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    Re: Refraction
    From: George Huxtable
    Date: 2008 Jul 4, 20:39 +0100

    I asked Dave Walden what he had taken as his standard for refraction, when 
    comparing various formulae, and he has replied-
    
    | I used the star link  R F R O subroutine by Wallace (head of the UK
    | Natu Alm Office.
    | ( It reproduces the critical table in the Naut Alm exactly.)
    |
    |
    |
    | *  P.T.Wallace   Starlink   3 June 1997
    | *     The routine computes the refraction for zenith distances up
    | *     to and a little beyond 90 deg using the method of Hohenkerk
    | *     and Sinclair (NAO Technical Notes 59 and 63, subsequently
    | adopted
    | *     in the Explanatory Supplement, 1992 edition - see section
    | 3.281).
    
    ================
    
    Fine. The refraction table in the almanac makes a good choice for 
    comparison. But, of course,  there's no "absolute truth" in it. It's chosen 
    to be a good fit to observations, in most circumstances.
    
    Indeed, the Hohenkerk / Sinclair predictions are based on a rather simple 
    model of the atmosphere, in which the air temperature falls by 6.5�C from 
    sea level to 11km up (that is, by 71.5�C), and then stays constant at that 
    value right up to the limit of the atmosphere, taken to be 80 km. For many 
    purposes, meteorologists now prefer a more complex model.
    
    I've implemented that routine on a pocket-calculator program, and also in an 
    Excel spreadsheet, but in a form in which the atmosphere can, if required, 
    be split into further bands, each with a particular lapse rate.
    
    George.
    
    contact George Huxtable at george@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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