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    Re: Parallactic retardation - don't give up so easily.
    From: George Huxtable
    Date: 2004 Jan 10, 23:05 +0000

    Bruce Stark said-
    
    >Try this perspective:
    >
    >Put the moon on the equator, or south of it, and yourself in fairly high
    >north latitude.
    
    All right; then the Moon doesn't attain a high altitude.
    >
    >During meridian passage the moon's altitude is hardly changing,
    
    I agree.
    
    >and what
    >change there is will have slight effect on her refraction and parallax
    >correction.
    
    I disagree here. Although the AMOUNT of the parallax (and also refraction
    too, but let's not bother too much about refraction now) is hardly
    changing, its DIRECTION is changing rapidly.
    
    Remember, the parallax displaces the Moon's position such as to reduce its
    apparent altitude, acting along a line between the zenith and the Moon.
    When the Moon is on the meridian, it is due South of the Zenith, so the
    displacement due to parallax is along the azimuth of 180 deg. Before
    meridian passage, the line between the zenith and the Moon (along the
    direction of the Moon's azimuth) is somewhat to the East of South, so has
    an Eastward component. After meridian passage, it will have an increasing
    Westerly component.
    
    It's these Easterly and Westerly components that are acting to displace the
    Moon along its path, and the rate at which the Moon is displaced along its
    path is a maximum when the Moon's on the meridian. At that point, the
    displacement of the Moon caused by parallax is exactly at right-angles to
    its path, so the actual displacement along its path caused by parallax is
    exactly zero, although the rate-of-change of that quantity is a maximum.
    
    George.
    
    ================================================================
    contact George Huxtable by email at george@huxtable.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    ================================================================
    
    
    

       
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