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DEFINITIONS AND PRINCIPLES
DIVISIONS OF THE SCIENCE
1. Navigation, as a general term, may be defined as the art of conducting a ship from one port to another, or that science which relates to the determination of the position of a ship at sea. It embraces two distinct methods, or branches, which, in practice, are carried on together, one serving as a check on the other. The first of these branches is known as navigation by dead reckoning, or simply navigation, and the second as nautical astronomy.
2. Navigation by dead reckoning consists mainly in recording the courses and distances sailed, or run, by the ship, and from the data thus obtained, determining the position of the ship with reference to some known point on land, or to some former position; it also embraces methods of computing courses and distances to be run in order to reach the place of destination. Hence, its execution requires a knowledge of the terrestrial globe and the imaginary lines and circles drawn upon its surface.
The principal instruments used in navigation by dead reckoning are the chart, representing the part of the sea in which the ship is navigating; the lead, to ascertain the depth and character of bottom; the mariner's compass, and the log, the former to determine the direction, or course, in which For notice of copyright, see page immediately following the title page
the ship is proceeding, the latter to measure the distance run in that direction.
3. Nautical astronomy consists in the measuring of altitudes and zenith distances of celestial bodies, and from the data thus obtained, in combination with the recorded time of observation, calculating the position of the ship by the application of astronomical laws and principles. Consequently, this branch of navigation requires a knowledge of the different systems of circles that are imagined to be drawn on the celestial sphere, as well as a familiarity with certain celestial phenomena and fundamental principles of astronomy.
The principal instruments used in nautical astronomy are the sextant, the azimuth compass, and the chronometer. The. Nautical Almanac and suitable nautical tables facilitate the solution of the various problems.
4. All problems appertaining to navigation are solved by the application of plane trigonometry, principally involving the solution of right-angled plane triangles, while those of nautical astronomy are solved by means of spherical trigonometry.
In this Course, we shall deal exclusively with such methods of navigation by dead reckoning that will be of practical utility in the navigation of vessels on the Great Lakes as well as Coast navigation in general.
THE TERRESTRIAL SPHERE
5. Form and Magnitude. - In geography, as well as in navigation, the earth is regarded as a sphere. From actual measurement at various parts of its surface, it has, however, been found to differ slightly from this, it being somewhat flattened at the poles. This departure from the true spherical form is so trifling and inconsiderable that no practical error can result from treating the earth as a perfect sphere when laying down positions and framing directions for sailing over its surface.
The dimensions of the earth, according to Col. A. R. Clarke, are as follows: Equatorial radius = 3,963.307 miles, or 20,926,202 feet.
Polar radius = 3,949.871 miles, or 20,854,895 feet. From this it is evident that the flattening at the poles does not amount to much, the difference between the two radii being about 13.5 miles. By using this value of the radius, the equatorial circumference of the earth may be approximately estimated at 25,000 miles, and the surface of the whole earth, land and water, at 49,243,000 square miles.
6. The rotundity of the earth is manifested in many ways; of these the most obvious is the curvature of the surface of the sea, which manifests itself in a striking manner. Assume a person to be on board a vessel at sea or at the summit of a hill along the coast. Then, when a steamer appears on the horizon, the first indication of its presence is the smoke from its funnel. After a while the
observer sees the upper parts of its spars and rigging, but the lower part of the masts, the smokestack, and the hull are invisible. As the steamer approaches, these lower parts gradually come into view, and later on the entire steamer can be seen, as illustrated in Fig. 1. In the same manner the successive appearance of the different parts of the coast are . manifested to the mariner, who from the steamer observes the land. The same appearances are observed everywhere, on the Indian Ocean and on the Atlantic Ocean as well as on the Great Lakes; hence, the rotundity is uniform and the earth globular. Many other proofs could be submitted, but the one already mentioned will perhaps suffice; they all tend to prove the same fact - the sphericity of the earth.
DEFINITIONS RELATING TO THE EARTH 7. The axis of the earth is that diameter around lich the earth daily revolves with a uniform motion from west to east, the revolution being completed in 24 hours.
8. The poles of the earth are the extremities of its axis or the points in which the axis meets the surface. Thus, if the
line P P', Fig. 2, represents the axis of the earth, the points P and P are the poles; the pole to which we in this country are nearest is the north pole, while the opposite is the south pole.
9. The equator is a great circle on the earth's surface equidis
tant from the poles. P'
It divides the earth
into two equal parts, or hemispheres - the northern hemisphere and the southern hemisphere. The poles of the earth are the poles of the equator, every point on it being 90° from either pole.
10. The meridians are great circles that pass through the poles of the earth and are therefore perpendicular to the equator. Thus, if Ebd E', Fig. 2, represents the earth's equator, the great circles Pa P', Pb P', etc., passing through and intersecting at the poles P and P', are meridians.
Of all the innumerable meridians that may be imagined as being drawn on the surface of our globe, one is always, for the purpose of reference, selected as the prime, or first, meridian. Most the maritime nations having a national observatory usually adopted the meridian that passed through that observatory as the first meridian, but at the conference held at Washington in 1884, for the