Lighthouse

I

Introduction

Lighthouse, structure from which light is projected at night, or which serves as a marker by day, to guide ships sailing in coastal waters. Lighthouses are constructed at important points on a coastline, at entrances to harbors and estuaries, on rocky ledges or reefs, on islands, and even in the water. Lighthouses help identify a ship’s location, warn ships of potential hazards, and notify them that land is near. Lighthouses differ from smaller beacons in that a lighthouse includes living quarters for a lighthouse keeper. Today, however, most lighthouses use automatic electric lights that do not require a full-time resident operator.

II

Beacons

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 bend 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 45 km (28 mi) away. A Fresnel lens used in a lighthouse beacon is an array of glass prisms and a “bull’s eye” lens that is usually encased in a frame of solid brass. The lens, which surrounds a single lamp, consists of a central glass disk surrounded by concentric glass rings that decrease in thickness moving away from the central disk. Horizontal rows of glass prisms are located above and below these concentric glass rings.

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.

III

Foghorns and 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).

IV

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.

	More from Encarta
	•  Presidential Myths Quiz •  Coffee break: Recharge your brain