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    Re: No Nautical Almanac data during Aug 21st eclipse
    From: Frank Reed
    Date: 2017 Aug 18, 23:17 -0700

    Bill B, you wrote:
    "the semi diameter of the moon at full totality will be larger than the sun's, so a pretty sweet view for those lucky enough to be centered in the path."

    In fact, that simple condition, SDmoon > SDsun, is the actual defining condition for a total solar eclipse. If it's the other way around, then you get an annular eclipse which is just a flavor of partial eclipse with none of the drama or beauty of a total solar eclipse. You only get a total solar eclipse when the Moon completely covers the brilliantly bright photosphere of the Sun leaving behind the ghostly corona. When the Moon is a bit too far from the Earth, implying a lower apparent diameter (and in CN terms quantified by a lower HP which is directly proportional to SD), it does not cover the photosphere and the Sun is only reduced in brightness by 5 or 6 magnitudes. An annular eclipse like that is interesting to the eclipse afficionado, another box to check off, but much less impressive than a total eclipse.

    Frank Reed

       
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