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    Star-sight discrepancy. was: [NAV-L] Multiple LOP using one sun sight
    From: George Huxtable
    Date: 2005 Aug 24, 10:13 +0100

    Tom Shanklin wrote-
    >   Shot star sights last night from the beach- fix was 5 n.m. off my actual
    >position.  First time in awhile I've been more than 1 n.m. off.  At first
    >was attributing it to the fog bank sitting far offshore which blurred up an
    >already rapidly darkening horizon.  Then I got home and realized my digital
    >wristwatch (that I had synchronized right before my sights) was inexplicably
    >6 seconds fast.  Of course I found this out only after I put the headphones
    >on my shortwave, turned it on and nearly blew out both eardrums (thanks to
    >my nephew)  Multiple agencies working against me.
    Not a good explanation of a difference of 5 n.m., I'm afraid.
    The Earth turns 360 degrees in a day, just about, which is 15 degrees in an
    hour, or 15 arc-minutes in a minute of time, or 1 arc-minute in 4 seconds
    of time.
    The fastest a star can rise or fall, when it's due East or West viewed from
    somewhere on the equator, is at that same rate of 1 arc-minute in 4 seconds
    of time, which corresponds to 1 sea-mile error if the time error is 4
    seconds. So to arrive at a position error from a star of 5 n.m. would
    require a time error of at least 20 seconds; much more that the 6 seconds
    watch error that Tom refers to.
    For a beginner, a star sight to 5 miles, even from a beach, is nothing to
    be ashamed of. Presumably, it was taken at twilight, but how clear was the
    view of the horizon?
    If Tom would like to post details of his observation, sextant reading,
    watch time, corrections made, it's likely that someone from this list will
    pick it up and see if there's an error in the figuring. It's the sort of
    discrepancy that might arise from applying index error or dip with the
    wrong sense.
    Contact George at george@huxtable.u-net.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.

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