lighthouse

The Needles and lighthouse, Isle of Wight. The Needles, at the western extremity of the island, are sea-stacks formed principally of chalk and flint; the unmanned lighthouse is on the furthest rock.

Watch Hill, Rhode Island, was used as a lookout during the American War of Independence, and in 1806 a lighthouse was erected on the site. Rebuilt after a devastating hurricane in 1838, the town of Watch Hill is today a resort community.

Built in 1867 and sited in the Hooper Strait, Maryland, this lighthouse was in 1877 pushed free from its pilings by ice and drifted out into Chesapeake Bay. Today the lighthouse is a very popular visitor attraction.

Faro Blanco lighthouse, Marathon, Florida Keys. The octagonal tower on the resort's marina was privately built, functioning both as a navigational aid and as a hotel.

Elevated structure equipped with a powerful flashing light for use as a maritime navigational aid. The light signals to ships that they are approaching land, or dangerous waters. Increasingly lighthouses are automated rather than staffed; later designs also emit radio signals that enable the ship to establish its position.

Lights may be either flashing, when the dark period exceeds the light, or occulting, when the dark period is equal to or less than the light; fixed lights are liable to cause confusion. The pattern of lighting is individually varied so that ships or aircraft can identify the lighthouse. In fog, sound signals may also be used, such as horns and sirens.

The light is magnified by one of two methods: catoptric, which uses reflection; dioptric, which uses refraction. Modern lighthouses are catadioptric, a combination of the two.

Early lighthouses used fire as the source of the signal, and include the Pharos of Alexandria (c. 280 BC) and those built by the Romans at Ostia and Ravenna in Italy, Boulogne in France, and Dover, England. In England, beacons burning in church towers often served as lighthouses until the 17th century. In lighthouses such as Smeaton Eddystone (1698), as few as 24 candles were used. The arrival of electricity in the 1860s introduced the ‘carbon arc’, a dangerous system that used a high-voltage spark for the signal. The tungsten filament light bulb, invented by Thomas Edison in 1879, standardized the use of electricity in lighthouses. By the end of the 20th century, bulbs of 1000–3000 watts were being used.

Where it is impossible to install a fixed structure, unattended lightbuoys equipped for up to a year's service may be used. Fog bells and whistles are operated by the movement of the waves. Where reefs or sandbanks make erection of a lighthouse impossible, lightships may be moored.

The numbers of lighthouses and lightships are declining with the increased use of navigational aids such as GPS. In the United Kingdom all lighthouses are now uncrewed and in the United States most are. Most British lightships are now operated by solar power, but the brightest ones are powered by diesel-electric generators.