The first lighthouses were built long before the time of Christ. The earliest known reference to a lighthouse dates back to 1200 BC. This reference appeared in the Iliad, Homer’s Greek epic poem. The first onshore beacons that were used to guide ships were bonfires. Eventually, bonfires were replaced with iron baskets filled with burning wood or coal and suspended on long poles.
It was not until the 18th century that these baskets were replaced by oil or gas lanterns. In the early to mid-20th century, electric beacons replaced oil and gas lanterns. One of the most important developments in the technology of lighthouse beacons was the invention of the Fresnel lens.
Developed in 1822 by French physicist Augustin Fresnel, the Fresnel lens is a collection of multiple glass prisms (transparent bodies consisting of two or more usually flat surfaces and used to bend a beam of light that bends nearly all the available light into a powerful central magnifying lens.
The magnifying lens projects an intense beam of light that is visible up to 28 miles away.
Modern lighthouse beacons vary in power from about 10,000 candelas to about 1 million candelas, depending on the prevailing weather conditions and the visibility requirements of shipping traffic in the particular area. (The candela is the unit used to measure the amount of light that a source radiates in a given direction – one candela is equivalent to about one two-hundredth of the brightness of a 50-watt light bulb.)
Every lighthouse emits a distinctive series of flashes known as its “characteristic.” These flash sequences allow ship captains to time intervals between flashes so that they can identify lighthouses, using a publication called the Light List. This publication assigns a number to each lighted beacon and describes its identifying characteristics. Flash sequences include the following: fixed, occulting, flashing, isophase, group flashing, alternating, and Morse code.
A fixed light is a light that shines continuously. An occulting light has longer periods of light than of darkness, while a flashing light has longer periods of darkness than of light. An isophase light has equal durations of light and darkness. A group flashing light has a specified number of regularly repeating flashes of light, while an alternating light shows light of different colors. Morse code shows short and long flashes that represent dots and dashes, respectively. These dots and dashes are grouped to represent code characters.
Foghorns & Radio Signals
Many lighthouses are equipped with a horn to help guide ships sailing in foggy weather. These foghorns, which make their sound by quickly releasing compressed air, can be heard for distances of up to 13 km (8 mi). Ship captains can determine their position by identifying distinctive combinations of long and short horn blasts specific to each lighthouse.
Some lighthouses are also equipped with radio beacons that transmit Morse code radio signals. These radio signals, which are distinguished by short (dot) and long (dash) combinations, have a range of up to 320 km (200 mi).
Lighthouse Structure and Appearance
By day, lighthouses often serve as landmarks for ship captains seeking to identify their position along a coastline. Thus, lighthouses vary in shape, size, and color. Lighthouses can be square, round, conical, rectangular, and even octagonal (eight-sided). Some are painted with identifying stripes, spirals, or diamond patterns. Some have twin and, in one case, triple towers. Most lighthouses range in height from 10 m (33 ft) to 63 m (208 ft).
Lighthouses are built from wood, stone, brick, reinforced concrete, iron, steel, or aluminum. They are designed to withstand local environmental conditions. For example, tall skeletal lighthouses located in the Caribbean offer less surface area to hurricane-force winds and waves. Lighthouse designs often reflect architectural styles of the time the lighthouses were built.
Lightships and Lesser Beacons
In the 19th century, lightships, which are essentially floating lighthouses, were stationed in coastal waters where lighthouses could not be built. A lightship displayed one or more lights from a mast or masts as an aid to navigation. At one time or another more than 120 lightships dotted the coastal waters of the United States.
By 1985 all U.S. lightships had been replaced by buoys equipped with an automated beacon and fog signal. These massive buoys, called Large Navigational Buoys, measure 12 m (40 ft) in diameter and are among the largest of a variety of a variety of navigational aids known as lesser beacons.
Lesser beacons include river lights, fog signals, and numerous other smaller navigational aids used to mark channels in rivers and harbors. Today hundreds of thousands of lesser beacons are in use throughout the world.