chart are true bearings, from 000° to 359°, given clockwise and from seaward: they are bearings of the light as you would take them from your boat. So a white sector 358°–002° means you see white while the light bears between 358° and 002° from you.
Which colour where
Where a sector light marks the edges of a channel, IALA says its colours follow the region’s lateral rule. In Region A you then usually see red on the port-hand side and green on the starboard-hand side, with white in the middle. Not every light follows this, and many sectors mark dangers, so read each light on the chart and in the list.
Worked example
A light shows green 352°–358°, white 358°–002° and red 002°–008°. The chart marks white as your approach sector. Heading about north towards it, you see green, and a hand-bearing compass gives the light at 355°T. That is below the white limit of 358°, so you are east of the white sector, on its starboard-hand side. Alter to port until the light turns white again.
Edges are not sharp
At each colour boundary there is a narrow arc of uncertainty, where the light may look white, mixed or dark. In cold or wet weather, frost and moisture on the lantern widen it, and coloured sectors may look white. Never steer along a sector edge, and check your position with a compass bearing of the light.
2Step by step
Using a sector light to enter
- 1
Find the light on the chart and in the List of Lights, and note each sector’s colour and limits.
You need to know which colour lies on which side before you see it.
- 2
Identify the light by its full character before using its colour.
Another light nearby may show similar colours.
- 3
Follow the sector shown for your planned safe approach, within its charted limits.
White alone does not guarantee safe water for your boat.
- 4
If the colour changes unexpectedly, check your bearing and chart before correcting along the safe route.
A colour change may indicate a planned turn or danger. Confirm your sector before altering.
- 5
Check with a hand-bearing compass and the echo sounder.
Sector edges are not sharp, and a light can be wrongly set.
3Key data
- Sector limits
- True bearings, clockwise, as seen from seaward
- White 358°–002°
- You see white while the light bears 358° to 002°
- Arc of uncertainty
- Narrow zone at each edge; wider in frost or rain
- Obscured sector
- Arc where the light cannot be seen
4Key points
- 1
Sector limits are true bearings from seaward, read clockwise.
- 2
The colour you see tells you which arc you are in.
- 3
Edges are blurred: confirm with a compass bearing, never steer on the edge.
- 4
Which colour lies where depends on the light: read the chart.
5Frequent mistakes
Wrong: Sector bearings are given from the light out to sea.
Right: They are bearings of the light from seaward, as you would take them from your boat.
Wrong: Green always means safe water in a sector light.
Right: Read the chart to see which sector, if any, marks your approach.
Wrong: The colour changes exactly on the charted bearing.
Right: There is an arc of uncertainty at each edge, wider in poor weather.
6Practice case & exam keys
A light’s sectors are green 090°–095°, white 095°–099° and red 099°–104°, from seaward. The chart marks white as the approach sector. Heading east towards it, you see green, and your hand-bearing compass gives the light at 093°T. Where are you, and what do you do?
Show the answer
The bearing, 093°, is below the white limit of 095°, so you are south of the white sector, on its starboard-hand side as you head east. Alter to port, northwards, until the light turns white and bears between 095° and 099°, then steer to keep it white. Confirm with the depth and further bearings.
Exam keys
- Sector limits: true, clockwise, from seaward.
- The colour shows your sector; the chart shows whether it suits your approach.
- Arc of uncertainty at each edge; check by compass bearing.
Finish this unit
When you've read it, match the terms, then answer three questions. Get them all right to finish the unit; opening the page alone doesn't count.
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