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    Re: Refraction
    From: Frank Reed CT
    Date: 2005 Aug 25, 14:21 EDT

    George H you wrote:
    "She tells me that the  refraction data were tinkered-with to conform with
    modern refraction values  used by astronomers, which are based on an
    integration made by Sinclair, of  JPL data provided by Standish."
    I suspect that this is exactly the same  integration procedure that we've
    been working with lately. Standish says he made  it widely known in the early 70s
    (did you get a chance to read the article? it's  here in gif format:
    http://www.HistoricalAtlas.com/lunars/ref.html ). There is  no sense in which these
    modern refraction integrations are superior to earlier  ones. They are somewhat
    more flexible and they converge faster, but they can  cover the same
    atmospheric complexity as earlier integrations. The difference  between the present
    Nautical Almanac refraction table and the earlier one  apparently comes down to a
    rather slight difference in temperature structure in  the atmosphere. The
    pre-2004 refraction table can be derived (using the Standish  integration) from
    an atmosphere model with a temperature lapse rate of 7.25  degrees C per
    kilometer up to 11km and constant temperature above that. The  post-2004 refraction
    table can be derived from an atmosphere model with a lapse  rate of 9.0 deg
    C/km up to 2km and 6.5 deg C/km up to 13km and constant temp  above that level.
    These are both reasonable temperature curves which occur  somewhere on the
    Earth every day. They are both "right" in that sense.
    It seems from the integrations that any plausible temperature curve will
    yield essentially the same refraction table for altitudes above 3.0 degrees with
    very little sensitivity to atmospheric structure. The refraction values
    within a  degree or two of the horizon, on the other hand, are very sensitive to
    the lapse  rate structure in the lowest part of the troposphere.
    And  also:
    "Although Frank's comment, that the changes are hardly relevant  to
    navigators, is broadly correct, it seems to me that the NAO are  being
    responsible in keeping their published refractions in line with  modern
    information, and in accord with values currently in use by  astronomers,
    rather than sticking to "traditional" values."
    Bear in  mind that the "traditional" refraction table is every bit as good as
    the "new"  table. The difference for celestial navigators from a practical
    standpoint is  completely insignificant, and in addition both are valid
    refraction tables under  the appropriate weather conditions. Even from a theoretical
    standpoint,  astronomers and marine navigators probably *should* use different
    refraction  tables. Astronomers tend to work on desert mountaintops while
    marine  navigators... er... don't! If you had to pick just one lapse rate
    structure for  the atmosphere in order to compile a refraction table for angular
    altitudes  below three degrees, you probably wouldn't pick the same structure for
    the open  ocean that you would choose for desert mountains.
    Finally, I think it's  worth pointing out that a difference in refraction
    values of less than 1% is  smaller than the refraction difference between the red
    and blue end of the  spectrum.
    42.0N 87.7W, or 41.4N  72.1W.

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