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    Re: Celestial Navigation without a sextant.
    From: Greg Rudzinski
    Date: 2008 Mar 11, 15:38 -0700

    George,
    
         I base my 6 minutes of arc figure on the refration variables as
    seen in the A4 table of the Nautical Almanac. The temperature and
    pressure extremes are -7.3' to +6.9' from the mean. In reality on the
    ocean and from the beach I get better than + or - 4' from a GPS
    position. Extreme refraction can occure after the passage of a cold
    front. In this case opposing LOP's or a back sight will be needed to
    cancel out the refraction variable.
    
    
    On Mar 10, 12:32�pm, "George Huxtable" 
    wrote:
    > Greg Rudzinski wrote-
    >
    > | � � �In a lifeboat with a chronometer,tables, and compass (no
    > | sextant), a navigator would be able to mark the GMT time of the sun's
    > | limb on the horizon. The elevation is zero. Correct for refraction,
    > | semi-diameter, and height of eye (dip). Perform normal sight reduction
    > | and expect 6 minutes of arc accuracy under normal weather conditions.
    >
    > What does Greg base his "6 minutes of arc" figure on, when it must include
    > the uncertainties in refraction for the Sun, as seen on the horizon? How
    > well does he know what that refraction will be, and how much it might vary
    > from the "book" value of 34 arc-minutes?
    >
    > I quote from Meeus, "Astronomical Algorithms", chapter 16 on Atmospheric
    > refraction (page 107 in my 1998 ed.).
    >
    > " ... According to Schaefer and Liller, the refraction at the horizon
    > fluctuates by 0.3 degrees around a mean value normally, and in some cases
    > apparently much more" The reference is to "Refraction near the horizon",
    > Publ. astron. society of the Pacific, vol 102, pages 796 - 805 (July 1990).
    > It would be nice to see more detail, but I don't have access to that
    > original paper. Brad Schaefer is a recognised authority on optics in the
    > atmosphere, and more recently has produced an authoritative series of papers
    > titled "Sunset Science".
    >
    > However, there are very divergent views on this matter. My 1977 edition of
    > Bowditch states, in vol 1 page 422 (para. 1606)- "Generally, the error in
    > tabulated refraction should not exceed two or three minutes, even at the
    > horizon". I regard that statement as absurd, but you can take your pick
    > between those views.
    >
    > George.
    >
    > contact George Huxtable at geo...---.u-net.com
    > or at +44 1865 820222 (from UK, 01865 820222)
    > or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    >
    > George.
    >
    > contact George Huxtable at geo...---.u-net.com
    > or at +44 1865 820222 (from UK, 01865 820222)
    > or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
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