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    Re: getting GHA from MICA
    From: Paul Hirose
    Date: 2014 Mar 19, 20:03 -0700

    I wrote:
    > For instance, at 2014 March 19 at 1700 UT1, compare Moon LHA from N89 59
    > 59 W000 00 00 to a GHA computed by subtracting the geocentric apparent
    > RA from Greenwich apparent sidereal time.
    >
    > 220 01 11.6  topocentric LHA
    > 220 01 11.5  RA - GAST
    
    Last line should say GAST - RA, as the paragraph above says.
    
    The topocenter should be near a pole to minimize parallax, because you
    are trying to emulate a geocentric observation. Of course an observation
    at a pole has parallax too, but the observer's displacement from the
    geocenter is along Earth's axis. Therefore hour angle is not affected. A
    terrestrial analogy is observing an azimuth from ground level vs. from a
    tower directly above. Elevation is different, but not azimuth.
    
    I chose the Moon for my example to make this more critical. The Moon's
    maximum parallax is about one degree.
    
    MICA is (or was, last time I looked) claimed to be suitable for
    celestial navigation. But unless improvments have been made, it gives
    unrefracted zenith distance in DMS.s format, a far cry from the
    refracted DM.m altitude a navigator needs. Tinyac, with its elaborate
    coordinate system options, was inspired by my disappointment with MICA.
    
    --
    I filter out messages with attachments or HTML.
    

       
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