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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Advice concerning sextants
From: Gary LaPook
Date: 2011 Jan 7, 12:56 -0800
From: Gary LaPook
Date: 2011 Jan 7, 12:56 -0800
| (Since I am never sure how the formatting is going to work out I am attaching this same post as a PDF file.) What I meant by we could “have worked the standard noon formula from the other end and compute what we would have measured if we had been at 25 degrees North” was to assume we were at 25 degrees north and then rearrange the standard noon formula to compute the altitude we would have observed had we actually been at 25 north. This is how it worked. Date: October 29, 2009 Time: 13:18:46 Declination 13̊ 36.4' south Height of eye: 33 feet Index error: +1.6' Lower limb observation Hs 51̊ 26.4' I.C. -1.6' Dip -5.6 Ref. -0.8' S.D. +16.1' Ho 51̊ 34.5' ( For instructional purposes, I like to do the semi-diameter and refraction corrections separately so the student can see where the numbers are coming from. The total of these two corrections was + 15.3'. If using the sun correction table the combined correction for the lower limb shot was + 15.5'. Doing the corrections separately is more accurate since the sun correction table doesn’t use the actual S.D for the sun on the day of the observation but an average S.D. for a six month period.) Normal noon sight computation 90̊ = 89̊ 60.0' Ho - 51̊ 34.5' ZD 38̊ 25.5' Dec -13̊ 26.4' Lat 24̊ 49.1' north Introducing the Marc St. Hilaire method by calculating computed altitude using the rearranged noon formula. A lat 25̊ 00.0' north Dec + 13̊ 36.4' Assumed ZD 38̊ 36.4' 89̊ 60.0' Assumed ZD -38̊ 36.4' Hc 51̊ 23.6' Ho 51̊ 34,5' Int 10.9 nm toward Noon Zn is 180̊ so 10.9' south of the assumed latitude places the 90̊-270̊ LOP at 24̊ 49.1' north latitude, the same result as the normal noon sight. Introducing the usual way of doing the same computation with H.O. 229 for the general case: LHA 0̊ A lat 25̊ north Dec 13̊ contrary name Tab Hc 52̊ 00.0' d -60.0' (per degree of declination change) d corr -36.4' Hc 51̊ 23.6' Ho 51̊ 34.5' Int 10.9 nm toward Zn 180.0̊ Using H.O. 249: A lat 25̊ north Dec 13̊ contrary name Tab Hc 52̊ 00' d -60 d corr -36' Hc 51̊ 24' Ho 51̊ 35' Int 11 nm toward Zn 180̊ Using a calculator with the normal Sine - cosine formula Hc= arc sin( (sin lat x sin dec) + (cos lat x cos dec x cos LHA)) --- On Wed, 1/5/11, Gary LaPook <glapook@pacbell.net> wrote:
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