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    Re: Navigation Spreadsheets now free
    From: Frank Reed
    Date: 2012 Sep 29, 11:18 -0700

    Peter, you wrote:
    " I was a little surprised to see the difference of 0.4' in the GHA of Venus developing toward the end of 2013"

    Hmmm. Since the HP of Venus is 0.4' at the end of the year, and since Venus is the nearly same size as the Earth so that its apparent angular semi-diameter is necessarily nearly equal to its horizontal parallax, I suspect that this is because the position of Venus listed in the Nautical Almanac is the position of the "center of light". This means that when Venus is a slim crescent, the position given in the NA is not the position of the physical center of the planet, but rather it is a point very close to the illuminated limb of the planet. This is an assumption which is reasonable for sextants equipped with 3x telescopes which cannot quite resolve the planet's disk, but it's really a rather arbitrary "editorial" choice. In software, I would suggest making this an optional setting.

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