Concepts of Maritime Navigation
The word “navigate” comes from the Latin navis, meaning “ship,” and agere, meaning “to move or direct.” The Latin word navis comes from the ancient Greek nafs, meaning “ship”.
There are four basic methods of navigation at sea:
- Dead Reckoning,
- Electronic Navigation, and
- Celestial Navigation
In piloting, the navigator directs a vessel from one place to another by observing such landmarks on the Earth’s surface as lighthouses, beacons, buoys, and prominent rocks and cliffs, and by measurements, called soundings, of water depths.
In dead reckoning, the navigator determines a ship’s position by keeping a careful account, or reckoning, of the distance and direction of travel from a known position called the point of departure.
In electronic navigation, the navigator determines a ship’s position with the aid of such devices as radar. These instruments variously make use of the directional properties of radio waves, of differences in the times of arrival of radio signals sent simultaneously from different locations, or occasionally of the difference in speed between radio waves and sound waves.
In celestial navigation, the navigator finds a ship’s position by observing the sun, moon, planets, and stars.
See also: ‘Examination Questions for Merchant Mariners’
Course, Heading and Track
The terms course, heading, and track are often loosely used. They should, however, be considered to have the meanings that follow. The course is the intended direction of the ship’s travel. The heading is the direction in which the ship is pointed at any given time. The track, or course made good, is the direction of a straight line between a point of departure and a present position.
The factors that together results in failure to make good an intended course are termed drift. The flow of ocean water, however, is only one of the factors involved.
Direction and Distance
On the Earth’s surface each meridian, or line of longitude, is half of a great circle which passes through the geographical poles of the Earth and lies in a true north-and-south direction (Latitude and Longitude). The center of a great circle on the Earth’s surface lies at the center of the Earth. The shortest distance between two points on the Earth’s surface is the shorter arc of a great circle passing through the points.
The track of a ship that sails along a great circle will cross each meridian at a different angle unless the ship is sailing directly along a meridian or the equator. The ship’s direction, then, would usually have to be altered constantly in order to maintain a perfect great-circle course.
In practice, however, a ship’s course is changed at regular intervals of perhaps several hours so that the ship follows a series of rhumb lines which approximates a great circle. A rhumb line is a line on the Earth’s surface that crosses all meridians at the same angle. A ship sailing a steady, true course is usually following a rhumb line, and the distance it covers is greater than that of a great-circle course.
The Instruments of Navigation
One of the basic tools of the marine navigator is the nautical chart. This is a representation, drawn to scale, of the water and land areas of a particular region of the Earth’s surface.
On the chart the navigator keeps a graphic record of the ship’s progress. Such a record is kept regardless of the method or combination of methods of navigation that is being used. Lines drawn between successive positions marked on the chart indicate at a glance the courses that the ship has followed. From scales on the chart the navigator can measure directly, without computation, the distance that the ship has traveled.
Traditionally, Mercator charts have been used at sea. Lambert charts may also be used for long sea voyages, though they were designed for air navigation.
A navigator needs other basic instruments to determine a course and plot it on a chart. A compass indicates direction. Dividers are useful in measuring distances on charts. Parallel rulers usually two straightedges connected by pivoted arms are used to transfer lines of direction from one portion of a chart to another. A transparent plotter is a combination protractor and straightedge employed in the measurement of angles and distances and in drawing course lines on a chart.
In directing a course by dead reckoning, a device which measures distance traveled is also essential to the navigator. These distance-measuring devices include taffrail logs, or patent logs, and engine-revolution counters.
In piloting, the navigator guides a ship largely by the bearings of landmarks. A bearing is the horizontal angle between an object and a reference point for example, true north is the reference point for true bearings. Bearings are usually measured clockwise from 000° at the reference point through 360° and expressed in three digits, as 028°.
Bearings are used to determine, or fix, a ship’s position. Drawn on a chart, a bearing forms a line of position a line on which some point must represent the ship’s location. Therefore, when two or more bearings intersect (cross-bearings), the intersection must represent the ship’s position.
