Leading lights

Range lights in Nantucket showing daylight markings

Leading lights (also known as Range lights in the USA) are a pair of light beacons, used in position fixing and navigation to indicate a safe passage for vessels entering a shallow or dangerous channel.

They consist of two lights that are separated in distance and elevation, so that when they are lined up vertically, with one behind the other, they provide a bearing. So leading lights are a form of leading line that can be used at night.

Leading lights are often confused as being lighthouses, as they are often built to look like lighthouse structures. Contrary to major lighthouses who usually warn offshore navigators of obstructions upon landfall however, leading lights assist the inshore navigation of ships, vessels and other watercraft into defined channels, lanes or port and harbour entrances.

Operation

Baileys Harbor Range Lights, with the rear light in a house and the front light on a skeleton tower. The red and white striped panel is a daymark designating a range light.

Two lights are positioned in front of one another. One, known as the front light, is shorter than the other one behind it, which is referred to as rear light. At night when viewed from a ship, the two lights become aligned directly above one another when the navigator of the vessel is heading on a straight course. If the vessel is on an incorrect course, the lights will not line up. The front light would appear slightly left of the rear light, for example.

During the day, the lights cannot easily be seen and therefore leading lights are often fitted with secondary visual aids, e.g. huge red flags with thick black lines running down them. When both red flags and black lines line up, the navigator will again know that the vessel is on the correct course.

For example, a narrow channel might have shallow water at each side but deep water in the middle. If the ship veers off to either side, it risks being grounded on the sea or river bed. But as long as it stays in the middle of the channel or river, it will stay out of danger because the water is deep enough. To help the navigator stay in the middle of the river, he or she lines up the front (bow) end of the vessel with the aligned leading lights or marks, and continues along that bearing.

Some rivers, such as the Elbe River in Germany, have a series of leading lines. Each time it is necessary to make a turn, the navigator lines up the next pair of leading lights. This provides guidance from Hamburg to the sea, using successive pairs of leading lights.

Leading lights and marks may also be used for pilotage to determine a vessel's exact location at sea.

The first set of range lights in the United States were privately established by subscription at Newburyport Harbor in Massachusetts in 1788. This technology was first used in Europe in 1837, where the term leading lights originates.

Unlike conventional lighthouse towers, leading lights are sometimes designed to be movable; consequently, their position can be shifted in the event of a change in the sea channel.