# NavList:

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

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Re: Group Project: USCG student sample problems
From: Antoine Couëtte
Date: 2013 Apr 19, 01:43 -0700

RE : http://fer3.com/arc/m2.aspx/Group-Project-USCG-student-sample-problems-PeterHakel-apr-2013-g23606

Hello Peter,

Thank you very much for your quite interesting remark : we look like having a difference of 1.9 NM in our Polaris intercepts.

On the other hand the Index Correction is 2.0 arc minute. Such Index Correction might very well be the main explanation for our differing results.

In " http://fer3.com/arc/m2.aspx/Group-Project-USCG-student-sample-problems-Cou%C3%ABtte-apr-2013-g23602 " I computed Polaris with the following data :

UT = 05h00m31.0s on June 02 nd, 2013 with TT-UT = +68.2 s, and
Position = N 24°00'0 W 143°40'0 , and
HOE 100 ft = 30.48 m , and T = 10°C = 50° F , and P = 1013.25 hPa = 29.92 " Hg , and
Hsc (Sextant Height corrected for only Instrument error) = 23°04'1

If I am not mistaken, Greg published Hs (Height as directly read from the Sextant) = 23°02'1 with I.C. (Index Correction) = +2.0' . From these data I have concluded that HSc = 23°04'1 and used this result in my computations. Is the value of HSc = 23°04'1 a correct one ?

I then got i = -36.4 NM, therefore Observer's Latitude (as derived from only the Polaris Height and assuming its Azimuth is exactly 360°) is N 23°23'6

If we perform this same Polaris computation with Hsc = 23°06'1, then i = -34.3 NM and Latitude = N 23°25'7, which is very close from your own Latitude determination (N 23° 25'5). The pending 0.2' difference should be attributed to slightly different Horizon Dip values as well as to possible round-off errors.

As a preliminary conclusion, if you did use Hsc = 23°06'1, it would essentially explain our results difference. I you did use Hsc = 23°04'1, then there remains an unexplained 2.0' or so difference between our results. If such were the case, I would suggest that we privately investigate this matter through direct e-mails.

It is very true that such comparisons can only be beneficial to all since they enable to find the "supposedly" :-) right answer.

Kermit
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