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    Re: Out of Date Almanac
    From: Gary LaPook
    Date: 2009 Dec 08, 03:36 -0800

    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 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.To improve on this you do not need a separate precession 
    correction for each star since the correction depends on latitude, LHA 
    Aries and azimuth. I have attached the precession and nutation 
    correction table from H.O. 249 volume one, epoch 2010. This volume is 
    based on the position of the stars on January 1, 2010. It shows no 
    correction is needed for the years 2009 or 2010, the period of one year 
    before to one year after the date of the star positions. Since H.O. 249 
    is accurate to one minute this means that there are no errors exceeding 
    one minute if no precession and nutation correction is applied for this 
    two year period. You can also see that the correction is only one minute 
    for an additional year on either side, 2008 and 2011.
    I have also attached the P&N table for epoch 2005 which displays the 
    correction to one-tenth of a minute ( I don't have the same table for 
    the epoch 2010 volume) to use as an example that we could also expect in 
    epoch 2010. You can see that the corrections for 2004 and 2005, plus and 
    minus one year from the January 1, 2005, the date of the star positions 
    for epoch 2005, don't exceed .6' and for the next year, 2006, they don't 
    exceed 1.3'.
    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.
    frankreed@HistoricalAtlas.com wrote:
    > Gary, you wrote:
    > "It's an exact correction to GHA Aries"
    > OK, but does it really do you any good? What you really need is a precession 
    table for each of the navigational stars. And we don't even need to think of 
    it as a pure precession table. If there's a longer interval involved, it 
    could include proper motion. But it's nothing complicated and not hard to 
    produce. The point is that it's some number of minutes of arc for each star's 
    SHA and Dec. And then you throw in the correction for GHA Aries and add them 
    up as usual to get the position of the star. But correcting GHA Aries alone 
    while leaving out the corrections for the individual stars doesn't help. So 
    instead, the Aries correction can just be lumped in with the other tabulated 
    corrections for the individual stars. Algorithmically, 
    > we can do either:
    > Dec*(2010) = Dec*(2006) + dDec*
    > GHA*(2010) = [GHA Aries(2006) + dGHA Aries] + [SHA*(2006) + dSHA*]
    > or:
    > Dec*(2010) = Dec*(2006) + dDec*
    > GHA*(2010) = GHA Aries(2006) + GHA*(2006) + dGHA*
    > It's not much of a difference --saves one step. But no matter what, we still 
    need a table of dDec* and either dGHA* or dSHA* for all the navigational 
    stars. And again, it's not hard to make one. Or you can just accept errors of 
    a few minutes of arc, or just not use the old almanac for the stars at all.
    > -FER
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