Bearings of visible objects may be measured with such instruments as the alidade, pelorus, or azimuth circle. These devices usually have sighting vanes and reference circles graduated in degrees. Bearings referred to a magnetic compass must be corrected for compass errors deviation and magnetic variation.
See also: ‘How to Apply the Compass Error’
When only one landmark is visible, a ship’s position can be fixed by determining the bearing of the object then measuring the distance to it with a range finder or a stadimeter. The range finder is an optical instrument for measuring the distance to any clearly defined object. If the height of the object is known, the stadimeter may be used. It operates on the principle that the closer an object is, the bigger it appears to be.
In shallow water, soundings help fix a ship’s position. Sonic, or echo, depth finders make use of the known speed of sound in water. Sound transmitted from the ship is reflected from the ocean floor to a receiver, which measures elapsed time and calculates distance. Some devices produce fathograms continuous profiles, or graphs, of the ocean bottom. An older depth-finding device is the hand lead and line a marked cord with a weight on the end.
See also: ‘Standard Marine Navigational Vocabulary’
Navigation Light and Buoys
Light stations and lightships are maintained along coastlines to warn approaching ships of potential dangers such as off-lying rocks. Most lights operate in on-and-off cycles. The length of time required for a light to complete a full cycle of changes is called the period of the light. Lights that are “off” longer than they are “on” are called flashing lights. Occulting lights are “on” as long as, or longer than, they are “off.”
Floating navigational aids (other than lightships and weather ships) which are anchored or moored are called buoys. United States waters are marked for safe navigation by the lateral system of buoyage. Simple arrangements of colors, shapes, numbers, and lights are employed to indicate the side of a buoy on which a ship should pass when moving in a given direction.
See also: ‘Buoyage’
In dead reckoning, the navigator estimates a ship’s position by keeping a careful record of its movement. The initial point of departure for dead reckoning is usually the last fix the navigator obtains from objects on land at the start of a voyage. From this point, true courses steered and distances traveled (as recorded by log) are plotted on a chart.
Points along the dead-reckoning line, representing successive positions of the ship, are labeled with the appropriate time and the notation “D.R.” Dead reckoning commonly begins anew each time bearings, celestial observations, or electronic aids provide an accurate fix. The dead-reckoning line on his chart is important to the navigator because it indicates at a glance the theoretical position of the ship, the track the ship should have followed, and the direction in which the ship is traveling.
Modern electronic devices are important aids in finding position at sea. For example, the navigator whose ship is equipped with a radio direction finder can determine the bearings of radio transmitting stations on shore. Special radio beacons for navigation are established at lighthouses, lightships, and prominent points along coasts. Radio bearings may be plotted on a chart to obtain a fix.
A variety of other electronic aids to navigation are in use or under development. Loran (long-range navigation) and shoran (short-range navigation) are among the most widely known. Radar is also of value, especially for a ship near the shore. Consol, by contrast, is designed for operation over relatively long ranges. A ship at sea can obtain a fix on its position from Consol shore stations with the use of an ordinary radio receiver.
‘Why Do You Need Celestial In These Days Of GPS?’
For centuries sailors have guided their ships across the oceans by celestial navigation, or nautical astronomy. This is the art of finding position by observing the sun, moon, stars, and planets.
As they journey for some distance, travelers observe that the celestial bodies appear to change their paths across the sky and to rise and set at new points along the horizon. Since the apparent positions of celestial bodies thus change with time and with changes in an observer’s position on the nearly spherical Earth, the location of a ship or other craft may be determined by careful observations of celestial bodies.
The Celestial Sphere
Celestial bodies, such as the stars, are so far from the Earth that they appear to be located on the inside surface of an imaginary hollow sphere. This sphere, which has an infinite radius, is called the celestial sphere. Its center coincides with the center of the Earth. All points on the Earth’s surface are considered to be projected onto the celestial sphere, as are the equator, the parallels of latitude, and the meridians.
For the purpose of navigation, a system of coordinates is required on the celestial sphere in order that the position of a celestial body at any time may be accurately described. One such system is the celestial equator, or equinoctial, system.
