Sailing Navigation: Sector Light Secrets You Need to Know – Part II

Here’s Part II of last week’s cruising tip from our friend Capt. John of

Build on the light sector secrets for safe sailing you learned in Part I. Now you will discover how to use sectors to avoid rocky shoals and how some lights show bright or dim lights to help you choose the best path to safety.

Look for major lights and lighthouses along the coast on your nautical chart or chart plotter. Some of these lights will show an arc–or “sector”–of light or dark to mark dangers or limitations. In the scenario above, the skipper could have sailed into the “obscured” cone of that specific sector (see description below). Light sectors are used by cartographers (chart makers) to :

- Lead the mariner clear of rocky shoals.

- Decrease intensity for easier identification.

- Increase intensity for easier identification.

Follow the examples below for a full description of these one-of-a-kind aids to navigation.

Lead the Mariner Clear of Rocky Shoals

Notice how one of the sectors in the illustration above shows no abbreviation (the arc between the two red sector cones). The sector without a color designation has been intentionally obscured (blacked out). Study your nautical chart to determine which lighted sector you need to enter for sailing safety.

Decrease Intensity for Easier Identification

Look for the word “Faint” inside the sector arc. Sector lights that show an intensity less than that of the main light are used in some countries (but rare in US waters). I would not put total faith in a light with a lower intensity sector. It may be difficult to tell the difference between the higher and lower intensity sectors of a light in certain atmospheric conditions.

You could use a danger bearing just inside the full intensity limit of the light to keep in safe water. Shoot the danger bearing as you approach with a hand-bearing compass. If the danger bearing tells you that you have drifted into the danger sector, change course toward safer water.

Increase intensity for easier identification

Take a look at the two designations of a higher intensity sector in the illustration above. The abbreviation “Intens.” will be used adjacent to the sector light color (left illustration above). Or, the range (distance) of the light will be greater than that of the light outside the arc of the intensified sector (right illustration above). Follow the same advice described earlier to shoot danger bearings if you have any doubt as to the location of the intensified sector.

Captain John’s Sailing Tip
Measure the exact true compass bearing of the sector limit lines (the dashed lines on each side of a sector) with a protractor or parallel rules. Use this information when plotting courses. Stay outside of the arc of any sector that marks dangers or shallows for sailing safety.


Now you know how to use nautical chart symbols that show light sectors to keep your small sailboat in safe water. Pass along these vital sailing navigation tips to your sailing crew or partner for peace-of-mind–wherever in the world you choose to sail or cruise!

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