Ships rely on lighthouses to warn them of danger and guide them to land or harbors. A traditional lighthouse is a tall, rounded tower on the coast, but a modern lighthouse may be a platform in the sea. Lighthouses now have horns, radio transmitters, and other navigation aids in addition to lights.

Construction

Lighthouses are built on the shore, on rocks or islands, or in the open sea. Some are made with stone or brick, but most modern lighthouses are made with concrete or steel. They usually have a tower that holds the light source. Lighthouses built on wave-swept rocks or islands must be very solidly built to resist the tremendous force of the waves.

Light, sound, and radio signals

The earliest lighthouses were lit by wood or coal fires. Later lighthouses got their light from lanterns holding groups of large candles. Lamps that burned oil or gas came later, and they are still used in some lighthouses.

Today, however, the standard source of light in lighthouses is an electric lamp. A special lens focuses the light into a narrow beam. The beam can be seen up to 28 miles (45 kilometers) away. The lamp and the lens rotate so the beam shines in all directions. Few lighthouses give off a steady light. Instead, most send out a series of flashes. Ships can identify a lighthouse by its pattern of flashes.

Lighthouses used to be operated by a lighthouse keeper who lived in the building and made sure that the light kept working. Modern lighthouses, however, have automatic lights that are operated by remote control.

In a bad storm or a dense fog, even the brightest of lights may not be strong enough to warn ships of danger. Therefore, lighthouses need other ways to send signals to ships. One method is sound. Lighthouses once used cannon or bells to warn ships, but today the most common sound device is the foghorn. The noise from a foghorn can be heard miles away. Modern lighthouses also send out radio and radar signals to passing ships. Ships carry special equipment to pick up these signals.

Lightships and buoys

Lightships and buoys are movable devices that serve the same purpose as lighthouses. They are used in places where lighthouses are impossible or too expensive to build. Their great advantage is that they can be moved to warn ships of shifting dangers, such as underwater sandbanks that move as the sea shifts the sand. Modern lightships are small ships equipped with lights, a foghorn, and radio devices. Buoys are smaller floating objects that provide light and information to navigators through their color, shape, and markings. Both lightships and buoys operate without a crew. They are held in place by anchors.

History

At first people simply built fires on hillsides to warn ships of land. The first true lighthouse may have been the Pharos of Alexandria, in Egypt. Built in about 280 BC, it stood about 350 feet (110 meters) high on the island of Pharos in Alexandria's harbor. In ancient times it was called one of the Seven Wonders of the World. It was destroyed by an earthquake in the 14th century AD.

The Phoenicians, Greeks, and Romans also built lighthouses during ancient times. Few lighthouses were built in the early Middle Ages, but construction picked up again as shipping increased in Europe around AD 1100. By 1500 lighthouses were a standard feature of navigational books and charts.

The first modern lighthouses were built around 1700. After that date improvements in construction techniques and lighting equipment came quickly. The great engineer John Smeaton made a breakthrough with his design for the lighthouse built in 1759 on the Eddystone Rocks off the southern coast of England. Earlier lighthouses on the site had been made of wood, but Smeaton's was built from interlocking blocks of stone. This technique strengthened the structure so it could withstand powerful waves. The Eddystone Lighthouse became a model for most lighthouses that followed.