It is always advantageous to get an observation of a star near twilight or dawn, in order that a well-defined horizon may be available; but, in taking a sight of Polaris, another important feature is to be considered. When the star’s hour angle is at or near 6 or 18 hours, that is, near that part of its orbit cut by a line passing through the pole and parallel to the horizon, it is rising or falling most rapidly, with the result that a small error in time will produce a considerable error in the hour angle, an error of 3 minutes introducing a difference of 1´ in the latitude.
It is quite worth while, therefore, to select a time for the observation of Polaris when this star is near either of its culminations, its highest or lowest positions, where the time need not be especially accurate; but by carefully noting the time it is possible to get good results at other times when the horizon is defined. By using the position of the star Mizar, as suggested above, however, the navigator will be greatly aided in selecting the most propitious time for observing Polaris.
CHAPTER VII
Azimuths and Amplitudes
Of all the navigational instruments now in practical use, there are few, if any, that exceed the mariner’s compass in usefulness to mankind. The part it has played in the development of the world has been most important, and its utility is no less to-day than in the past, for the intercourse of nations is still guided by the compass needle. With such a responsibility depending on this instrument, it would naturally be supposed that its indications must be very accurate, but, on the contrary, the needle is swayed by the slightest magnetic influence and points North only on rare occasions; and in steel vessels only by mere chance.
The needle is drawn from true north first by the direction of the earth’s magnetic force which is not coincident with the meridian owing to the position of the magnetic poles. The north magnetic pole being in the extreme northern part of Canada, all the lines of force in the northern hemisphere converge toward this locality. The needle when otherwise undisturbed lies in the direction of these lines of force and takes an angle with the meridian depending on the locality.
The amount of divergence from the true north, or variation, as it is called, differs in different localities but is readily obtained by a glance at the chart where each compass rose shows the amount of variation at that place. From a magnetic course, or bearing, the true course, or bearing, is readily found by the proper application of this variation, which may be either easterly or westerly. The true course is to the right of the magnetic course, when considered from the center of the compass, in easterly variation; T. R. E.—True-Right-Easterly. Remember these three words and the whole lesson is learned, for if true is to the right in easterly variation it must be to the left in westerly; and if true is to the right in easterly, the magnetic course must be to the left of true course in easterly and to the right in westerly. In this way the true and magnetic courses are converted one to the other at will.
If we were to sail always in an entirely wooden ship, our compass troubles would be very few, for the above would include every phase of the situation. As wood is non-magnetic the compass would be uninfluenced by outside disturbances. Wood, as a ship-building material, having been so much displaced by iron and steel, the use of these metals has brought many problems to solve in connection with the deflection of the compass needle.
The effect on the compass needle of the magnetism in a vessel and her cargo is known as deviation and is very complicated, owing to many influences which are at work at all times giving an ever-varying value to this element of error.
The causes of deviation and its treatment in the way of compensating the compass are subjects much too extensive for this little book; furthermore, they are carefully dealt with in a half dozen of the well-known works on navigation, so we will touch only on the every-day side of compass work.