Lighthouse is a structure, most commonly a tower, with a powerful light that serves as a navigational aid for mariners. Lighthouses help sailors determine their position at sea. Lighthouses may also inform sailors that land is near and warn them of dangerous rocks and reefs.
Lighthouses are built at ports and harbors, on capes and peninsulas, and on isolated rocks. They can be situated on land near the sea, or they can be wave swept—that is, exposed to the sea or in it. In areas considered too difficult to build in or with water levels too deep to permit building, a lightship—a ship fitted with a powerful light—might be anchored where needed and used instead of a lighthouse.
Identifying lighthouses.
For lighthouses to be useful for navigation, mariners must be able to identify which lighthouse they are seeing. Therefore, lighthouses have traits to make each unique.
At night, a lighthouse gives off a distinctive light pattern, known as its characteristic. The light signal may be constant, or it may alternate between light and dark, or it may alternate from white to colored light. The unique signal allows sailors to identify a particular lighthouse at night. In the daytime, lighthouses can be identified by their appearance, or daymark. Most lighthouses are towers made of stone, brick, wood, iron, steel, or concrete. Some are simple metal towers, and others consist of houselike structures atop a multilegged platform. In areas where there are several lighthouses of similar construction, each may be distinguished by a noticeable paint pattern, such as a design of spiral stripes.
The characteristics and daymarks of lighthouses are recorded in publications called light lists. Mariners determine which lighthouse they are seeing—and therefore their own location—by observing a lighthouse’s characteristic or daymark and consulting a light list.
Development of the lighthouse.
The ancient Phoenicians and Egyptians were probably among the first to use a dedicated light to guide ships. They kindled fires on top of hills or perhaps in tall structures that may have been among the first lighthouses. The first definitive record of lighthouses dates to around 300 B.C., when people began to build elaborate towers to house beacons.
For many centuries, wood was the primary fuel used in lighthouses. By the A.D. 1500’s, coal fires or multiple candles were also being used. For centuries, lighthouse engineers tried to use mirrors or other reflectors to intensify the brightness of the light. But until the 1700’s, scientists remained generally inefficient at producing the appropriate kind of light.
By the 1700’s, lighthouse designers began to focus on using oil-based lamps in lighthouses. Around the mid-1700’s, engineers began attaching parabolic (curved) reflectors to the lamps to redirect and strengthen the light. These reflectors improved the strength of the light beam, but the flame created by oil-based lamps at the time was relatively weak and generated large amounts of smoke. In the early 1780’s, Aimé Argand, a Swiss scientist, created a new kind of oil-based lamp. The Argand lamp used a hollow, circular wick that generated a steady, strong flame with little smoke. A single Argand lamp generated as much light as seven candles. Designers attached cone- or parabolic-shaped reflectors to the lamps. Lighthouses would use up to dozens of such devices to generate a strong light signal.
In 1822, Augustin Fresnel, a French physicist, invented the Fresnel << FREHZ nuhl or fray NEHL >>, or classic, lens. The Fresnel lens was a thin, light, compound lens that could be formed into a large structure encircling a lamp. This structure resembled a barrel or beehive made of glass. The multiple prisms in a Fresnel lens focused the light into a single beam, making it visible from a great distance. Using just a single lamp in its center, a Fresnel lens could project light more than 20 miles (32 kilometers).
By the late 1700’s and early 1800’s, lighthouses could be seen from such long distances that mariners were likely to see the lights of two or more lighthouses at the same time. It became more important for a lighthouse to have an identifiable characteristic. By rotating either the lamp itself or the lens surrounding a lamp, it was possible to vary the light pattern. The number of times the light could be seen in a certain period depended on the rotation speed. Flashes of different colors could be created by using shades of colored glass.
Argand-style lamps that burned kerosene were widely used in the second half of the 1800’s, but experiments using electric current created by generators began around the same time. By the turn of the century, most lighthouses had begun converting to electric power. In the late 1800’s, the Argand lamp design was altered to run on acetylene instead of oil. Acetylene, a colorless gas, produces a bright, relatively smokeless light. Some lighthouses today still use acetylene-based lamps. Many remote lighthouses now run on solar power.
Some lighthouses have kept their Fresnel lenses. But many working lighthouses produce light with self-contained weatherproof beacons that use acrylic lenses instead of glass. These modern beacons can often be monitored remotely and are generally easy to maintain.
Several thousand lighthouses remain in use worldwide. But the development and increased use of advanced navigational aids have led to a decrease in the number of working lighthouses. Many retired lighthouses are regarded as historic monuments and are maintained by preservation organizations and communities.
The lighthouse keeper.
Many lighthouses had keepers who lived in or near the lighthouse. The keeper’s duties included trimming and lighting the lamp wicks, polishing the reflecting mirrors or lenses, and cleaning soot off the lamp chimneys and lantern glass. Keepers also replaced the fuel and rescued shipwrecked sailors.
Although most lighthouses worldwide are now fully automated, a few still have a keeper. The last station in the United States to have an official keeper was the historic Boston Light. Its last keeper, Sally Snowman, retired in 2023, though its light had been automated since 1998. Some lighthouses in the Bahamas are still fueled by kerosene, with resident keepers who still wind machinery to turn the lenses.
Notable lighthouses.
The Pharos of Alexandria, one of the Seven Wonders of the Ancient World, was perhaps the most famous of ancient lighthouses. Completed by the Egyptians during the reign of Ptolemy II (283-246 B.C.), this structure guided ships for 1,500 years.
The world’s oldest operating lighthouse is at La Coruña in Spain. This lighthouse, also known as the Tower of Hercules, was built by the Romans about 2,000 years ago, but it has been significantly altered over the years. Another of the oldest operational lighthouses is the Cordouan Lighthouse in France, originally completed in 1611.
