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    Re: Out of Date Almanac
    From: Gary LaPook
    Date: 2009 Dec 08, 11:30 -0800

    I wrote:
    
    
    "So using the December 31, 2009 star positions for all of 2010 will not 
    result in "errors of a few minutes of arc"  that you are worried about."
    
    Frank wrote:
    "That's good data, but not relevant to this question. He asked about using a 
    2006 almanac in 2010. That's not one year."
    
    I had sugested that he zerox the page for the last day of 2009 which has the 
    positions of the stars, fold it up and tuck it inside his 2006 almanac which 
    seems like a simple work around. Of course if he didn't do this and attempted 
    to use the star positions from 2006 then the errors would be larger since it 
    would be a five year difference.
    
    gl
    
    
    
    
    
    
    frankreed@HistoricalAtlas.com wrote:
    > Gary, you wrote:
    > "It looks like you are making it too hard. I always come back to to the 
    > question, "how accurate do you need your position to be?" "
    >
    > If we can tolerate thirty mile errors, then hey, just use a kamal, etc. :-) 
    But that isn't the point here, right? What kind of errors can one expect from 
    using a 2006 almanac in 2010? It's a rather specific question.
    >
    > And you wrote:
    > "If you can tolerate an error of up to one NM then you can use one set of 
    > star positions for at least one year on both sides of the date of the 
    > star positions."
    >
    > That's good data, but not relevant to this question. He asked about using a 
    2006 almanac in 2010. That's not one year.
    >
    > You concluded:
    > "So using the December 31, 2009 star positions for all of 2010 will not 
    > result in "errors of a few minutes of arc"  that you are worried about."
    >
    > But, Gary... You know that he didn't ask about 12-31-2009. He asked about a 
    2006 almanac, which is a very reasonable thing to ask considering the leap 
    year cycle for solar position data. As for the stars, if you use a 2006 
    Nautical Almanac in 2010, you need to correct for precession (primarily 
    precession, with other smaller effects). And to correct for precession, the 
    simple correction for GHA Aries which you gave is insufficient, though it 
    might be a good place to begin.
    >
    > Let's try two specific cases: Capella and Kochab. What are their positions 
    in the Nautical Almanac in 2006? What would they be in 2010?
    >
    > -FER
    >
    >
    >   
    
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