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    Re: Eclipse maps & solar semidiameter
    From: Frank Reed
    Date: 2017 Aug 15, 19:56 -0700

    The difference in the diameter that they're talking about is around 0.3 seconds of arc out of an average of just about 960 seconds of arc (that's just the usual 16.0' SD of the Sun multiplied by 60). The mountains and valleys in the limb of the moon have differences from highest to lowest of as much as 4 seconds of arc or more. So this question about the Sun's diameter is a very small difference. It doesn't mean that they're wrong, and in fact re-aligning eclipse observations with the new precise lunar limb profiles derived by spacecraft in the past decade should probably lead to a small adjustment in the solar radius. Obviously there couldn't be any substantial uncertainty in the apparent solar radius since celestial navigation observations, especially including lunars, have been critically dependent on this parameter on a scale of some arcseconds since the late eighteenth century.

    This tiny detail all depends on the quite peculiar nature of the solar photosphere with its almost instant transition from opaque to transparent gas when the density of H- ions (hydrogen atoms with an extra electron loosely bound) falls below a magic, critical level. We're very lucky to have such a well-behaved star... in more ways than one!

    Frank Reed

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