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    Re: Refraction
    From: George Huxtable
    Date: 2005 Aug 24, 09:55 +0100

    I've just been away for a week in Holland, to discover much productive
    activity on Nav-L in my absence. It's good to see that refraction questions
    have been thoroughly thrashed.
    A fall-out from this is a perceptive mailing from Frank Reed on 16th
    August, stating-
    >George H wrote:
    >"Bennet has provided a   formula which is an empirical attempt to fit that
    >averaged data. At large  angles of altitude, it becomes proportion to the
    >tan of the zenith angle, as  Snell's law requires. Near the horizon, where
    >refraction rises sharply, the  divergence from Snell's law shows up in
    >correction terms which turn out to be  remarkably simple."
    >Of course, Snell's Law always applies. It's just that  a simple "slab" model
    >of the atmosphere doesn't work near the  horizon.
    Very true.
    >"In some publications, such as the Nautical Almanac, it  appears that
    >formula itself is used as the basis for the refraction  tables"
    >I don't think there's any reason to suppose that Bennett's  formula is used
    >in the Nautical Almanac. The tables predate the formula.
    Also true.
    >"(though the constants have recently been tinkered-with a bit to  improve the
    >fit to
    >recent Pulkova data)"
    >I know that someone (possibly  you --?) previously speculated that the
    >changes in the almanac refraction tables  were based on "recent Pulkova
    >data" but
    >has that been confirmed?
    Yes, it was by me, and no, it hasn't been confirmed: It was indeed no more
    than speculation, and I should have qualified my comment to say so.
    Since then, I've been in touch with Catherine Hohenkerk, of HM Nautical
    Almanac Office. That office has for some years now been effectively a
    "one-man-band", run, most competently I must say, by Catherine, who I
    regard as Miss Nautical Almanac.
    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. Methods are
    described in a book by Andrew Murray, "Vectorial Astronomy", from the "70s
    or 80s".
    That's a book I haven't come across myself. Catherine promises to send me a
    NAO technical note on the topic, and if it adds any further useful
    information I will inform the list.
    She tells me that the Bennett constants, as given in the back pages of  the
    latest almanacs and in the NAO publication "NavPac and compact data
    2005-2009" (in the US, "AstroNavPC and compact data 2005-2009", published
    Willmann-Bell), have been slightly adjusted to accord with the changed
    refraction values, with agreement from George Bennett.
    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.
    >42.0N 87.7W, or 41.4N 72.1W.
    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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