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Doug Royer wrote:

> In coastal pilotting these are known as Horizontal distance bearings.Pick 2
> or more terrestial objects.Turn the sextant on it's side and take the angles
> between the 2 or 3 objects by superimposing one of the objects over one of
> the remaining objects.Dip is not a consideration in taking Horizontal
> angles. Write the angles down and go to the chart.With dividers find the
> distances between each of the objects.Your position will be tangental to the
> distance between the objects.Calculate the distance off,using dividers from
> each point draw in the radius and where the lines intersect is your
> position.


Not quite.

It is possible to find distance off by observing the vertical angle, if
the elevation of some charted object is known, but the method Doug is
trying to describe is more commonly known as "horizontal sextant
angles". It does not involve estimating distance, nor bearings.

Pick three charted objects. For convenience here, I'll name them a, b
and c, from left to right as you view them. Using your sextant on its
side, measure the angle between your lines of sight to a and to b, then
likewise the angle between b and c.

On the chart, draw a construction line from a to b. Subtract your a-to-b
observation from 90 degrees and draw another line through a that makes
an angle (90 minus a-to-b) with the line from a to b. Do the same thing
with a second line passing through b, thus forming an isosceles
triangle. Where the two lines cut at the apex of the triangle is the
centre of a circle that passes through a, b and your position. Draw that
circle on your chart.

Repeat that plotting for a second circle passing through b and c, using
your observation of the b-to-c angle.

Since your position is on both circles, it must be where they cross.

There are some extra wrinkles, dealing with cases where a, b,c and you
are all on (or close to) the same circle or where your observation is
greater than 90 degrees but the above is the essentials of the method.

As has been noted, it is much the most accurate of the non-electronic
position-fixing methods available to a navigator and was much used by
hydrographic surveyors for that reason. Indeed, I rather suspect that
the more upmarket sextant manufacturers are kept in the business by
surveyors' needs for precise instruments rather than by the few seagoing
officers who are willing to pay for something more accurate than an Astra.


Trevor Kenchington



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Trevor J. Kenchington PhD                         Gadus@iStar.ca
Gadus Associates,                                 Office(902) 889-9250
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