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    Re: DIY long-term almanac for calculators
    From: Antoine Couëtte
    Date: 2018 Jan 24, 03:37 -0800

    Dear Paul,

    You wrote, "Curiously, his Sun GHA is as good as any method despite the greater error in GHAA."

    I had long observed this.

    Although I am far from any document to elaborate in further details, and to the best of my memories, the Sun Longitude is initially computed in Ecliptic 2000.0 . To get it into Ecliptic of date, you simply add "+ Delta Psi".

    Then you transform the Ecliptic Longitude into Right Ascension in the Equator of date.

    The precession effect in Equatorial coordinates has then become equal to "+ Delta Psi Cos (Obliquity)".

    The Calculation of GHA Aries also involves the very same term, i.e. "+ Delta Psi Cos (Obliquity)".

    When you compute GHA Body = GHA ARIES - Body RA, any error on the precession term then cancels out.

    Hope it may help,

    Antoine Couëtte

       
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