History of Navigation
	The navigation of rivers, lakes, and oceans began before recorded history. Navigation, due to its relationship and importance to transportation, has played a leading part in the advancement of civilization. Men learned early that travel by water was a convenient means of transporting their goods of trade to other lands. The people living near the Mediterranean Sea the Sumerians, Cretans, Egyptians, Phoenicians, and Greeks became able mariners, as did the Scandinavians in northern Europe. The early mariners did not venture very far from the coasts. Skirting the coastlines, they could identify objects on land and thereby know the positions of their ships. Usually they traveled by day and went ashore at night. They did not have nautical charts, but sometimes they found their way by a list of directions. The Romans called such a list a periplus. It gave details of landmarks, good anchorages, and such hazards as shoals and reefs.
	Early Developments in Navigation
	The Phoenicians and Greeks were the first of the Mediterranean sailors to navigate far from land and to sail at night. They made primitive charts and knew a crude form of dead reckoning. They used observations of the sun and the North Star, or polestar, to determine directions. They estimated distances from the time it took to cover them. Advances in seamanship the art of handling a ship kept pace with advances in navigation. The Egyptians used rowers, and the Phoenicians and Greeks increased the number of tiers of rowers. The Greeks added a second mast, in the bow, and the Romans a third mast, in the stern.
	The First Navigational Aids
	One great aid to navigation was the development of the magnetic compass. Although men had known of the magnetic properties of the lodestone for centuries before the Christian Era, the first use of the magnetic compass by navigators appears to have been in the 12th century. In the next century the Italians learned to make a chart called a portolano. It showed an outline of the coast and had crosslines to aid in finding directions. Navigators at this time also used the cross-staff and the astrolabe, two devices that the Greeks had invented to measure the altitudes of celestial bodies. From these measurements it was possible to determine the approximate latitude of the vessel as well as approximate local time. The simplest version of the cross-staff was a stick, or staff, about one yard (0.9 meter) long with a shorter sliding stick set at right angles to the staff. The navigator pointed the staff at a spot about halfway between the horizon and the sun or a star. The crosspiece was then moved until the sights at its ends were in line with both the observed body and the horizon. A scale along the staff showed the altitude, or angle above the horizon, of the body. The astrolabe was a disk of brass or bronze, from 4 to 20 inches (10 to 50 centimeters) in diameter. A pointer, called an alidade, was pivoted at the center of the disk. One person held the astrolabe by a small ring at the top while another person knelt facing the rim of the instrument. The person kneeling pointed the alidade at the sun or a star and read the angle from the markings on the disk. Such great explorers as Christopher Columbus and Ferdinand Magellan made their voyages with these aids to navigation. The instruments, however, were not satisfactory, and for some two centuries after Columbus, no clock could keep time well enough to aid in fixing longitude.
	From the 17th Century to the Present
	In the 17th century, Britain, France, and other maritime countries actively began to aid the development of navigation. Astronomical observatories were established to provide almanacs. Mapmaking and the invention of required navigational instruments were also encouraged. In 1731 John Hadley, an Englishman, and Thomas Godfrey, an American, simultaneously invented a quadrant that made it possible to obtain accurate observations of celestial bodies. The instrument was similar to the sextant in common use today. The problem of fixing longitude was solved when John Harrison in England produced several chronometers between 1730 and 1763. Pierre LeRoy in France built an improved chronometer in 1766. Captain James Cook's voyages of discovery in the Pacific Ocean at this time proved the accuracy and reliability of navigational instruments and techniques. Early in the 19th century, Nathaniel Bowditch of Salem, Mass., devised many improved methods of navigation. In 1837 Capt. Thomas Sumner devised a trigonometric method of obtaining from celestial observations the lines known as Sumner lines of position. In 1875 Frenchman Marcq St. Hilaire improved upon Sumner's trigonometric calculations. These calculations were later used to supplement dead reckoning, a more precise method of correcting for drift using triangular calculations of velocity. Dead reckoning allows a navigator to plot where a craft will be at any time, making it possible to plan a journey in its entirety before the start of the journey. Matthew Fontaine Maury made famous studies of wind and weather and helped in the development of government aids to navigation. Today, electronic devices such as radar and loran are widely used in navigation. Most vessels use an automatic pilot. Since 1978 a satellite system managed by the United States Air Force, called Global Positioning System, has been providing continuous worldwide coverage adequate for determining latitude and longitude to within about 30 feet (10 meters) and, in many places, altitude, with the same accuracy. The digital computer, another tool, works so fast that it can provide continuous information. It also has a memory to store information for use when needed. The human navigator is becoming more and more a manager of computer systems; however, there is no substitute for human judgment to deal with the occasional unexpected situation.
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