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    Re: Parallactic retardation - don't give up so easily.
    From: Bruce Stark
    Date: 2004 Jan 9, 17:22 EST

    This was posted in the wrong thread. Here it is in the proper thread:
    
    I believe George was right when the concept of parallactic retardation first
    occurred to him. That is, it can affect the accuracy of GMT taken from the
    moon.
    
    But I never did understand what parallax in altitude had to do with it. The
    parallax in this problem comes, it seems to me, from the rotation of the
    earth.
    If you are at one of the poles the earth's rotation doesn't move you from
    west to east, and the effect disappears. But if you are at the equator,
    rotation
    moves you 900 nautical miles east every hour. To the extent the motion is
    perpendicular to the direction of the moon, parallax shifts the moon in the
    opposite direction.
    
    Nor do I understand what the lack of an effect on the cleared distance has to
    do with it. The difficulty is in measuring the distance we see. If not, why
    do we use scopes on our sextants? If we can only see and measure a shift of,
    say, 1' in the moon's apparent position, the accuracy of our resulting GMT
    will
    be affected if that 1' shift occurred in four minutes instead of two.
    
    Actually I hope I'm wrong in this, as some people might feel constrained to
    take lunars only at "appropriate" times. And experience suggests I am wrong.
    Seems to me the lunars I've taken when the moon is on or near the meridian
    have
    been as good or better than average. I intend to keep taking observations when
    it's convenient and comfortable to do so. Although I appreciate the
    discussions of these technical considerations, I'm not going to let them
    limit my
    pleasure.
    
    Bruce
    
    
    

       
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