Leading lights

Leading lights (also known as range lights in the United States) are a pair of light beacons, used in navigation to indicate a safe passage for vessels entering a shallow or dangerous channel; and may also be used for position fixing. At night, the lights are a form of leading line that can be used for safe navigation. The beacons consist of two lights that are separated in distance and elevation, so that when they are aligned, with one above the other, they provide a bearing. Range lights are often illuminated day and night.

In some cases the two beacons are unlighted, in which case they are known as a range in the United States or a transit in the UK. The beacons may be artificial or natural.

Operation

Two lights are positioned near one another. One, called the front light, is lower than the one behind, which is called the rear light. At night when viewed from a ship, the two lights only become aligned vertically when a vessel is positioned on the correct bearing. If the vessel is on an incorrect course, the lights will not align.