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    Re: Accuracy of sextant observations at sea
    From: Henry Halboth
    Date: 2010 Nov 26, 19:46 -0800

     

    Alan,

     

    Sorry to be so long I adding my 2-cents worth to the many responses to your posts on the subject, as I am sure you have received enough expert opinion by now. Neither will I endeavor to check your math as you can do that for yourself by use of the various calculators provided on the USNO website.

     

    It is, however most interesting to note that your commentary states position results “ well within 5 NM of GPS position”. The question is, of course, how “well” within 5 NM? Frankly speaking, I am inclined to think that your results are adequate, given the constraints under which you are working, especially as related to 2-body fixes.


    I am very familiar with Emerald Isle (NC) and have taken many sights from a position in Latitude 34-40-00.28 N and Longitude 77-00-24.10 W, quite close to your observation position, and would comment as follows:

     

    1. Usually, there are problems relating to the sea horizon offshore at EI, presenting to the observer what I would term a “Gulf Stream” horizon, i.e., generally somewhat hazy and ill-defined with low lying white clouds behind. Having taken a myriad of Noon solar transits, my accuracy of derived latitude over a period of some 6-years has varied from within 14.8 (arc) seconds to within 02 (arc) minutes. At the high end, such variation can raise hell with a 2-body fix, particularly at lesser angles of LOP intersection. As all observations at EI have to be taken in the same semicircle, perhaps even in the same quadrant, horizon errors are not readily apparent in plotting,  as well as they might be with a "full round" of sights.

     

    2. As respects your star sights. How early or late are you observing? Are you identifying your stars visually before observing? If so you are observing too early (AM) or too late (PM) to maximize your horizon and obtain the best results. The secret to good star sights, generally learned by experience at sea, as opposed to from books, is pre-computation of altitude and azimuth and observing as soon as your selected stars become telescopically visible, while you still have an optimal horizon - simply preset your sextant or octant to the calculated altitude and sweep the horizon in the area of the calculated azimuth; your selected star will pop up reasonably close to the horizon. For quick selection of optimally positioned navigational stars, pre-compute using HO 249 (downloadable for free from the internet). For shoreside use, you might well consider orienting a small compass rose to the meridian  assist in azimuth determination.

     

    3. You seem to indicate no better results by use of the artificial horizon, and this concerns me. Should this be true, you might be well advised to check your sextant or octant for undetermined errors, as a possible cause of poor fixes. Don't use only one method of determining IC - sometimes our eyes play tricks on us - particularly if one uses eyeglasses, has uncorrected astigmatism, cataract surgery, or simply age related deterioration.

     

    4. I would also suggest that, on your next session, you concentrate on noon sights for Latitude in a known location. This should give you a greater sense of accuracy as related to your observational skill, as well as a possible instrument error, devoid of possible tabular misuse or plotting error involvements.

     

    5. Lastly, do not be misled by the claims of accuracy appearing on this List. Certainly they are attainable with excellent horizons and otherwise good instruments in the hands of experienced observers. They are generally not attainable on questionable horizons, with less than perfect instruments, imprecise time, or inexperienced observers. Also conditions vary dependent upon location - the North Atlantic can be far different from the Topics, the South Atlantic, the Pacific Ocean, or the Indian Ocean, etc. You really don't seem to be doing as bad as you may think - practice makes perfect.

     

    Regards,

     

    Henry

     

    .

     

     



    --- On Fri, 11/26/10, Alan <alan202---.net> wrote:

    From: Alan <alan202---.net>
    Subject: [NavList] Re: Accuracy of sextant observations at sea
    To: NavList@fer3.com
    Date: Friday, November 26, 2010, 5:54 PM

    FrankReed---com
    Date: 25 Nov 2010 17:44
    Frank:

    Re your request for data, please note the following:

    1. Shooting location, within a few yards, all shots listed.
    GPS Coord. Lat 34D 39.3Min North/Long 77D 03.6Min West,
    (Emerald Isle, North Carolina)

    2. Altair
    Date 23 October 2010
    UT 22:58:34
    Ho 64D 12.4M
    3. Fomalhaut
    Date 23 October 2010
    UT 23:23:17
    Ho 16D 13.3M
    4. Jupiter
    Date 23 October 2010
    UT 22:54:37
    Ho 24D 29.0M

    I used USPS SR 96a sight reduction form, USPS CLS 98 for plotting, and Daily Pages, 2010 Nautical Almanac. The above sights appear to be the best 3 body fix I shot. Distance from DR position (GPS Coordinates) to Fix measures inside 2 NM, looks like about 1.8NM, measuring with dividers. I can send more data if you like or desire.

    Interesting that you recall my having mentioned a metal sextant, Astra 111B.

    Hope you and yours had a happy thanksgiving holiday.

    Later.

    Alan
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