A lighthouse is a building, often in the form of a tower, with a light at the top to guide vessels at night. Lighthouses are built on land and at sea. Some mark ports and harbors, while others warn of shallow waters or dangerous rocks. Originally, a lighthouse keeper was always in attendance, but modern lighthouses are usually unmanned and operated by remote control. The light system may produce a steady beam, a rotating beam, or a pattern of flashes, sometimes of different colors. Sailors can identify any lighthouse by the pattern of light it emits. Knowing the location of a lighthouse enables navigators to work out their own positions. In poor visibility, a siren, horn, or bell may sound to warn vessels to keep clear of rocks. In addition, many lighthouses transmit radio signals for use by ships' direction-finding equipment. Today, lighthouses are becoming less important as many vessels are equipped with the global positioning system (GPS), a highly accurate, satellite-aided navigation system.
The illustration shows the Eddystone lighthouse, off the coast of Devon, England, as it was before it became computer operated. Built of stones that dovetail into one another, the lighthouse was completed in 1882 and has a focal plane 4 m (133 ft) above high water level.