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    Re: Accuracy of sextant observations at sea
    From: Jeremy C
    Date: 2010 Nov 27, 05:35 EST
    I did the reduction as well and the biggest suspect line is Fomalhaut which was shot quite a bit later than the other two.  Civil twilight was 22:49 UTC that evening at Alan's location.  Jupiter and Altair were shot soon after so he probably had a good horizon at that point.  Fomalhaut was shot at 23:23 UTC, after Nautical twilight.  I am quite certain that the horizon was of poor quality at this point so my calculated intercept of -4.7 nm (with assumed 0.0 index error and 6 ft of HoE) is not unexpected. 
     
    If we don't use Fomalhaut, the cross of the two remaining lines has significant error in longitude.  My next question is how accurate was the timepiece or the recording of time?  Was it set to GPS time rather than UTC?  Did Alan subtract a second or so from his recorded time to compensate for the delay between the "mark time" and when he could look at his watch?
     
    Greg's points about index error are also good points that should be explored.
     
    Lastly, there might be errors in actually placing the body on the visible horizon.  This is an error that can only be corrected with practice.  The true art of celnav is being able to skillfully place the body on the horizon correctly and ensuring that the sextant is vertical.  This skill took me some time to master.
    Jeremy
     
    In a message dated 11/27/2010 4:54:55 A.M. Central Asia Standard Tim, alan202---.net writes:

    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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