The whys and wherefores of navigation

by Gershom Bradford

The work of computing a time sight is more or less laborious to everyone and with some seafarers forms their most arduous mental exercise. At any rate no one wants to work any more than is necessary to insure accurate results. So when establishing a position line it will often be found convenient to use the short cut known as the tangent method.

With the latitude by account work the observed altitude as in the ordinary time sight, instead of assuming two latitudes. Seek the true azimuth in the tables or on diagram, using the latitude and declination employed in the time sight and the local apparent time gained from it, as the arguments. The true azimuth, it will be remembered, always bears at right angles to the position line. Hence if the azimuth is laid down through the position furnished by the time sight, the position line may also be readily plotted at right angles to the line of azimuth at the time sight position.

The navigator now-a-days is expected to think in position lines when he is clear of the land, as a pilot thinks in shore bearings and marks. That is, he must see these imaginary lines of the different visible bodies, and keep track of their availibility for his particular use. It is easy to get into the habit of this, for they are simply astronomical bearings instead of bearings on distant terrestrial objects, with the distinction that the celestial bearing allows of a 90° correction to produce a position line.

The morning sun on the prime vertical with a sufficient altitude to avoid any dangerous refraction, will produce a north and south line of position. During the forenoon as the sun passes toward the meridian, the northern end of the position line will move in direct proportion with the body’s change in azimuth to the eastward and the southern end to the westward, until at noon with the sun on the meridian we have an east and west position line.

It will be seen that at one moment of the day it is a very easy matter to establish a line of position; the mere working of a meridian altitude does this. This simple expedient of finding a position line was utilized a great deal as a means of making a landfall in the days before chronometers were perfected. In those good old days, before the clipper ship era, time was not held at such a premium as in the present hustling period, and a few days more or less at sea mattered but little. The shrewd shipmasters then would keep well offshore until in the latitude of Boston or the Virginia capes, as the case might be, when they would haul due west and let her go, making, no doubt, first rate landfalls, if the old pig yoke was in good working order.

The value of a position line was demonstrated to the writer some years ago when bound in from the eastward and running into a heavy and very extensive fog bank somewhere southeastward of Halifax. During a break in the prevailing conditions the navigator succeeded in securing an ex-meridian sight and fortunately got a fairly good idea of the latitude. The vessel was under sail and making but slow progress, and as a result of the protracted period of overcast sky the longitude became considerably a matter of guess work. The vessel, however, was kept on a west course with a careful allowance made for the set. “Sir William Thompson” was kept going at regular intervals and it was surprising to see the soundings check up with the chart as the vessel approached, crossed, and left astern the Roseway Bank, southward of Cape Sable. One felt as sure of the position as did the old Nantucket sailor in crossing “Marm Hackett’s garden.”

In cases where the soundings do not check so precisely as in this instance, it will sometimes be found a great help to lay off to scale the depths obtained on the edge of a piece of draftman’s transparent linen. Place it on the chart in the line of the course, and, should the soundings fail to agree, move the scale forward and back or to either side, always preserving the direction of the course, until a position is found where the soundings on the scale agree with the depths given by the chart.

Progress has been made in the science of navigation as in all other sciences, and the modern shipmaster is not obliged to hold aloof from Nantucket Shoals and Georges Bank under ordinary conditions as our ancestors were compelled to do, for with a correct chronometer and a knowledge of the position line such outlying dangers have been robbed of many of their anxiety-producing elements. Before showing the method of working around such places another point of value of the position line is called to the reader’s attention.

A line of position extended until it reaches the land or some danger will indicate to the mariner the bearing of that particular point of the coast or danger. If it so happens that this point is not the place of destination, the navigator, not being able to lay a course direct for his objective port through inability to determine the vessel’s distance offshore, overcomes the difficulty by sailing a sufficient distance at right angles, then hauling on to a new position line parallel to the original one. This is similar to what our ancestors did in the simple way cited above. If the line lies in the direction of an off-lying or isolated shoal that is dangerously near the course, an offset like that shown above will allow a course parallel to the position line to be sailed in safety. Here is an example to show its useful application:

A steamer sailing from St. John, N. B., for New York proceeded but about 10 hours on her voyage, when she ran into a terrific gale. The master was soon forced to heave his vessel to and ride it out as best he could. The driving snow and mountainous seas occupied the attention of the officers in their efforts to save the steamer and in this way the dead reckoning position became a matter of mere guesswork. The wind after some 20 hours in the northeast quadrant hauled to the northward, at length blowing out in the northwest with clearing weather.