Navigation is the art and science of conducting a ship, airplane, or spacecraft safely and expeditiously to a specific destination. In the broadest sense anyone faced with the problem of "finding the way" is a navigator, whether he or she travels on land, at sea, in the air, or in space. EARLY SEA NAVIGATION Before navigators ventured to cross the ocean, sea trade was confined to inland seas and coastal voyages; a navigator may have been out of sight of land for only a few days at most. The primary concern in this situation was familiarity with coastal features along the route followed. Knowledge of the relative positions of prominent headlands, the distances between them, and the directions from one to the other, together with information on local winds, currents, and such hazards as rocks and shoals, formed the basis of the mariner's sailing directions. At first these directions were passed down orally from experienced master to novice apprentice. Later, directions were recorded in writing; and still later, following the invention of the printing press, printed sailing directions became readily and widely available. During the primitive period of navigation there was hardly any need for sophisticated instruments and techniques. Indeed, the only navigational instrument commonly used by the first navigators was the sounding pole or sounding line that enabled them to measure underkeel depths. It was not until the 12th century that the magnetic compass (see \Tcompass, navigational\t) was introduced, first to Mediterranean seamen. From that time it has been considered the most important aid to navigation. Before the advent of the compass, seamen estimated directions largely with reference to the wind, although strong evidence indicates that some Mediterranean voyagers used elementary astronomical principles for this purpose. Soon after the invention of the magnetic compass, marine charts appeared. The earliest form was the portolan chart drawn on a prepared animal skin. The portolan chart is characterized by a maze of sets of intersecting lines, each set formed by radial lines drawn from a common point. The lines, commonly called rhumb-lines, were used for ascertaining the direction in which to steer in order to reach a destination. Knowledge of the rate at which a ship is traveling through the water is important if the navigator needs to estimate the time of arrival at a destination. In modern times any device used for this purpose is called a log, of which there are many varieties. The earliest printed reference to a log, dated as late as 1574, appears in the first native English manual of navigation, written by William Bourne of Gravesend. In earlier times the navigator judged the speed of the ship, without instrumental assistance, merely by observing the relative motion of the ship and the sea. A record of courses steered and distances sailed on each course forms the ship's "reckoning"; a position obtained from such a record is a "dead reckoning" position. Because of the difficulty of determining actual courses and distances traveled, and because of the effects of wind and current, dead reckoning positions are usually inexact. Although this was not a serious handicap in the days when navigation was confined to the enclosed Mediterranean or coastal waters, when Portuguese seamen began their methodical exploration of the open Atlantic in the early 15th century, the primitive dead reckoning techniques were recognized as inadequate for ocean navigation. The cosmographers whom the Portuguese prince \THenry the Navigator\t gathered around him during the early days of the Age of Discovery are credited with providing seamen with a technique for finding latitude when out of sight of land. This technique represented the beginnings of modern nautical astronomy, the science by which terrestrial positions may be found from observations of heavenly bodies. The advantage of being able to use the latitude of the ship in these calculations, together with the record of courses and distances kept in the reckoning, enabled the seaman to increase the reliability of the ship's positions. Determination of Latitude at Sea The earliest astronomical method used by mariners for finding latitude involved measuring the altitude (the angle in the vertical plane between a heavenly body and the horizon) of the Pole Star (alpha Ursae minoris). The Pole Star is located near the north celestial pole (the point on the sky vertically above the Earth's north pole), so that its altitude is approximately equal to the observer's latitude. Prince Henry's cosmographers devised tables that provided the necessary correction to be applied to the altitude of the Pole Star to obtain the latitude. Portuguese navigators discovered that when the equator was crossed, the Pole Star ceased to be visible for use in finding latitude. To solve this problem, solar declination tables were first devised. By observing the Sun's altitude at noon when the altitude is at a maximum for the day, and then combining the noon Sun's angular distance from the zenith (the complement of the altitude) with the Sun's declination obtained from the tables, the navigator was able to find the ship's latitude. The earliest instrument used by navigators for measuring the altitude of the Pole Star or the Sun was the seaman's \Tquadrant\t, a device with the shape of a quarter circle made of wood or brass and having two sights on one radial edge. A plumb line, suspended from the center of the circle of which the arc of the quadrant forms a part, indicates the altitude of the Sun or star on the graduated arc when the quadrant is held in the vertical plane and the Sun or star is observed through the sights. The instrument, however, was not accurate when used on board a ship at sea. Another primitive instrument for measuring altitude that was designed for sea use was the mariner's \Tastrolabe\t. It consisted of a massive metal ring fitted with a diametrical \Talidade\t provided with two sights. When in use the instrument was suspended from a thumb-ring and the alidade set so that the heavenly