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    Re: formula for refraction
    From: Bill B
    Date: 2007 Mar 22, 20:08 -0400

    Frank
    > Yes, it turns out that the VLOOKUP function in Excel has a slightly
    > different definition from the one in the spreadsheet I was using. Just
    > change the third argument in each VLOOKUP from "1" to "2". Then it
    > works.
    
    You're good. You not only answered my questioned but anticipated my next
    question.
    
    At this point I have the following:
    
    =VLOOKUP(INT(B2),G10:H100,2)*(1-(B2-INT(B2)))+VLOOKUP(INT(B2)+2,G10:H100,2)*
    (B2-INT(B2))
    
    It seems to be working a lot better, but Excel (or me) still seems to have
    problems.
    
    B2     C2
    
    40.0  1.132 (matches column H)
    40.1  1.124
    40.2  1.117
    40.3  1.109
    40.5  1.094
    40.6  1.086
    40.7  1.078
    40.8  1.071
    40.9  1.063
    40.99 1.056
    41.0  1.093 (matches column H)
    
    Mean of 40.0 and 41.0
    1.113
    
    There seems to something wrong.
    Note: "INT" in my Excel rounds down the nearest integer.
    
    As a follow up, what is your opinion of the Saemundssen quoted in Meeus for
    Hc to H (below) as opposed to 57"/tanH
    
    R = 1.02 / tan ( H + 10.3/ (H + 5.11)) where H is the CALCULATED altitude.
    
    Thanks again
    
    Bill
    
    
    
    
    
    
    
    
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