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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...@huxtable.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...@huxtable.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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