Range lights are a navigational aid designed to help ships stay on course. Range lights are commonly used in dangerous waters so that captains can find a safe route through the water, and they can also be used for position fixing. When range lights are present, they are indicated on nautical charts so that captains and navigators know to look for them.
Two separate beacons are used to create range lights. A front beacon is low to the ground when compared with a rear beacon, and the distance between the beacons can vary. When someone is approaching the lights and is on a safe course, they align vertically, with the rear beacon appearing to be stacked on top of the front beacon. If someone is off course, the range lights will not be aligned, indicating that the ship may be on an unsafe course.
At night, the beacons will be easy to use as long as conditions are clear enough to make them visible. During the day, the beacons can be hard to spot, and as a result, range lights are usually brightly painted and may be hung with flags so that they will be clearly visible. As long as there is no fog, navigators will be able to see the colorful support structures for the beacons, which should stand out from the surrounding environment.
Sometimes, lighthouses are arranged in a range light configuration. In other times, range lights are simply mounted on posts, poles, and other structures. They must be maintained so that they will operate at all times, as navigators may make mistakes if they think that range lights should be present and functional and they are not available. One common use for such lights is in harbors, where they are used to mark safe passages, and they can also appear on inland waterways like rivers, guiding navigators through twists and turns.
Range lighting is only one among many navigational aids designed to help ships of all sizes move safely through the water. The major drawback to range lights is that if visibility is poor, navigators will not be able to see the lights. This is one reason why range lights are marked on charts, so that if a navigator cannot see the lights, she or he knows that visibility is bad and it may be advisable to stop or slow down until conditions improve. Devices like foghorns can pair with lighting to warn navigators off if they approach dangerous areas in the water.