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    Re: Irradiation
    From: George Huxtable
    Date: 2004 Nov 29, 10:49 +0000

    Alex is demonstrating, to himself and to us, the uncertainties of
    refraction at low altitudes.
    
    He tells us that his recent measurements of Sun semidiameter were taken, in
    one case "sun very low, just before sunset" , and on another day "few
    minutes before sunset", without saying what the Sun altitude actually was.
    But anyone can see, at times near sunrise and sunset, that the Sun's disc
    is often distorted, squashed or expanded in a vertical direction. And Alex
    mentioned a paper about refraction, in a posting labelled "dip" on 27 Nov,
    
    "their main conclusions seem to imply that the refraction near the horizon
    (and thus the dip) is uncertain to more than 1/2 of a degree."
    
    The only thing wrong with that quotation was Alex's interpolation "(and
    hence the dip)".
    
    The problem is due to horizontal layers of air at differing temperatures,
    through which the light has to pass. Those layers can be there at any time
    but they become obvious only when a distorted Sun disc shows them up.
    
    Trevor had written-
    
    >> The better demonstration of irradiation, for members of this list,
    >> might be to measure the semi-diameter of the Sun, under various conditions
    and Alex responded-
    >Actually, this was the main resaon for my questions:
    
    But it might have been better to qualify that suggestion-
    
    >> might be to measure the semi-diameter of the Sun, under various
    >>conditions, but NOT at low altitudes.
    
    If it's necessary to measure Sun semidiameter at low altitudes, an
    observation with the sextant horizontal (as Alex made) is likely to give
    better answers, as refraction affects altitudes rather than azimuths. If
    measured horizontal angles differ from tabulated values, then that
    discrepancy is likely to be due to a combination of instrumental error and
    the optical effect of irradiation.
    
    It is possible to calculate an allowance for vertical compression of the
    Sun near to the horizon, but this applies only to conditions of standard
    refraction, and can not account for the transient local distortions which
    can have such an important effect, so is not worth much. The vertical
    compression can change from minute to minute, as the image of the Sun's
    disc rises or sinks through these air-layers.
    
    In comparing predicted Sun semidiameter (from the Nautical Almanac
    tabulations, presumably) with his observations, a significant source of
    scatter may be in those tabulations, which are given to the nearest 0.1
    arc-minute. The semidiameter is of course known to a much higher precision,
    and given to a hundredth of an arc-second in the Astronomical Almanac, if
    Alex is seeking the ultimate.
    
    I agree with all three of Alex's conclusions.
    
    George.
    
    ================================================================
    contact George Huxtable by email at george@huxtable.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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