A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2017 Sep 24, 09:31 -0700
John Howard, you wrote:
"Just as an exercise, I used a globe and string to get an AP at 60° N and 115° W."
But that's way off! May I suggest that you try this again? With a common globe, you should expect to get a position this way within one or two degrees. Even if you mark up a common ball (like a 10-inch diameter ball that you might find in the toy department of Walmart) with a bit of care, you can get a position this way within 3-5°.
And by the way, for others following along who may not realize this, this is not technically an "AP". It's a fix. The globe is serving as an analog computer, and the result, when done right, is your position. Given that the accuracy is rather low, you could then feed that into a standard celestial calculation and use this as a starting point or "assumed position".
To reiterate, in the real world, you're never this lost. These are game problems, not navigation. Also in the real world, a sight consists of the altitude as well as an approximate azimuth, which really ought to be given in the statement of the problem. You know what direction you were facing within 10 or 20 degrees, and this significantly constrains the calculation, whether you're working by direct computation or by analog on a globe. If you know that Spica, e.g. was in the SSE at some altitude, h, then you measure off a distance of 90°-h away from Spica's GHA and dec (its "GP") and draw an arc of a circle. That much is obvious. But you don't need to draw the whole circle; you just draw that portion that "sees" Spica in the south-southeast, or equivalently, that portion of the circle which is perpendicular to SSE.