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    Re: Out of Date Almanac
    From: Frank Reed
    Date: 2009 Dec 8, 00:26 -0800

    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*]
    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.
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