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

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Re: Hardly ever get an LOP than 20 NM - why?
From: Fred Hebard
Date: 2020 Jan 17, 09:29 -0500
The list would be glad to check your calculations.  If they are by hand, take a picture of them with a phone and attach it to an email.  If they are computerized, paste them into the email.

The Sun is moving at 15 seconds of arc per second of time, so if you mistime your sight even a little it will through you off.

Judging tangency of the limbs of the sun is also critical.  You can get an estimate of how accurate your are by measuring the semi diameter of the sun.  Take measurements when the top limb is approaching the bottom and vice versa, always twisting the dial in the same direction.  The index error is half the arc distance between the measurements and the semidiameter is one fourth of their sum.  The Nautical Almanac tabulates the sun’s semidiameter on the daily pages.  You should be able to match that within 0.1 seconds of arc, and usually be spot on.  Average about 3 to 6 sets of measurements.

Finally, if you are judging coincidence of the direct and reflected images by superimposing them, then do not use the sun corrections from the Nautical Almanac. They incorporate the semidiameter of the sun and you will be off plus or minus about 16 minutes of arc.  That may be your source of error.

Welcome aboard!

Fred

Fred Hebard

On Jan 16, 2020, at 23:16, Laurence HOlden <NoReply_HOlden@fer3.com> wrote:

With somewhat over 200 sun sights with my AstraIIIB now, I rarely come up with an LOP under 20NM, and I can'tfigure out why. I've checked and rechecked the setting of my mirrors (I'm using the whole horizon mirror) and and they are correct. I am doing my sights in a pan of water. All I have left to wonder is if my calculations are somehow wrong. Would someone look over a recent sight reduction of mine and see if I am doing something wrong?

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