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    Re: formula for refraction
    From: Frank Reed CT
    Date: 2007 Mar 22, 18:41 -0700

    Bill,
    You changed one too many ones to twos when you edited the formula:
    toward the end, you've got "INT(B2)+2" where it should be "INT(B2)+1".
    That seems to be the only problem.
    
    You wrote:
    "Note: "INT" in my Excel rounds down the nearest integer."
    
    This you can count on. INT(x) goes back to the earliest programming
    languages. By contrast, VLOOKUP(x,a,i) is a relatively modern addition
    to spreadsheet functionality so you have to double-check the way it
    works.
    
    And you asked:
    "As a follow up, what is your opinion of the Saemundssen quoted in
    Meeus for Hc to H (below) as opposed to 57"/tanH"
    
    Well, I'm not quite sure what you're asking here. Those are not used
    for the same purpose. One thing to remember: equations like the one by
    Saemundssen that you mentioned are not "derived" from anything.
    They're just numerical "fits" to the tabulated data. Somebody just
    monkeyed with a simple equation and adjustable parameters until the
    equation's ouput looked like the table, more or less. If you have the
    table, you should *never* use those fitted equations. Likewise, if you
    have entered the table into a computer and implemented code or a
    spreadsheet function to interpolate, then you would always prefer that
    to an equation like Saemundssen. By contrast, the short equation for
    refraction above 15 degrees (r = 57"/tanH) is derived from fundamental
    physics given certain limiting conditions so we know *exactly* where
    it begins to be inaccurate (at some specified level of accuracy).
    
    -FER
    
    
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