A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2016 May 19, 15:25 -0700
Paul Jackson, you wrote:
"would the near shore I used as the horizon be the reason I got a 0.0 intercept with this shot of a low altitude Sun."
Here's how I would put it: you got an intercept of 0.0 because everything worked out nicely, partly because you did it right (I assume) and partly because of inevitable good luck in accidental cancellations. Your observation and the corrections happened to net out to exactly zero. This doesn't mean that the sight was perfect, nor does it mean that the corrections were perfect. But it's fun to see a "0.0" like that. At minimum, you can give yourself a gold star!
When you get an apparently perfect result like this, it can be useful to do a little sanity check by varying some of the parameters a bit. Since you're using a dip short correction, you might check to see how much things would change if you used a height of eye that's one foot different from your original choice. Or you might try changing the distance to the shoreline you're using as a surrogate horizon by a quarter of a mile. Those differences are probably both within measuring error. Also you might want to look at the temperature/pressure correction table since the Sun was at a relatively low level in the sky. And of course you should remember that it all comes down to the accuracy of your index correction. Do you really believe that you know your index correction exactly to the nearest tenth of a minute of arc? If not, any error in the I.C. feeds directly into an equal error in the intercepts you get. It's not unreasonable to believe that your I.C. uncertainty is at least smaller than half a minute of arc. Normally, for celestial altitudes, I would count anything with an intercept less than half a mile as "negligible error" or "close enough to call it perfect".
Conanicut Island USA