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    Re: Hardly ever get an LOP than 20 NM - why?
    From: Frank Reed
    Date: 2020 Jan 18, 19:08 -0800

    Laurence, I think it's clear now what the issue is. You took your artificial horizon Sun sight by superimposing one image of the Sun directly on top of the other. That's fine, but then you need to do the altitude correction differently.

    A majority of people shooting the Sun in an artificial horizon place the two images just in contact one above the other, limb to limb. It's not wrong to shoot it superimposed as you have done, though it's arguably a little bit less accurate (not enough to worry about right now). But what you have after correcting for index error and then dividing by two is the altitude of the Sun's center. To get the proper altitude correction, you can either take the UL and LL values from the table and average them, or you treat the Sun as an object with no diameter and use the star table for the altitude correction. Those two methods will give the same result usually (around 1.7' for this specific sight) though you may notice a tenth of a minute of arc difference sometimes.

    For the sight you posted, you can accomplish the same thing by subtracting the Sun's SD, which is about 16.3' this time of year, from the Ho. And the net effect of that is to reduce your intercept to about 7.3 n.m. Plot that out, and you should find that it passes about 2.3 nautical miles NNW from your known position. So that's good. :)

    You mentioned your whole horizon mirror a couple of times. This doesn't affect the analysis of the sights.

    You wondered about your height above sea level affecting your sights. Air pressure drops about 1 inch of Hg for every 1000 feet of altitude above sea level. If you're at 2000 feet, that's two inches out of a normal mean of 29.8 inches or about 7%. So the refraction in altitude is reduced by about that amount. The refraction at sea level would be 1.7'. Reduce that by 7% and you get 1.6'. Really nothing to worry about.

    Finally, your steps using the 229 tables all appear to be accurate. No problems there. 

    Frank Reed
    Clockwork Mapping / ReedNavigation.com 
    Conanicut Island USA

    PS: Try out my app next time you shoot the Sun. It will immediately tell you your error on sights as you shoot. You get immediate feedback which will allow you to focus on the sights and sextant use and save the paperwork for another day.

       
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