# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Horizontal Angle and the Hand Bearing Compass
From: Greg Rudzinski
Date: 2011 Jun 4, 11:38 -0700

Micheal Dorl wrote:

" it would seem that the other angles would enter
into the solution. Surely there are other positions from which the
angle between A & B are the same but with different bearings."

Every point on the arc of position will have the same difference between the bearings to mark A and B. When determining the radius of the circle half the distance between A and B is used so that two right triangles are formed with the hypotenuse of each being the radius of the circle. The half distance represents the opposite side so simple trig will give you the Radius(hypotenuse) = 1/2 distance(opposite) A-B divided by SIN of observed angle A-B. The radius is then used to plot the circle center on the chart arcing an intersection off A and B. From the circle center the full circle can be made which will have the observer, mark A and B on the perimeter. Getting a fix requires plotting a second circle between mark C and A or B. If variation and deviation are known then a single bearing to A or B can be plotted to intersect the circle to produce a fix. The intersection of two horizontal angle circles is the better fix though. See David Fleming's previous post for additional explanations and a description of the standard method for plotting the horizontal angle circle.

Greg Rudzinski

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