Definition
Also called range lights in United States practice, they are two lights at different heights and distances; when the higher rear light appears directly above the lower front light the vessel is on the leading line, whose true bearing is printed on the chart. Sensitivity is high because a small lateral error swings the lights apart visibly. The nearer front light swings further, so if the rear light appears to the right of the front you are right of track and must alter to port: steer towards the lower light.
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