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    Appy Day is here again
    From: Frank Reed
    Date: 2009 Jul 4, 17:07 -0700

    O Appy Day. Aphelion Day, actually...
    
    More correctly, it was yesterday -- the Earth at aphelion. And so the Sun's 
    semi-diameter is at its lowest for the year at 15.74 minutes of arc with no 
    change to the nearest 0.01' for about four weeks centered on this date (full 
    diameter = 31.48'). Around perihelion in early January, the SD is 16.27' 
    (full diameter = 32.54').
    
    Incidentally, back in the 19th century, an apparently common confidence check 
    on index corrections derived from Sun limb-to-limb observations was to check 
    the measured apparent diameter of the Sun (which you would normally throw 
    away) with the tabulated values in the almanac. 
    
    But what about refraction? When the Sun is low in the sky, you have to worry 
    about the flattening, and, while that's easy enough, for the moment let's 
    stick with the case where the Sun is above 45 degrees. In that case, since 
    refraction shrinks all angles by almost exactly the same proportional amount, 
    the Sun is smaller both vertically and horizontally (and in any other 
    direction in which you would like to measure the apparent diameter) by a 
    factor of 1.00034 so the apparent diameter would be 31.47' instead of 31.48' 
    even when the Sun's center is exactly in the zenith. Needless to say, a 
    change in angular size of 0.01' is completely insignificant for sextant 
    observations.
    
    Tricky question: since the Sun is reduced in size by refraction, is it fainter 
    (for the same true angular size if there were no refraction)?
    
    -FER
    
    
    
    
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