Aligning two fixed points on land provides a navigator with a line

Aligning two fixed points on land provides a navigator with a line of position called a range in the USA and a transit in Britain. Ranges can be used to precisely align a vessel within a narrow channel such as in a river. With landmarks of a range illuminated with a set of fixed lighthouses, nighttime navigation is possible.

Such paired lighthouses are called range lights in the USA and leading lights in the United Kingdom. Two lights are used in this scheme. The one closer to the vessel is named the beacon or front range; the furthest away is called the rear range. The rear range light is almost always taller than the front.

When the vessel is on the correct course, the two lights line up above one another. But when the observer is out of position, the difference in alignment indicates the proper direction of travel to correct the problem.

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