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    Re: Lunar trouble, need help
    From: George Huxtable
    Date: 2008 Jul 12, 00:08 +0100

    Kent and I seem to be disagreeing, somewhat, about the principles involved 
    in correcting for refraction, and I had written-.
    
    " In practice, in the case we are considering, the difference in refraction 
    between the Moon's centre and upper limb is pretty infinitesimal: no more 
    than 0.4 arc-seconds. Presumably, Kent is keen on such details because he 
    wants to be completely rigorous, and get the
    principles right. And it seems to me that he hasn't got the principle right 
    here. I will try to convince him.
    
    He had written, and my comments are [interpolated]-
    
    "Consequently in my way to do the reduction I start with refraction on the 
    UL ..."  [all right so far...  ] "and then I calculate the difference in 
    refraction for the UL - SD for the moon." [that's the bit that's 
    unnecessary, and indeed wrong].
    
    ========================
    
    Kent has replied-
    
    I am still of the opinion that the refraction correction used in altitude 
    reduction must be referred to the geocentre of the moon. I find this 
    correction  by calculating the refraction both on the UL/LL and the 
    geocentre, which means as in Jeremy�s case that I add a small value due 
    measurement of the UL. George means that this approach is unnecessary 
    because the correction (in Jeremy�s case) is so small. Even if George has 
    tried to convince me I still keep my opinion."
    
    =========================
    
    NO! I do NOT argue that "this approach is unnecessary because the correction 
    (in Jeremy's case) is so small". I argue that it is wrong in principle, no 
    matter how big or small the refraction might be! If the upper limb is being 
    observed, then there is only one light-ray involved, that from the upper 
    limb to the observer, and that is the ray which the atmosphere is 
    refracting, and that is the ray to be corrected for that refraction. The 
    refraction from the centre and from the lower limb play no part, because 
    there's no light-ray from those directions being observed. Kent is welcome 
    to keep his opinion, but unless we can somehow persuade each other into 
    agreement on something so fundamental as this, we will not get any further 
    forward.
    
    =======================
    
    As a separate issue, Kent has written-
    
    "Furthermore, it seems that we do not yet agree on correction for clearing 
    the LD. The corrections I use are similar to what can be found in Henry 
    Raper, 1840, Table 45. This correction is for small deviations in refraction 
    due to the angle between the distance and vertical (and when reducing 
    altitudes this angle is 0d).  Maybe this convince George that my way of 
    treating these corrections are not completely off-road?"
    
    Actually, Kent and I agree about that correction, when working a lunar with 
    maximum rigour. My edition of Raper dates from 1864, and in that, it's table 
    53, "Correction of the lunar distance for the contraction of the vertical 
    semidiameter". Perhaps Kent will confirm that's what he is referring to. 
    That's used, just as it says, to correct the measured distance for the 
    apparent vertical shrinkage of the Moon, and that certainly does depend on 
    the difference in refraction between the Moon's centre and limb. Indeed, I 
    took a look at that table, and noted that for all Moon altitudes above 30�, 
    it would be less that 1", so disregarded it. So in this case, (unlike for 
    the corrections to altitude, above) that correction really was a matter of 
    being right in principle but numerically trivial. But it isn't to be used 
    when correcting for altitudes above the horizon.
    
    George.
    
    contact George Huxtable at george@huxtable.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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