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    Re: October Lunar
    From: George Huxtable
    Date: 2008 Oct 6, 17:28 +0100

    Jeremy wrote-
    
    | I have repeated this experiment the next day with Venus, and then
    | again today (5 Oct) with the Sun.  I am getting a consistant error of
    | 1 minute on of my averaged sights, so I am guessing I have an
    | alignment issue with my sextant.  Fortunately, since the error is
    | constant, it is easily compensated for.
    
    =================
    
    Presumably, with the second Venus observation, on the next night, the angle 
    being measured on the sextant was significantly greater, by at least 10�. So 
    that's two widely-spaced data points on a "calibration curve" of his 
    sextant, that are both 1' out. And presumably the Sun lunat was well-spaced 
    again, from those two, and was still 1' out (presumably, in the same 
    direction).
    
    If all points on a sextant calibration are out by 1', that's not so much an 
    error in the overall scale as an index error of some sort. It could be a 
    built-in error, due to the zero-mark on the sextant's arc being a bit out of 
    place, but that seems a bit unlikely. More likely is some difference between 
    the way the index-error is being assessed, compared with the way the angles 
    (lunar distances) are being measured.
    
    And this takes us into rather deeper waters. The discrepancy that Jeremy 
    reports, though consistent, is only a small angle, of 1'. To measure a 
    planet against the Moon's limb calls for a bit of judgment, because the 
    image of the star is not a precise point, and the image of the limb is not 
    an infinitely-sharp boundary. The eye does its best with them, but it's 
    beset with the problem of "irradiation", in which brighter objects such as 
    the Moon disc, against a darker background, appear to be slightly enlarged. 
    And this affects different observers' eyes to a different extent. It may be 
    that after long practice, a lunar observer gets to know just how much 
    overlap (or alternatively, gap) to allow for, to end up with just the right 
    answer. I think this may be what Jeremy was getting at when in [6343] he 
    referred to a "personal observation error".
    
    Whenever we near the limits of what we can perceive, some such degree of 
    personal judgment comes in. It sets a limit to the inherent accuracy 
    available in an observation, and being systematic rather than random, 
    repetition and averaging aren't going to help.
    
    George.
    
    contact George Huxtable, now at george{at}hux.me.uk
    (switched from george---.u-net.com)
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. 
    
    
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