Using Lights for Navigation

Identifying a light positively requires matching both its characteristic (colour and flash pattern) and its period (the time for one full cycle). Use a watch to time the interval from the start of one pattern to the start of the next and compare with the chart or light list. A single observation is not enough — time at least three complete cycles to be confident.

Light sectors are a powerful tool for pilotage approaches. By staying within a white sector, you remain on the safe track. If the light changes to red or green, you know immediately that you have strayed off the intended line. Sector lights are particularly valuable for harbour approaches at night where the channel is narrow or bounded by dangers.

The Admiralty List of Lights (ALRS Volume A–F) is the definitive publication listing all navigational lights worldwide, including their positions, characteristics, ranges, heights, and sector details. For passage planning, nautical almanacs (such as Reeds or the Admiralty Nautical Almanac) provide light lists for a particular region. These should be kept corrected from Notices to Mariners.

When timing a light's period, start your stopwatch as the flash begins and stop at the beginning of the next identical flash pattern. For group-flashing lights, time from the first flash of one group to the first flash of the next. Compare the result to the charted period. If the period and characteristic match, you have confirmed the identity of the light.