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    Re: Almanac data
    From: Frank Reed
    Date: 2009 May 30, 16:08 -0700

    Peter H., you wrote:
    "Our recently expanded suite of Navigation Spreadsheets can now calculate 
    almanac data and process lunar distance sights.  A free sample (Moon GP, SD, 
    HP, and phase) can be downloaded "
    
    I tried out some dates and noticed that the Moon's position is right on the 
    money for 2009, but it's a few tenths of a minute of arc off c.1950 and a 
    couple of minutes of arc out if I go back two centuries. Those are fairly big 
    error for lunars. It appears that you're using a simple expression for 
    delta-T that's quadratic in time. That's fine for current dates within a 
    decade or two of the present (considering that a few seconds difference in 
    time is not important for lunars), but it's no good for historical lunars. 
    Folks who experiment with lunars usually expect the tools to work correctly 
    with historical observations, or at least the software should provide some 
    indication when the inputs are outside acceptable limits for the code. Maybe 
    you should set up a little "lookup table" for delta-T. That's what the code 
    on my web site does (lunar distances data as well as common "Nautical 
    Almanac" ephemerides here: www.HistoricalAtlas.com/lunars).
    
    By the way, I tried your advice from an earlier message and ran your 
    spreadsheet on the "pocket" version of Excel on my cell phone. And yes 
    indeed, it works great!
    
    -FER
    
    
    
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