body being observed could be seen through the sights. The edge of the alidade then indicated the altitude of the observed body on the graduated arc of the ring. The astrolabe was of little use except in calm conditions. The first instrument of practical use for measuring altitudes from the deck of a ship in rough seas was the cross-staff, also known as the \TJacob's staff\t, consisting of a graduated rod with a crossbar for sighting. The cross-staff was superseded for measuring Sun altitudes in the late 16th century by a novel device in the form of a shadow-staff, invented by the Elizabethan navigator Captain John \TDavis\t and first described in his book The Seamans Secrets (1595). The Davis quadrant, known on the Continent as the English quadrant, evolved from the Davis backstaff. It was in general use at the time of the invention of the Hadley reflecting quadrant, developed in the early 18th century. (See longitude section below.) Determination of Longitude at Sea Finding latitude at sea is far simpler than finding longitude. Indeed, the problem of determining longitude at sea was not solved until the mid-18th century, when two methods came into use, both of which reached a state of practicability after a long period during which the methods were gradually improved. The first of these, known as the lunar-distance method, became available with the publication of the first official British Nautical Almanac for the year 1765. This almanac gave for the year, at intervals of 3 hours of Greenwich time, predicted angles (known as lunar distances) between the Moon and each of a small number of selected stars located on or near the Moon's monthly circuit of the heavens. The mariner merely measured a lunar distance and compared it with the predicted lunar distances given in the almanac. The comparison enabled the mariner to determine the Greenwich time of the observation. The Greenwich time compared with local time (obtained from a simultaneous altitude observation of a heavenly body) gave the ship's longitude, the longitude of a place being proportional to the difference between Greenwich and local times at any instant. It was to facilitate the lunar method for finding longitude at sea that the mariner's \Tsextant\t was invented. This instrument, in the form of a sixth of a circle, was developed from the reflecting instrument designed by John Hadley about 1730. The Hadley instrument, in the form of an octant (eighth of a circle), was capable of measuring angles up to 90 deg. Because lunar distances sometimes exceeded 90 deg, a British naval officer, John Campbell, suggested (1757) extending the limb of the Hadley octant to a sixth of a circle (sextant) so that angles up to 120 deg could be measured. In the second method for finding longitude at sea a delicate mechanical timekeeper, now known as a \Tchronometer\t, is used. The first satisfactory marine timepiece designed for determining longitude at sea was constructed by John \THarrison\t in about 1750. When improved manufacturing methods made it possible to produce chronometers at relatively low prices, the lunar method became secondary for finding longitude at sea. Predicted lunar distances ceased to be given in the Nautical Almanac in the early part of the 20th century, and since that time the chronometer method for finding longitude has been standard in astronomical navigation. Until the mid-19th century, the usual practice of the nautical astronomer had been to observe the Sun at about 8:00 AM local time and then again at noon, the second observation allowing the observer to find the ship's noon latitude and hence (from the reckoning) the latitude of the ship at the time of the morning observation. The observer would then be able to compute the longitude of the ship at the time of the morning observation, and hence (from the reckoning) the longitude of the ship at noon. To find longitude requires knowledge of latitude, so that in the event of a failure to observe the Sun at noon (because of fog or cloudiness) the data of the morning observation was rendered useless. In 1837 an American captain, Thomas Sumner, discovered what became known as position-line navigation. Using this technique, a navigator is able, whether or not latitude is known, to determine from an altitude observation a line somewhere on which the ship lies. Such a line, when projected on the chart, is called a position line. The plotting of two such intersecting lines enables the navigator to determine the ship's position. Sumner's discovery, published in 1843, revolutionized astronomical navigation. MODERN AIR, SEA, AND SPACE NAVIGATION In the early days of piloted flying machines, navigational techniques in the air were analogous to those of early seamen navigating by following the coastline: it was the recognition of ground marks that enabled the flying navigator to find the way. When long flights over the sea were first made, aviators, like seamen, used astronomical navigation for position finding. The need for speed in position finding on fast-flying aircraft led to a number of astronomical navigational improvements, especially in the field of navigation tables. These improvements were eagerly adopted, in turn, by mariners. In more recent times, when the relatively slow procedures used in astronomical navigation ceased to be suitable for air navigation (especially in wartime), the application of electronics and radio techniques to position finding brought about a new revolution in marine, as well as in air, navigation. Radio Techniques In addition to their increased use of sonic depth sounders and gyrocompasses, mariners soon took advantage of other technological advances. The first radio aid to navigation, initially used in the opening decade of the 20th century, was the radio time signal; this, in effect, made the chronometer redundant. In the 1920s medium-frequency-radio direction finding gave a new dimension to navigation by making it possible to find a ship's position from radio bearings of land-based transmitters. The world wars, especially World War II, gave great impetus to the advancement