# NavList:

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

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Re: Advice concerning sextants
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 andcompute what we would have measured if we had been at 25 degrees North” was to assume wewere at 25 degrees north and then rearrange the standard noon formula to compute the altitudewe would have observed had we actually been at 25 north. This is how it worked.Date: October 29, 2009Time: 13:18:46Declination 13̊ 36.4' southHeight of eye: 33 feetIndex error: +1.6'Lower limb observationHs   51̊ 26.4'I.C.         -1.6'Dip         -5.6Ref.        -0.8'S.D.     +16.1'Ho   51̊ 34.5'( For instructional purposes, I like to do the semi-diameter and refraction corrections separatelyso the student can see where the numbers are coming from. The total of these two correctionswas + 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’tuse the actual S.D for the sun on the day of the observation but an average S.D. for a six monthperiod.)Normal noon sight computation90̊ = 89̊ 60.0'Ho   - 51̊ 34.5'ZD     38̊ 25.5'Dec  -13̊ 26.4'Lat     24̊ 49.1' northIntroducing the Marc St. Hilaire method by calculating computed altitude using the rearrangednoon formula.               A lat             25̊ 00.0' northDec                    + 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 towardNoon 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           0̊A lat          25̊ northDec            13̊ contrary nameTab 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 towardZn              180.0̊Using H.O. 249:A lat             25̊ northDec               13̊ contrary nameTab Hc          52̊ 00'd                                 -60d corr                -36'Hc                51̊ 24'Ho                51̊ 35'Int                       11 nm towardZn                180̊Using a calculator with the normal Sine - cosine formulaHc= arc sin( (sin lat x sin dec) + (cos lat x cos dec x cos LHA))--- On Wed, 1/5/11, Gary LaPook wrote:From: Gary LaPook Subject: [NavList] Re: Advice concerning sextantsTo: NavList@fer3.comDate: Wednesday, January 5, 2011, 10:14 PMHere is a link to the article, read the third from last paragraph.http://www.oceannavigator.com/content/ad-hoc-celestial-teacher-royal-clipper-0gl----------------------------------------------------------------NavList message boards and member settings: www.fer3.com/NavListMembers may optionally receive posts by email.To cancel email delivery, send a message to NoMail[at]fer3.com----------------------------------------------------------------

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