In this system the celestial equator, or equinoctial, is the base, or primary, circle. It corresponds to the Earth’s equator. At right angles to the celestial equator are the hour circles. An hour circle is a great circle on the celestial sphere that passes through the poles and through a celestial body or point. Each meridian of the celestial sphere is identical with an hour circle.
The declination (dec.) of any point on the celestial sphere is its angular distance north or south from the celestial equator, measured along the hour circle that passes through the point. Declination on the celestial sphere corresponds to latitude on the Earth’s surface.
The Greenwich hour angle (GHA) of any point or body is the angle, measured at the pole of the celestial sphere, between the celestial meridian of Greenwich and the hour circle of the point. The angle is measured along the celestial equator westward from the Greenwich celestial meridian, from 000° through 360° . The GHA differs from longitude on the Earth’s surface in that longitude is measured east or west, from 000° through 180°, and remains constant. The GHA of a body, however, increases through each day as the Earth rotates.
The Theory of Celestial Navigation
At any instant of time every celestial body is directly above or in the zenith of some point on the Earth’s surface. This point lies on a line connecting the body and the center of the Earth. It is called the geographical position, or GP, of the body. Sometimes the GP of the sun is called the subsolar point; that of the moon, the sublunar point; and that of a star, its substellar point.
A line from the center of the Earth through the GP of an observer would extend to a point on the celestial sphere. This point is called the zenith of the observer; the line is his local vertical.
The altitude of a celestial body is the angle, measured by an observer on Earth, between the body and the horizon. Were a celestial body say, a star directly above, or in the zenith of, an observer, its altitude would be 90° . The observer would be at the GP of the star.
Were the observer a distance away from the GP of a star, however, the altitude of the star would be less than 90° by an amount proportional to the distance. On the celestial sphere, the observer’s zenith would be apart from the star by a distance called the zenith distance, or ZD.
All points a given ZD from a star would form around the star a circle of radius equal to the ZD. Were lines from all points on the circle extended to the center of the Earth, a similar circle would be formed on the Earth’s surface. From any point on this circle, the observed altitude of the star would be the same; hence, it is called a circle of equal altitude. Its center is the GP of the star. A second circle of equal altitude would exist around the GP of a second star. Ordinarily, the circles would intersect in two widely separated points. One of these points, of course, would be the position of the observer on the surface of the Earth.
Celestial Navigation in the Sea
To put this theory into practice, a navigator measures with a sextant the altitudes of two or more celestial bodies. He carefully notes to the second the time at which he made his observations. He obtains the time from radio signals or from accurate clocks called chronometers. These are kept set to Greenwich mean time, or GMT, for this is the time the navigator must know as he turns next to the Nautical Almanac.
The Nautical Almanac is a book of astronomical tables from which may be found, for every second of every day, the positions on the celestial sphere of the sun, the stars, the moon, and the planets used in navigation. The positions are given in declination and GHA. From them, of course, the latitude and longitude of the bodies’ GP’s may be found.
Knowing the altitudes of the bodies he observed and their GP’s at the time, the navigator has the information necessary to construct the circles of equal altitude which define his position. Actually, the navigator does not plot on his chart the full circles. From dead reckoning or other means, he knows his approximate latitude and longitude. All he needs, then, are segments of the circles so short that, without practical loss of accuracy, they may be drawn as straight lines. Like the lines obtained from bearings in piloting, they are called lines of position.
The Continental Shelf
Around each continent is an area, of varying distance from shore that lies in water of relatively shallow depth. It is called the continental shelf. In some of these areas, submerged river channels can be traced well out to sea. Mariners and ocean navigators use those submerged channels that have been charted as aids in navigation.
The ocean currents of the world have also been charted. The ocean floor is being charted in greater detail and at greater depths than ever before. This is being done partly to meet the requirements of antisubmarine defenses. Electronic depth-finding instruments have produced much data for charts of the ocean floor. In recent years major advances have been made in underwater archaeology, in oceanography, and in meteorology.