Range light

Range lights, also called leading lights, are a pair of fixed lights set at different heights and distances apart so that when the mariner brings them into a vertical line he is on the safe leading line of a channel or approach. The rear light is higher than the front, so the two appear in transit only on the centerline; drifting off track opens the lights to one side and shows which way to steer back. The leading line’s bearing is printed on the chart, and the lights carry stated characters and sectors in the light list. Range lights give a precise track without instruments, which is why they mark narrow dredged channels and harbor approaches.