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    Sextant accuracy with short distance to horizon
    From: Dan Allen
    Date: 2001 Jun 21, 7:51 AM

    I was out sailing yesterday through the San Juan islands and in the Strait
    of Juan de Fuca.  It was a gorgeous day aboard a friend's Nauticat 44 and we
    took out an Astra IIIb sextant to work on sites from a rolling ship--the
    swells at the east end of the Strait kept us rolling +/- 20 degrees either
    way for about an hour.
    Anyway, we wanted to measure our use of sextant accuracy so we did this: we
    would record our exact GPS position, exact time, and the angle of our sun
    shot and then calculate our position via an HP-48 program of mine to see how
    close we came to the truth, the truth being approximated to within 12 feet
    by the GPS position using a Garmin GPS III.
    My friend hadn't used his sextant much before so his first shots were off by
    300 miles, but quickly they got to within 5 miles of our actual position, a
    great improvement.  I took one shot myself and got to within 4 miles.
    Later we did the noon shot and his results were about 68 miles off for a
    bunch of shots.  These were taken in Guemes channel with a local horizon
    less than a mile away.
    This led me to think more about possible inaccuracies of using a horizon
    that is very close rather than naturally far away.  That is, typically you
    use a horizon that is at the limit of your vision, perhaps 20 miles away.
    Our local horizon was within a mile.  I don't remember reading much about
    any errors with regard to this.  Do any on this list have anything to add
    about possible errors of using a very near horizon?  I plan to go back and
    reread the usual suspects on the topic, but I thought it may be of interest
    to others.
    Simply put, is there a formula or correction to give greater accuracy to
    sextant shots taken using a very close horizon?  If one knows the exact
    distance to the local horizon, can this be applied to the shot to give a
    more accurate reading of the angle of a celestial body?
    Or, is there a rule of thumb that says if the local horizon is closer than X
    miles, one should use an artificial horizon instead due to big inaccuracies
    that cannot be corrected for?
    Thanks in advance,
    Daniel K. Allen
    Navigate | Calculate | Sail

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