Purposes of a Lighthouse
An aid for navigation and pilot age at sea, a lighthouse is a tower building or framework sending out light from a system of lamps and lenses or, in older times, from a fire. More primitive navigational aids were once used such as a fire on top of a hill or cliff.
Because of modern navigational aids, the number of operational lighthouses has declined to less than 1,500 worldwide. Lighthouses are used to mark dangerous coastlines, hazardous shoals away from the coast, and safe entries to harbors.
A problem with a single lighthouse is that it is only a point at night. For many navigation problems, this is insufficient.
If the navigator is trying to find the path into a river, he needs a better way, such as the Range light. Two lights are used in this scheme, one is named the Front Range, and the other is called the rear range. The rear range light is always taller than the Front Range light. When you are on the path into the river, the two lights line up one on top of the other.
This provides better aid for the navigator. When the vessel is on the correct course, the two lights line up. But when on the wrong course, the difference in height makes it very obvious whether to travel left or right to correct the problem.
This technology was first developed in Europe in 1837 where they are referred to as Leading Lights. [1] The first usage in the US is not well defined, but there are some listed in the 1854 Light list ( Cherry Hill, NJ, North Point, MD, and Beacon Island). [2]
This technology works not only for finding the way into a river; it can be used to navigate the entire river. For instance, it is possible to sail on the Elbe River in Germany at night, and each time it is necessary to make a turn, the navigator lines up the next range lights. This takes you all the way from Hamburg out to the sea, using one pair of range lights after another.
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