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    Re: Old style lunar
    From: Fred Hebard
    Date: 2004 Dec 13, 12:28 -0500

    On Dec 13, 2004, at 12:00 PM, Ken Muldrew wrote:
    
    >> Now suppose we measured both distances EXACTLY with our sextant
    >> and want to deduce longitude (or chronometer corr., does not matter)
    >> Averaging the two reduction results will give you nothing.
    >> Both will give the same error in time/longitude.
    >
    > I don't see how that follows. One error puts you too far West of your
    > true
    > position, the other error puts you too far East. Averaging the two
    > cancels
    > the systematic error of the moon being in the wrong place.
    >
    
    I believe Alex is correct about this.  First, assume the errors are in
    the RA not the declination, which is reasonable (and that the positions
    of the stars are accurate).  Essentially, the almanac puts the moon too
    far along in it's orbit or not along far enough.  If it's too far
    along, it will be closer to the star in the east and farther from the
    star in the west.  Thus the error in the eastern distance will be of
    opposite sign from that of the western distance, but both distances
    will give a time that is greater than the correct time.
    
    I wanted to work out a numerical example before responding to Alex, but
    haven't had the opportunity.
    
    Fred
    
    
    

       
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