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    Re: Sun semidiameter
    From: Herbert Prinz
    Date: 2007 Apr 15, 00:13 -0400

    Bill,
    
    After overcoming my natural resistance to take a web site seriously that
    offers "ephemerii", I must admit that their astronomy is better than
    their English. The tabulated value for the sun's semidiameter on
    2007-05-14 is correct, in so far as it is based on a physical sun radius
    of 696000 km.
    
    The slightly larger value given in the N.A. is also correct. It is
    apparently based on a value of nominally 16' 01.18" at unit distance.
    This standard, which was also used in older astronomical almanacs is
    supposed to account for an irradiation effect.
    
    There may or may not be something in the archives of Oct 2004, when
    Alexander Eremenko sought help because a certain procedure for checking
    index error involving the sun's SD would not match up with the more
    reliable one using a star. My reply was on 2004-oct-10.
    
    Alexander's recent notion (msg 2568) that Her Majesty's almanac
    computers don't know how to round properly is quite amusing. His
    assertion that they would not care to do it properly borders libel.
    Accuracy of  the almanac data is outlined in detail in section 24 on
    page 261 (not on p. 254).
    In no way does the explanation suggest that the last digit of any
    tabulated value may be incorrect. It never is (with one deliberate
    exception which is specifically addressed).  The statement that the
    tables are correct to the nearest 0.1' means that the maximum error is
    nominally 0.05', not 0.1' as Alex would have us believe. A nominal error
    of 0.07' for the SD, were it real, would be unacceptable and would have
    to be reported.
    
    Herbert Prinz
    
    
    
    
    
    
    Bill wrote:
    
    >Looking at Frank's site
    >
    >http://www.clockwk.com/lunars/nadata_v5.html
    >
    >and Sun and Polaris Ephemerii for Land Surveyors
    >
    >http://www.cadastral.com/2007ephs.htm
    >
    >for 14 May 2007 I find agreement in the semidiameter of the sun; 15!83
    >(Frank) and  15' 49.6" = 15!83 (Cadastral).  Yet the nautical almanac lists
    >SD as 15.9.  I see this happening on other dates as well.  I would expect it
    >could happen near a rounding, point, say 15!9 one page and 16!0 the next,
    >but that thought does check out.
    >
    >Why the difference in SD?
    >
    >Bill
    >
    >
    >>
    >
    >
    >
    >
    
    

       
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