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    Re: Sun semidiameter
    From: Frank Reed CT
    Date: 2007 Apr 14, 21:31 -0700

    Bill, you wrote:
    "If we take the average of the 23,24.25 page, 15.83
    If we take the average of the 26, 27, 28 page, 15.84
    No way either rounds up to 15.9"
    And so that looks like a difference of 0.07 minutes of arc --maybe
    something to worry about. But actually it results from a much smaller
    difference. After all, that 15.9 displayed value covers everything
    from just above 15.85 to just below 15.95. All those values round to
    15.9. So if the result of the underlying calculation for the value in
    the N.A. was 15.853 (as an example), then the difference in the value
    from the other values you've found is only about 0.015 minutes of arc
    or one SECOND of arc. That's all you're seeing here. Because the Sun's
    semi-diameter changes so slowly, at the level of a tenth of a minute
    of arc, the displayed result of a calculation may differ by 0.1
    minutes of arc for some days even though the underlying calculations
    are only one second of arc apart. Just doing some rough comparisons,
    this seems to be about right. It appears that the Sun SD in the
    Nautical Almanac is derived from a mean of 961 arc seconds while the
    value that I use (and Meeus, and lots and lots of other software and
    text sources) is 960 arc seconds. To get the Sun SD on any other date,
    you simply divide by the Sun's distance in AUs on that date.
    Of course, if you really want to drive yourself nuts with the
    variation of the Sun's semi-diameter, go to the table for the Sun's
    total altitude correction on the inside front cover of the Nautical
    Almanac. Suppose the Sun's observed altitude was 10d 00'. Suppose the
    date is March 31st locally, but April 1st in Greenwich. So what's the
    altitude correction? According to the table, interpreted very
    literally (!), the correction is 11.0 minutes of arc if it's March 31,
    but it's 10.7 minutes if the date is April 1. Take your pick.
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