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    Re: Refraction
    From: Frank Reed CT
    Date: 2005 Aug 27, 00:51 EDT

    Marcel, you wrote:
    "Does the Nautical Almanac  have a different table than the previously
    mentioned table 6 from Pub. No.  249? If so, which title, page number etc.?
    Or, is there an other possibility  to obtain it, instead of ordering it from
    "my" library abroad?"
    It's  distinct from the table in pub. 249. The latter tables are intended for
    aviators  so they include calculations for higher observer altitudes and they
    are rather  low accuracy. The refraction tables in the Nautical Almanac are
    precise to the  nearest tenth of a minute of arc and extend right down to the
    horizon in small  steps. They are limited to observations made close to sea
    level. Do you want  copies?
    You asked:
    "Wouldn't it be possible to find the authors of the  table and ask them which
    model they used und why? Or, are there some  clarifying comments in the
    There is no clarification in the  almanac. The refraction tables published
    there were fixed and unchanging for  fifty years. They were modified very
    slighly just a couple of years ago. Myself,  I'm not too worried about how they
    calculated either the original table or the  new version since we can calculate
    almost exact matches using simple atmospheric  models. And that's good enough,
    as far as I'm concerned. Additionally, the  difference between the new table
    and the old table is considerably smaller than  the expected daily variations
    from layering in the lower  atmosphere.
    42.0N 87.7W, or 41.4N  72.1W.

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