# NavList:

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

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Re: Death to the Intercept Method!
From: Sean C
Date: 2016 Nov 5, 03:16 -0700

Frank,

You had mentioned this method before without going into too much detail. That piqued my curiosity, but I had forgotten to ask about the specifics. Before you elaborated, I decided to try and work it out for myself. It looks as though I pretty much got it right, although I used the equation: cos-1((sin(h) - sin(Dec)sin(Lat))/(cos(Dec)cos(Lat))) = LHA.

This is an interesting alternative to the intercept method. Although, personally, I'm not sure if I find it any more or less convenient. After all, it's not exactly difficult to construct a scaled plotting sheet from scratch, and it is certainly quicker than hand ruling a sheet of graph paper if you don't have any handy. Also, the need to calculate a longitude for each of two assumed latitudes doesn't seem to offer much of an advantage over the altitude and azimuth calculations of the intercept method. Then (and I realize I'm reaching here), there's the possibility that, for whatever reason, the use of a calculator is suddenly inaccessible. I could get by in a pinch with nothing but the N.A., a sextant and a watch (and, of course, some plotting tools). On the other hand, if I planned to use this method I guess I might think to bring along some time sight tables, too. :) The only benefit obvious to me is that, when using Sumner lines and graph paper, one doesn't need a protractor/pre-printed compass rose or a ruler/scale of any sort. I suppose I'll have to try some more examples to see how I really feel about it. Thanks for pointing out this additional option.

One question, though: Since computational power is cheap these days, why not go 'all out' and use the formula for calculating a fix found right in the N.A.? With a calculator like the TI-30X IIS, it's easy to do, and with a programmable calculator it would be even easier. If it weren't for logging the altitudes/times of observations and the ship's position, one could almost do away with using any paper altogether.

Regards,

Sean C.

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