The Whys and Wherefores of Navigation/Chapter I
In embarking upon the study of navigation familiarity with the compass is the first logical step: the quick mental conversion of a course or direction given in points to the same direction in degrees expressed in quadrants as S. 35° E., and again into a system by which direction is indicated by degrees from 0° to 360°. A mariner will encounter all three of these systems and will find constant conversion necessary back and forth for various purposes. The 0° to 360° system is the most up-to-date and the simplest form of handling direction.
Following the compass may be taken up the use and description of other nautical instruments with which every mariner is supposed to be familiar.
Dead reckoning is the first calculation to appear and this involves the correction of the compass courses back and forth between true, magnetic and compass directions. This is dealt with under Azimuths and Amplitudes. In practical navigation a vessel commences her voyage and attempts to sail in a certain direction, but the well-known elements of compass error, variation and deviation, current, wind, seas and poor steering all divert the vessel from the projected course. In dead reckoning a navigator strives to keep track of his position by keeping a record of actual courses steered and distances run. He then is obliged to guess at the amount the vessel has diverted both in direction and distance until an astronomical observation sets him straight again. It is here particularly shown that navigation becomes an art of estimating position and the better the navigator’s bump of locality, the greater his success. This is a peculiar gift and usually is born in the man, at least it cannot be learned from books. The process of finding latitude and longitude by dead reckoning is supposed to be already well known to the reader and will not be detailed at length. However, every course angle is laid off from a meridian (which is true N. and S.) and terminates in a parallel of latitude. This meridian and parallel intersect at right angles; hence these with the distance run (which is the hypothenuse) form a plane right angle triangle, plane because the curvature of the earth is not considered in short distances. To solve this triangle, we have the course angle and one side—the distance run. With these the other two sides are easily found by computation, but more easily by tables No. i and 2, Bowditch. The side along the meridian is represented by the column headed Lat. (difference of latitude) and the side lying in the parallel is in the column headed Dep. (departure). Thus the values of the sides of the triangle are given in miles and tenths, showing the distance good made N. or S. and E. or W.
It will be noted that at the top of the pages of these tables are four different courses and at the bottom are likewise four courses making the same page serve for eight different courses. This is accomplished by the fact that triangles formed by these particular eight courses are the same in shape. Thus N. 30° E., for instance, makes a similar triangle to N. 30° E. (330°); S. 30° E. (150°); or S. 30° W. (210°). They have identically the same difference of latitude and departure. If this fact is not clear draw a diagram and be convinced. In the cases of N. 60° E. (60°); N. 60° W. (300°); S. 60° E. (120°); S. 60° W. (2400), the same shaped triangle as above is found, but reversed in that what was the difference of latitude side now has become the departure side. The values of these sides are read from the bottom of the page and are found in the reverse columns to fit the reverse triangle. The latitude value read from the top of the page as 30° becomes a departure value when read from the bottom with 6o°. The subject of Sailings is one of the early problems confronting the student of nagivation and will be considered briefly. The above remarks on dead reckoning cover the principle of plane sailing, the simple method where the spherical surface of the earth is ignored and a flat ocean substituted. This method will not serve for anything but short distances of a few hundred miles without sufficient error to render it impracticable. Traverse sailing is a series of plane sailing courses made, for instance, by a sailing vessel beating to windward.
In parallel sailing the vessel pursues a true E. or W. course and runs along a parallel of latitude. Thus all her progress is in the terms of departure with no difference of latitude. As all meridians converge from the equator towards the poles the length in miles of a degree of of longitude keeps on diminishing as the poles are approached and, conversely, miles of departure have an increasing value in degrees of longitude. So in parallel sailing what we desire to know’ is what is the value in the particular latitude of our course of our departure (miles) in ° ’ ” of longitude. Having this and applying it to the longitude left will give the longitude in. Middle latitude sailing is very similar to parallel sailing in that it is assumed, for the purpose of getting the difference of longitude, that the whole departure of the course or courses sailed has been made in the mean or middle latitude, because the greater value (in the northern hemisphere) in difference of longitude of a mile northward of the middle latitude is counteracted by the corresponding lesser value southward of the middle latitude.
Mercator sailing is perhaps the most extensively used, as the Mercator principle is employed almost universally in the construction of navigational charts. It is described under Charts in this book. Also under Charts is a description of the Gnomonic Chart which is also called the Great Circle Chart and used in Great Circle Sailing, also referred to in those pages.
The young navigator is counselled never to know where his vessel is, lest through over confidence he be led into close and dangerous quarters.