More sextant piloting technique

[HenryC]

While we're on the subject, Here's another tip on piloting with a sextant from Eric Hiscock's "Cruising under Sail".

We saw how to use a sextant as a range finder on a vertical object in the posts below, so it stands to reason it can also be used in the same fashion on horizontal ones.  An example would be finding the angular size of a distant island, or angular horizontal distance between any two objects of known separation on the horizon (for example, two very similar tangent bearings) to get a distance. 

For the algebraically challenged, Hiscock suggests this totally graphical variation of the same technique:  Shoot a compass bearing to one end of a distant island or other geographical or human-built feature marked on the chart.  Using your sextant, determine the angle between the island's two tangent bearings, or the ends of the feature.  A plotted pair of two very similar compass bearings will not give a good fix, compasses simply cannot be read that precisely.  However, if you plot that angle you measured with your sextant OFF  the compass bearing with a protractor,  You will see the two lines (the bearing and the angle) will cross at a very shallow angle on the chart.

Using your parallel rulers, slide the angle line along the bearing line, preserving the angle between the two, so that the angle line points to one tangent, and the bearing LOP points to the other,  Your position will be the intersection of the bearing LOP and the plotted angle line. Again, it is not as accurate as two compass LOPs crossing at a large angle, but it will give you a very good estimated position.  This can be very helpful whenever widely separated charted landmarks are scarce.

The procedure takes advantage of the fact that a sextant can measure a small angle much more accurately than a pelorus or alidade, or any other compass bearing shooter.  A variation of this technique is to use a station keeper, AKA as three-arm protractor (an inexpensive but surprisingly good plastic one is made by Davis and can be purchased at most chandleries), to simultaneously plot two sextant angles on a chart for a good position without using a compass at all!  The Davis station keeper actually has a vernier, so angles can be plotted quite precisely. This method is rarely seen in practice, since under those circumstances the compass/sextant combo described above works just as well, but it never hurts to know an alternative method.  Someday it may come in handy as a work-around.