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navigation, science and technology of finding the position and directing the course of vessels and aircraft.

Early Navigational Techniques

In ancient times, mariners navigated by the guidance of the sun and stars and landmarks along the coast. The Phoenicians were probably the most daring of the ancient navigators. They built large ships and, traveling out of sight of land by day and by night, probably circumnavigated Africa. The Polynesians navigated from island to island with the help of guide stars, knowledge of which was passed from generation to generation.

In England, Queen Elizabeth I did much to establish navigation laws, giving additional powers to Trinity House, a guild that had been created in 1514 for the piloting of ships and the regulation of British navigation. During this period the study of bodies of water, or hydrography, was given much attention, and harbors and the outlets of rivers were surveyed and buoyed. A tremendous advance in navigation had taken place with the introduction of the compass. Early in the 15th cent. there was progress by the Portuguese under the leadership of Prince Henry the Navigator, who built an observatory and formulated tables of the declinations of the sun; collected a great amount of nautical information, which he placed in practical form; made charts; and sponsored expeditions that led to numerous discoveries.

Introduction of Navigational Instruments

With the development of shipbuilding and the increase in knowledge of astronomy, there was increased use of instruments. The cross-staff was used to find latitude early in the 15th cent. It consisted of two pieces of wood, the cross at right angles to, and sliding on, the staff. At each end of the 26-in. (66-cm) cross a small hole was bored, and at the end of the staff a sight was fixed. To measure the altitude of a heavenly body, the instrument was sighted in that direction, and the cross was moved forward or back until the heavenly body appeared through the upper hole and the horizon through the lower. The altitude could then be read on a scale marked on the staff. Another device used for finding latitude was the astrolabe. Both were far from accurate.

The navigating equipment carried by Columbus probably was simply a compass, a cross-staff, and a table of the sun's declination. Vasco da Gama on his first voyage around the Cape of Good Hope in 1497 used an astrolabe. The Flemish geographer G. K. Mercator's work in improving charts at the end of the 16th cent., the works of the Spanish scientist Martín Cortés during the same period, the determining of the earth's circumference, and the introduction of logarithms at the beginning of the 17th cent. by the Scottish mathematician John Napier all helped advance navigation.

By the middle of the 18th cent. a quadrant could be used to find latitude and a log line and half-minute glass could help keep track of distance traveled; but the problem of finding the longitude remained unsolved until the invention of the chronometer. The appearance of the Nautical Almanac (see ephemeris) in 1767 was a great step forward in navigation, and the 19th cent. saw the development of books on navigation that far surpassed any earlier instructions, such as the standard book by Nathaniel Bowditch, an American mathematician. The system of dead reckoning, which was much refined, is the art of finding a position by calculating the point of departure (i.e., the last known point of latitude and longitude), the course (as shown by the compass), the speed and the distance traveled according to the log, and the time elapsed. The use of buoys and the making of careful charts made navigation easier, while the fixing of positions by sextant and astronomical charts was greatly improved.

Modern Navigational Tools

The next great revolution in navigation occurred in the 20th cent., when radio signals came into wide use. The development of radar, loran, and radio direction finding during World War II caused fundamental changes in navigational practice; a mariner or pilot today can turn on a Loran or Global Positioning System receiver and determine position and course to within a few yards. Inertial guidance systems, most often used to navigate submarines, aircraft, and spacecraft, allow navigation without contact with a ground base. In such systems, a computer navigates the vehicle with the aid of an inertial navigator device, which consists of a gyroscope to indicate direction and an accelerometer to measure changes in speed and direction. Inertial guidance systems and terrain-following radar allow a cruise missile to fly a thousand miles and hit its designated target.

See also air navigation; navigation satellite.

Bibliography

See latest edition of Bowditch's Practical Navigator; see also D. Sobel, Longitude (1995).

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navigation

Science of directing a craft by determining its position, course, and distance traveled. Early mariners followed landmarks visible on shore and studied prevailing winds for clues to direction. The Phoenicians and Polynesians sailed out of sight of land and used the stars to set their course. The compass (first used by the Chinese c. 1100) was the first navigational aid that gave a constant reference point, though its accuracy was limited, especially in heavy seas. Modern compasses are stabilized by gyroscopes and housed in binnacles that compensate for the craft's motion. Ship speed was first calculated by dropping overboard a log attached to a reel of line knotted at regular intervals; the number of knots exposed while the log drifted and a sandglass emptied gave the vessel's speed in knots (nautical mph). Charts are another essential navigational tool. Fixing a position requires charts detailing known locations, together with instruments that calculate a vessel's bearing relative to them. The earliest instrument for determining latitude was the quadrant, which measured the altitude of the polestar or the noonday sun. Other early instruments included the sextant and the astrolabe. Longitude (used for navigation with increasing success in the 17th–18th century) was fixed using chronometers and tables showing positions of celestial bodies throughout the year. In the 20th century, radio beacons and satellite networks allowed aircraft and ships to determine their position. Dead reckoning uses an accurate history of a vessel's headings and speeds drawn from gyroscopes and from computerized measurements of the craft's acceleration. See also Global Positioning System.

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navigation [‚nav·ə′gā·shən]

(computer science)

In a database management system, the techniques provided for locating information within the system.

(engineering)

The process of directing the movement of a craft so that it will reach its intended destination; subprocesses are position fixing, dead reckoning, pilotage, and homing.