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    Re: Sun semidiameter
    From: Bill B
    Date: 2007 Apr 17, 23:14 -0400

    > From: Frank Reed 
    
    > Herbert Prinz wrote:
    > "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. "
    >
    > Could this really be connected with the so-called irradiation
    > correction? It's only a bit more than 1 second of arc larger than the
    > more commonly used value. Surely the irradiation phenomenon was
    > supposed to amount to several tenths of a minute of arc --15 to 20
    > times larger than this measly arcsecond.
    
    Indeed, we did not all receive Herbert's post, but off list he directed me
    to it in the archives.  I too had questions about the small difference from
    irradiation.  Herbert's response was:
    
    "I am not sure what you have in mind here, but I am talking about the
    1.55" irradiation constant that Auwer derived from the analysis of over
    6000  transits observed in Greenwich versus 3000 heliometer
    measurements. He got 961.18" from the former and  959.63" from the
    latter. Other people may have come to other conclusions, but those are
    the values that ended up in the various almanacs and that I posted. When
    you reduce the latter value at unit distance to the physical radius you
    obtain 696000km."
    
    My response:
    
    "...I was thinking of irradiation as an adjustment to observed body
    elevation.  The thinking being that the sun would appear larger against a
    dark background and the horizon (light sky) would appear lower against the
    darker sea. Hopefully this would cancel out in lower limb observations but
    be a problem in upper limb shots.  In discussions of this I have seen
    correction values in the 1!2 range, hence my confusion."
    
    Am either tangled up in words again, or are we talking about two different
    animals running under the "irradiation" flag?
    
    Bill
    
    
    
    
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