of navigation, as it did to other areas of science and technology. The problem of enemy detection, for example, in the air and at sea, led to the invention of \Tradar\t techniques, using high-frequency radio energy. Radar in peacetime has been of great importance for safety in air and sea navigation when visibility is low. Again, the problem of position finding in the air when sky conditions rendered astronomical navigation useless led to the invention and development of several systems of navigation, some of which, at the present time, have all but superseded astronomical navigation. In particular the invention in the 1940s and '50s of the method known as hyperbolic navigation has had the greatest influence on sea as well as air navigation. Hyperbolic navigation, so called because the method involves lines of position in the form of hyperbolas, is based on the accurate measurement of the difference in times taken by radio signals transmitted from each of two fixed stations to reach an observer. The velocity of radio energy is considered constant, so that distances traveled by radio energy are proportional to the amounts of time taken. Included in hyperbolic systems are the Decca Navigation System, Loran (Long-Range Navigation), Omega, and the satellite-based Global Positioning Systems. The primary international \Taircraft\t navigational system is VOR (Very High Frequency Omni-Directional Range). Omnidirectional radio ranges are high-frequency signals that give bearings in all directions and--unlike earlier Low-Frequency radio beacons--tell the pilot whether the plane is moving toward or away from the signal, as well as how distant the signal is. VOR has been combined with the military system known as TACAN (Tactical Air Navigation System), which provides distance, azimuth, and identification information. Soon after the first artificial satellite had been launched in 1957, scientists, noting the change in the frequency of the radio signals transmitted from the satellites--the well-known \TDoppler effect\t--invented the navigational system now known as \TTransit\t. This system employs six satellites, each of which circles the Earth in polar orbit at a distance of about 965 km (600 mi) with a period of about 108 minutes. Each satellite is equipped with a magnetic memory that stores details of its orbit and position (its ephemeral data) for 12-hour periods. The ephemeral data are provided by a ground station that transmits to the satellite each time it makes a passage at or near the station. Before the 12-hour ephemeris is transmitted for storage in the satellite, the satellite's memory of the previous ephemeral data is erased. The ephemeral data are determined through the agency of a number of tracking stations, at which Doppler data are collected and transmitted to a computing center, which analyzes the data and computes an updated ephemeris for each of the satellites. Details of the satellite's position--which a navigator receives direct from the satellite--together with an observed Doppler shift as the satellite passes the observer's position, enables the navigator to find the ship's position to an accuracy of about 0.1 mile. vessels. The \TNAVSTAR\t satellite system, part of the Global Positioning System (GPS) developed by the U.S. military, is available for civilian use as well. Scientists have discovered how to use the satellites' signals to measure distances with a precision of 1 part per million, and further refinements may reduce the error to 0.1 part per million. Inertial Navigation The system known as inertial navigation is a sophisticated form of dead reckoning in which the motion of the ship, airplane, or spacecraft is sensed so that the craft's position relative to its starting position is known at all times. Inertial navigation, which plays a vital role in \Tspace exploration\t, employs \Laccelerometer\ls to measure accelerations of the craft in each of three mutually perpendicular planes. The accelerometers are fitted to a device known as a stable platform, which has three planes of freedom, thereby maintaining a fixed orientation in space irrespective of the motion of the craft. The platform is stabilized by \Lgyroscope\ls that sense the rotation of the platform relative to space. The inertial system employed in submarines and surface craft is a self-contained system that functions independently of weather conditions that may hamper astronomical navigation and independently of radio energy, which may suffer natural interference or that caused by people. For these reasons such a system is of great strategic importance for military vessels. Its high cost, however, precludes its use in most commercial vessels and aircraft, although it has become standard equipment in large commercial airplanes. Effects of Space Research Recent research into \Tguidance and control systems\t for space exploration has resulted in a large array of sophisticated navigational techniques being made available for terrestrial navigation. The navigation of space vehicles is conducted almost entirely by ground control. The navigation of aircraft also is becoming increasingly a matter of ground control, with the process of takeoff and landing largely in the hands of air traffic controllers. It is only in the case of ships that the navigators on board still control the movements of their respective vessels, but this, too, may change before long. \TCharles\t H. COTTER Bibliography: Bowditch, Nathaniel, The American Practical Navigator, 2 vols. (1977); Chapman, C. F., and Maloney, E. S., Piloting Seamanship and Small Boat Handling, 53d ed. (1977); Clausing, D. J., The Aviator's Guide to Modern Navigation (1987); Cotter, C. H., Elements of Navigation and Nautical Astronomy (1977); Maloney, E. S., ed., Dutton's Navigation and Piloting, 14th ed. (1985); Moore, D. A., Basic Principles of Marine Navigation (1979); Shufeldt, H. H., and Dunlap, G. D., Piloting and Dead Reckoning (1981); Sonnenberg, G. J., Radar and Electronic Navigation (1988); Toghill, Jeff, Celestial Navigation (1987). See also: \Tairport\t; \Taviation\t; \Tship\t.