NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Brad Morris
Date: 2014 Apr 2, 00:26 -0400
Hello Greg
That 20° wedge to bias the mirror needs to be a known angle, getting precisely 20° will be interesting. I'd suggest that the true angle be known to one or two tenths, as this contributes directly to your error.
As to the azimuth alignment, that's known (to me anyway) as cosine error, as the angle will fall off with the cosine of the angle in error. Suppose the wedge is a perfect 20° (good luck with that.). If the azimuth is 0° in error, then you have 20°x cos(0°). If the azimuth error is 1° then you have 20°x cos(1°). And so on. It will be up to decide how much error is tolerable. Here's my thought. If you are within 5° then the cosine error reduces the 20° wedge to a 19°55'.4 wedge. You divide your angle measured by two, so you will bias you measurement by 2'.3 (that's the error contribution due to azimuth).
There's one other thing to try. You mentioned that you live on LI. drive to the south shore (pick any beach) for a sea horizon!
Brad
Hi all, Just a thought - the time that I can do latitude from a noon sight is
rapidly ending, by next month my Hc will be approximately 65 degrees. I am
purchasing a precision level with 20" sensitivity / resolution, this should
allow for some very accurate star sights; I had a thought if I levelled a table
with the level and placed the 1st surface mirror on it, could I use a 20 degree
machined metal 'wedge' under the mirror to bias my sun Ho to be 20 degrees less
(and thus readable on the sextant) ? just a thought. Another question: how
accurate does the azimuth need to be for a meridian passage sight if I want to
try getting latitude that way?
many thanks in advance,
~Greg----------------------------------------------------------------
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