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    Re: GHA of the Moon
    From: Roger W. Sinnott
    Date: 2020 Oct 2, 00:49 +0000

    The Moon's orbit around the Earth is severely perturbed by the Sun (mainly).  For $5, willbell.com sells a booklet called "Low-precision Formulae for Planetary Positions" by Van Flandern and Pulkkinen.

    To calculate the Moon's GHA to 1-arcminute accuracy involves these steps:

    (1) Evaluate 7 preliminary arguments, each from a short polynomial in time.
    (2) Use them in a 55-term trig formula for the Moon's ecliptic longitude.
    (3) Use them again in a 38-term formula for the Moon's ecliptic latitude.
    (4) Transform the Moon's ecliptic long and lat to right ascension (RA).  
    (5) Obtain the Greenwich sidereal time (GST) from a short formula.
    (6) Combine the RA and GST to get the GHA.

    For greater accuracy than 1 arcminute, you'll need MANY more trig terms in steps 2 and 3 above.  Those can be found in other books besides Van Flandern and Pulkkinen.

    Roger S.

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