The whys and wherefores of navigation

by Gershom Bradford

The deviation changes with every alteration of the vessel’s head, owing to the change in position of prominent parts of the vessel’s hull relative to each other, to the compass, and to the terrestrial lines of force (magnetism).

As a result of these influences on the compass needle, the mariner has three courses to deal with. The first is the true course, which is based on a compass whose needle points true north. The second, the magnetic course, is taken from a compass affected by variation alone and therefore pointing to the magnetic pole. The third is the compass course, or that course actually shown by an ordinary standard compass in a steel ship, affected by the error of variation combined with the error of deviation.

The combination of the deviation and the variation is the compass error and is obtained by adding the deviation and variation if both are of the same name, the compass error taking that name; for instance suppose we have a variation of 2° W. and deviation of 10° W., the combined error is 12° W. If, however, the variation and deviation are of different names, it becomes necessary to find the difference between the two and name the result after the greater quantity; thus, with a deviation of 4° E. and a variation of 10° W., the error is 6° W.

The compass error is applied to compass course to obtain true course and vice versa by the same rule as for variation.

The navigator in planning his course between two positions lays the parallel rulers on these positions on the chart and carries this direction to the nearest compass rose. This may be a true rose, in which event he remembers his T. R. E. rule, reversing it in this case, and with the variation given on the chart secures the magnetic course. In an iron or steel vessel, the deviation for that course must be ascertained from the deviation card by trial or from a Napier Diagram direct and applied to the magnetic course in order to obtain the compass course. This is accomplished precisely as in finding the magnetic from the true course (to the left if deviation is easterly and to the right if westerly). The course by standard compass is now at hand by which we can steam from one selected point to the other.

The deviation as has been said is an ever-varying error, and consequently it is quite impracticable to depend wholly on a fixed deviation card. We may take aboard some magnetic cargo or change our latitude to a great extent, the vessel may be pounded excessively by heavy seas, a stroke of lightning or by stranding; all these are causes liable to affect the deviation more or less.

In order to forestall the serious consequences that are liable to attend such a derangement of the normal and expected deviation, the careful navigator takes azimuths or amplitudes on every course when practicable. Azimuths and amplitudes are nothing more nor less than astronomical bearings of heavenly bodies; they indicate the true bearing of the body, and the difference between this bearing and the bearing taken simultaneously by standard compass is the compass error.

The azimuth of a body is the angle at the zenith between the meridian and the vertical circle passing through the body. It is customary, however, to consider the azimuth as measured by the arc of the horizon rather than by the angle at the zenith. It is measured from the north or south point according to the latitude, toward either the east or west point, through 180°.

An amplitude, unlike an azimuth, is restricted as to time of observation, for the body must be on the horizon either rising or setting; and should be observed when the sun is about its own diameter above the horizon and with a not excessive height of eye. The amplitude is measured from the east or west point through 90° to the north or south point. If the body observed has a south declination and is rising, the amplitude will be East so much South; if declination is north, East so much North, for a body rises in the East point when its declination is 0°—on the equator.

The principle of the amplitude lies in the solution of a right-angled spherical triangle, whose sides are the body’s polar distance, the co-latitude, and the zenith distance which is 90°. We desire the complement of the angle at the zenith. It is unnecessary to compute an amplitude, for in Table 39, Bowditch, will be found the desired bearings for different latitudes and declinations. The sun will be found the most satisfactory of the heavenly bodies to utilize for amplitudes.