Editing Lighthouse (section)

===Design===

For effectiveness, the lamp must be high enough to be seen before the danger is reached by a mariner. The minimum height is calculated by trigonometry (see [[Horizon#Distance to the horizon|Distance to the horizon]]) as <math alt="D is 1.22 times the square root of H"> D = 1.22 \sqrt {H} </math>, where ''H'' is the height above water in feet, and ''D'' is the distance from the lighthouse to the horizon in nautical miles, the '''lighthouse range'''.<ref>{{Cite web |title=How to Calculate the Distance to the Horizon |url=https://www.boatsafe.com/calculate-distance-horizon/}}</ref>

Where dangerous shoals are located far off a flat sandy beach, the prototypical tall masonry coastal lighthouse is constructed to assist the navigator making a landfall after an ocean crossing.  Often these are cylindrical to reduce the effect of wind on a tall structure, such as [[Cape May Light]]. Smaller versions of this design are often used as harbor lights to mark the entrance into a harbor, such as [[New London Harbor Light]].

Where a tall cliff exists, a smaller structure may be placed on top such as at [[Horton Point Light]]. Sometimes, such a location can be too high, for example<!-- Should this be "too high; for example,"? --> along the west coast of the United States, where frequent low clouds can obscure the light. In these cases, lighthouses are placed below the clifftop to ensure that they can still be seen at the surface during periods of fog or low clouds, as at [[Point Reyes Lighthouse]]. Another example is in [[San Diego]], [[California]]: the [[Old Point Loma lighthouse]] was too high up and often obscured by fog, so it was replaced in 1891 with a lower lighthouse, [[New Point Loma lighthouse]].{{Citation needed paragraph|date=May 2021}}

As technology advanced, prefabricated skeletal iron or steel structures tended to be used for lighthouses constructed in the 20th century. These often have a narrow cylindrical core surrounded by an open lattice work bracing, such as [[Finns Point Range Light]].

Sometimes a lighthouse needs to be constructed in the water itself.  Wave-washed lighthouses are masonry structures constructed to withstand water impact, such as [[Eddystone Lighthouse]] in Britain and the [[St. George Reef Light]] of California.  In shallower bays, [[Screw-pile lighthouse]] ironwork structures are screwed into the seabed and a low wooden structure is placed above the open framework, such as [[Thomas Point Shoal Lighthouse]]. As screw piles can be disrupted by ice, steel caisson lighthouses such as [[Orient Point Light]] are used in cold climates.  [[Orient Long Beach Bar Light]] (Bug Light) is a blend of a screw pile light that was converted to a caisson light because of the threat of ice damage.<ref>{{cite web|url=http://www.nps.gov/history/maritime/constype/constype.htm|title=Maritime Heritage Program - National Park Service|access-date=6 April 2017}}</ref> Skeletal iron towers with screw-pile foundations were built on the [[Florida Reef#Shipwrecks and lighthouses|Florida Reef]] along the Florida Keys, beginning with the [[Carysfort Reef Light]] in 1852.<ref>{{Cite book|last=Dean|first=Love|title=Reef Lights|publisher=The Historic Key West Preservation Board|year=1982|isbn=0-943528-03-8|location=Key West, Florida}}</ref>

In waters too deep for a conventional structure, a [[Lightvessel|lightship]] might be used instead of a lighthouse, such as the former [[United States lightship Columbia (WLV-604)|lightship ''Columbia'']].  Most of these have now been replaced by fixed light platforms (such as [[Ambrose Light]]) similar to those used for offshore [[oil exploration]].<ref name="nps">{{Cite web|url=https://www.nps.gov/maritime/index.htm|title=Maritime Heritage Program &#124; National Park Service|website=www.nps.gov}}</ref>