NAVIGATION. The science or art of conducting a ship across the sea. The term is also popularly used in connection with boats on rivers and lakes and with flying-machines, although in this latter case the science is often and correctly termed air navigation. Navigation is technically limited to the art of con ducting a ship from one port to another across, and on the surface of, the open sea, and it is in this technical sense that the subject is dealt with below.
Before the introduction of the mariners' compass in the 14th century, the only practical means, among western nations, of navigating ships was to keep within sight of land; or to steer for short distances out of sight of land by reference to the sun or stars, more particularly the pole star. In eastern waters, long voyages out of sight of land were possible owing to the steadi ness in direction of the monsoons, which enabled vessels in these localities to keep their course by running directly before the wind. The act of fixing positions by astronomical methods progressed far more rapidly on shore than at sea. The few rude appliances avail able to the navigator were limited, until the 18th century, to find ing only the latitude of the ship at sea. No means of finding the longitude was devised, except the very rough method of estimating the run of the ship—known as dead reckoning—until the introduc tion, for practical purposes, of the lunar problem early in the 18th century. This complicated calculation made the finding of longi tude one of considerable difficulty, even after the introduction of the sextant ; because the moon passes the stars lying in her course through the heavens at a mean rate of only 33" in one minute of time, and her motion in regard to the sun being less than this, it is obvious that a very small error in measuring the distance pro duces a correspondingly large error in the resulting longitude. The rise of what may be termed modern methods of astronomical navi gation may be fairly dated from the invention of the chronometer in 1735, although it was not until some 4o years later that exten sive trials of Harrison's chronometer proved to navigators the value of this instrument. The method of finding the apparent, or sun, time at the ship's position was well-known; but until the chronometer came into use afloat, and enabled the navigator to carry Greenwich mean time with him on board, it was not possible to find the longitude by simple calculation. (See CHRONOMETER.)
Rapid strides were made in the 19th century in the improve ment of instruments and the publication of text-books useful to the navigator ; also in the production of tables of logarithms com piled to facilitate the solution of trigonometrical problems asso ciated with navigation. The Nautical Almanac, first published under authority for the use of seamen in 1767, came into general use. Machines for taking soundings were introduced and various methods were invented whereby the errors of the magnetic com pass, due to the permanent and induced magnetism in the iron of the ship, could be very largely eliminated. Methods were applied whereby that important factor in obtaining an accurate position, the speed of the ship through the water, could be serviceably ascertained. The introduction of steam vessels went far to simplify the problems of navigation as the dead reckoning could be calculated with greater accuracy than was the case with sailing vessels. The progress of the art of navigation was, and is, of course, still inseparably connected with chart drawing. The steady and continuous improvement in the number and accuracy of charts is due to the marine hydrographic and coast survey service conducted by the principal maritime nations.
With the above brief summary of the progress of navigation, the methods in use at the present time will be described, referring the reader for a more detailed and technical description to the text-books available on the subject. When mak ing a passage from port to port situated at no great distance apart, the navigator ordinarily selects a course on a Rhumb line, which, on a Mercator's chart, the chart normally used, is a straight line. Between ports situated at great distances from one another, and differing widely in longitude, the course followed should be on the Great Circle, which, on a Mercator's chart is a curve, the shortest distance between any two places on the surface of the earth.