# 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: Michael Dorl
Date: 2011 Jun 04, 06:56 -0500

```On 6/3/2011 3:54 PM, Greg Rudzinski wrote:
>
> Try taking a horizontal angle using a hand bearing compass the next
> time out sailing. This only requires taking the difference between two
> bearings and plotting the arc of position. This adds a third line of
> position to the pair of magnetic bearings and will act as a deviation
> check. My favorite way to plot a horizontal angle is to take half the
> distance between mark A and mark B then divide by the SIN of the angle
> between mark A and mark B. The result is the radius of the circle
> which passes through the observer and marks A & B. Plot the circle by
> first marking off the circle center using the radius distance arced
> off marks A & B. From the circle center then trace the cirlce that
> will pass through mark A & B and the observer. I find this technique
> very quick using only a divider compass and pocket trig calculator.
>
> Greg Rudzinski
>

Is it really as simple as this?   I realize that since you know he
bearings of the lines from your position to marks A & B, the angle
between A & B and the bearing of A-B, you essentially know the three
angles in the triangle (position - A - B). Therefore the problem is
solvable but off hand 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.

```
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