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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Beginner with inaccurate results
From: Lu Abel
Date: 2005 Aug 30, 10:56 -0700
From: Lu Abel
Date: 2005 Aug 30, 10:56 -0700
Looks like this was a simple case of getting d correction wrong. For those on the list who are not aware of it, a great source for checking one's Hc and Zn calculations (assuming Internet access) is the US Naval Observatory's interactive Nautical Almanac. Enter UT and assumed L/Lo, and it will give the Zn and Hc for every above-the-horizon navigational body for that time and position! http://aa.usno.navy.mil/data/docs/celnavtable.html Lu Abel Geoffrey Kolbe wrote: > Hello Asbj?rn > > I checked your calculation for measured altitude with the data you gave > below. I get a sextant altitude, with refraction, of 29 degrees 20.2 > minutes, which is in close agreement with yourself. However, I get a GHA of > 23 degrees 53.2 minutes and a declination of 9 degrees 11.7 minutes. (This > is calculated using the Stormy Weather software, which has all the bells > and whistles and is very accurate.) Your GHA is close enough - perhaps > there is a typo in your declination....? > > Your procedure seems fine - but how are you levelling your coloured glass > artificial horizon? > > Try some star shots and see if there is refraction in the shade. (You will > not need the shade for star shots.) > > Geoffrey Kolbe. > > > > > >> Hi. >> >> I have just startet learning celestial navigation, and have bought a >> Davis 15 sextant. Even though it has been rather cloudy here in >> Trondheim after I got the sextant, I have managed to get some >> "shots". >> >> I am however not satisfied with my results, my observed height is up >> to 6.5' different from calculated height. This is strange as I can >> easily notice a 6.5' index error. And my observed height is always >> (with no exceptions yet) less than calculated height. One would think >> that if I am just inaccurate, my results should be on both sides of >> the calculated values. So I am starting to thing I do something >> wrong. >> >> Here is what I do, hopefully a kind person reads through it and finds >> an error: >> >> I use a Davis artificial horizon with colored glass. >> >> 1. First I adjust mirrors. Horizon mirror is adjusted so that when >> setting sextant to 0 degrees and looking at the sun, both reflected >> and real sun overlaps completely. >> >> 2. Then I remove some filters from horizon mirror (could there be some >> refraction here?) and measure height. Lower limb on sun is taken >> down to touch upper limb of sun's reflection in the artificial >> horizon. >> >> 3. I note height, GMT and then check index error with same procedure >> as in 1. >> >> 4. Then I calculate, this is an example I took yesterday: >> >> date: 29/8-2005 >> GMT: 13-52-23 >> body: sun, lower limb >> GPS long: E10� 24.7 >> GPS lat: N63� 25.4 >> >> Sextant heigth: 57� 58.4' >> index error: 0' >> halve height: 28� 59.2' >> correction: 14.4' >> observed height: 29� 13.6' >> >> GHA hour: 14� 47.3' >> GHA inc: 13� 5.8' >> GHA 27� 53.1' >> LHA: 38� 17.8' >> >> Decl. hour: N9� 12.4' >> d-correction: 0.8' >> Decl: N9� 13.2' >> >> Calculated height: 29� 20.0' >> Height difference: 6.4' >> >> >> >> Asbj�rn > > >
