A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Beginner with inaccurate results
From: George Huxtable
Date: 2005 Aug 30, 22:59 +0100
From: George Huxtable
Date: 2005 Aug 30, 22:59 +0100
Here are a few comments about Asbjorn's mailing and some replies he has received from this list. Fred Hebard wrote- >It's more accurate to determine the index error by first touching the >reflected lower limb to the upper limb, recording the result, then >reversing and recording that result. Index error is half the >difference. Usually three measurements on each limb is more accurate >than one on each; measure one then the other three times. As a check >on your interpretation of contact, you also can calculate the sun's >semidiameter from these measurements; it's one fourth of their sum. It >should be no more than 0.1 arcminutes of that recorded in the Nautical >Almanac. Just a word of clarification, as Fred's explanation might mislead an observer who is using reflection in an artificial horizon to observe altitude. The reflection Fred refers to as "reflected lower limb" is within the sextant, and determining the index error does not invole the art. horizon. However, the simple method Asbjorn has chosen to zero his index error, of superimposing the two Sun images, is a perfectly valid way to do that job, if a bit less precise than the way Fred has suggested, and would be expected to check the index error to within less than a minute of arc. It would never give rise to an index error of 13', which is what would be needed to explain the 6.5' discrepancy that has plagued Asbjorn. I agree completely with Fred where he advises against the practice of correcting the index error to zero by adjusting a sextand mirror. Don't do it. Leave that adjustment alone, measure the index error, whatever it happens to be, and correct for it by subtracting it out. Geoffrey Kolbe has put his finger on a discrepancy, which I estimate to be 1.5', from applying the d-correction the wrong way, but then implies that correcting that error "will get you close to the right number". And Lu Abel seems to agree, in writing- "Looks like this was a simple case of getting d correction wrong." But that's not the case. It still leaves a discrepancy, now reduced to 5', unresolved. I'm not familiar with the sextant that was used, but would be very surprised if any sextant, even a plastic one, showed an error in its scale calibration of as much as 5'. My suspicion falls on the glass artificial horizon, and the spirit level that was used to set it horizontal. What's important is the levelling with a spirit level placed along the same direction as the azimuth of the Sun; alignment in the direction at right-angles to that matters much less. A very sensitive level is needed, and needs to check both ways, turned through 180 degrees.. And a light one too, to avoid the weight of the level changing the tilt of the glass as it's placed on it, and removed. For the same reason, a very firm base is required, on a firm floor that doesn't flex at all as the observer shifts his feet. All these problems can be avoided if a liquid horizon is used instead. Just plain water, or oil, in a shallow bowl, is fine for Sun observations, but not for stars. Just avoid using a shiny flat container, like a tin lid, because reflections from the bottom can confuse the image. It has to be kept out of the wind, to stop ruffling of the surface, and that may be a problem in Trondheim; in which case, oil will do better than water. Other than that correction for declination, I can find nothing wrong with the rest of the calculation. I hope that with these hints from Nav-l, the discrepancies will resolve. George. At 11:24 30/08/2005, you wrote >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 =============================================================== Contact George at email@example.com ,or by phone +44 1865 820222, or from within UK 01865 820222. Or by post- George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.