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    Which diameter of the sun in digital photos ?
    From: Marcel Tschudin
    Date: 2009 Aug 21, 14:49 +0300

    For investigating the refraction near the horizon I perform systematic
    observations by photographing sunsets. So far I used the horizontal
    diameter of the sun as a reference scale (pixel per degree) for
    measuring the sun's position above the apparent horizon.
    
    Recently I compared this scale to one derived from pictures of
    landscapes and found to my surprise that they differ by around 8%; the
    sun is systematically slightly larger.
    
    The scale in landscape photos has been measured with distinct features
    and the distances measured with Google Earth. The scale for (not
    overexposed) sun photos has been calculated with the angular size of
    the sun for the corresponding days; this size is based on the sun's
    diameter (of the photosphere) and the earth's distance.
    
    The difference is not related to refraction since the measurements are
    made in horizontal direction.  Any idea, why those two scales differ?
    It is known that some CCDs have a leak in the IR, also the one in my
    camera. Could this be responsible for it? Does anyone know more on
    this?
    
    Marcel
    
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