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    Re: Dip
    From: George Huxtable
    Date: 2004 Dec 1, 14:59 +0000

    Alex wrote-
    
    >From what I read, error in the dip due to refraction
    >is usually much less less than 1', that is not totally
    >negligible but small.
    
    Response from George-
    
    Yes, that USUALLY is the case. But occasionally, the dip is significantly
    greater, as in Blish's own report of observations off Southern California,
    and you have no way of knowing that it's so. So, presumably, the prudent
    navigator has to allow for a possible error in his observations up to the
    maximun possible (or likely) value of anomalous dip, whatever he considers
    that might be. Unless he has a dipmeter to check it, he is working in the
    dark, as far as dip is concerned.
    
    He can null out the effects of anomalous dip by taking a round of
    star-sights at dusk that include stars at many different azimuths, or,
    better, star-pairs in nearly opposite directions.
    
    Referring to Schufeldt's refraction corrections, Alex mentions-
    
    >He uses some Russian tables of refraction for such altitudes
    >published in Moscow in 1930.
    
    Yes, the astronomers at Pulkova Observatory (near Leningrad, I think) took
    a special interest in refraction, and there has been a series of
    publications on the topic from Pulkova over many years. They have long been
    regarded as the authorities on this matter.
    
    The most recent reference I've found (in Seidelmann's "Explanatory
    Supplement") is to "Refraction Tables of Pulkova Observatory, 4th ed.",
    (Academy of Sciences Press, Moscow, Leningrad), 1956, though I haven't seen
    this publication.
    
    >He says that he took 380 sights at such altitudes, and with very
    >careful correction of the sights (including water and air temperature
    >and other meteorological info, irradiation etc.) his position
    >line was never mostly within 2 miles of his true position.
    
    But Alex has to accept that no matter how good are tables of mean
    refraction and any meteorological corrections, the evidence of his own eyes
    showing distortions in a low-level Sun-disc proves that occasionally
    refraction can deviate locally and substantially from any predictions.
    
    George.
    
    ================================================================
    contact George Huxtable by email at george---.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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