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    Re: Moon's 4SD
    From: Alexandre Eremenko
    Date: 2006 Nov 2, 00:05 -0500

    
    
    The value in the almanac for SD of the Moon
    is not only rounded too much but it is also too small.
    Apparently they compute it "as seen from the center
    of Earth".
    The real value depends on the Moon's altitude and it
    is given by approximate formula
    
    SD=0.27277HP(1+sin(HP)sin(ALT)),
    
    where HP is the Moon Horizontal Equatorial Parallax
    (rightmost column of the Moon section of the daily page)
    and ALT is the Moon altitude.
    The second term is non-negligible if we talk about 0.1'
    precision!
    
    Computation with this formula shows that all
    my (and Frank's) recently mentioned measurements
    of IC using Moon are acceptable, that is 4SD is
    within 0.4 from the true value.
    
    It remains to explain why star and Sun tests give
    consistently
    smaller IC than the Moon test (at least in my measurements).
    It is interesting how various authors rate the
    various tests
    for IC in the order of decreasing precision.
    
    Heath (of Hezzanith): Star, Horizon, Sun.
    Russian manual: Sun and Star equally good, then Horizon.
    (They quote the statistics that the typical
    accuracy of the Sun/star test is 0.1 to 0.2, while
    the Horizon gives only 0.5.
    
    I've never seen the Moon mentioned for IC tests,
    before Frank's messages.
    
    Today I also tried wires. Computation shows that to achieve
    0.1 accuracy of the test, the wires have to be at
    least 1 nautical mile away. It is really hard to see
    any wire
    at this distance., even with a 7x telescope.
    At least at my age:-(
    
    Alex
    
    
    
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