# NavList:

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

**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.