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    Re: Accuracy of sextant observations at sea
    From: Gary LaPook
    Date: 2010 Nov 27, 12:58 -0800
    "Practice makes perfect." Actually is should be "perfect practice makes perfect" since if you do the practice the wrong way you will not improve. So be careful when taking your practice sights from a known GPS position.

     I have some suggestions.

    Get yourself one of those hand held, pocket sized, dictaphones or audio recorders (some cell phones can be used this way.) When you start a series of sights on the same body simply start the recorder running, place it in you shirt pocket, and then simply speak the times and sextant altitudes. Doing it this way allows you to take many more sights in a given time period than having to set the sextant down to use a pen and paper to record the observations. It's actually quite easy to get three or four observation per minute this way.

    Use a digital watch, it is all too easy to get the minute wrong when reading from an analog watch. Read the second first into the recorder then the minute and second again. If you read the minute first it is easy to forget what the second was by the time you get around to recording it. Wear the watch on your left wrist with the face of the watch on the palm side of your wrist so when you have brought the body to the horizon you can immediately shift your gaze to the watch face and the delay will be less than a second so you should not have any timing error.


    You can get a small light that is designed to illuminate a bicycle
    speedometer, costs about ten bucks. Simply attach it to your wristwatch
    with a rubber band. It uses an LED, provides a green light that is
    plenty to light up your watch dial.
    
    http://www.icyclesusa.com/catalog/sigma-sport-bike-computer-nite-light.htm
    
    see my previous post at:
    
    http://www.fer3.com/arc/m2.aspx?i=109869&y=200909
    
    
    this eliminates any time delay needed to turn on a flash light or to push a light
     button on the watch. If there is going to be a time delay
    then start counting seconds until you can read your watch.
    
    Get a small portable short wave radio, about fifty bucks, so you can receive the time
     signals from WWV or WWVH on 5, 10 and 15 megahertz to ensure your watch is set correctly.
    Another way to use the radio is to let it run while you take your series of sights, follow
    the body and then stop when the radio gives the beep marking the exact minute which will also
    be recorded in the dictaphone and then simply read the sextant altitude into the recorder. WWV
    omits the tick for the 29th second so you can repeat this process at 30 seconds after the minute
    just follow the body, wait for the omitted tick, and then read the sextant on the next tick
    at the 30 second mark. This allows you to take two accurately timed sights per minute.
    
    Use the navy website to check you sight reduction:
    
    
    http://www.usno.navy.mil/USNO/astronomical-applications/data-services/cel-nav-data
    
    See prior discussion:
    

    http://www.fer3.com/arc/m2.aspx?r=1&y=200710&i=103486&hi=737869.33475.qm%40web80401.mail.mud.yahoo.com&s=Re%3A+[NavList+3486]+Re%3A+Single-handed+sights

    gl


    On 11/27/2010 2:35 AM, Anabasis75---.com wrote:
    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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